Introduction to VLSI Design

3.2.5 Metal Etch ... 3.0 Introduction ... reaction in which the etch species react with the film forming soluble byproduct...

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Chapter 3

UEEP2613

Microelectronic Fabrication

Etching

Prepared by Dr. Lim Soo King 24 Jun 2012

Chapter 3 ............................................................................................83 Etching ................................................................................................83 3.0 Introduction .............................................................................................. 83 3.1 Basic Theory of Etching .......................................................................... 84 3.2 Wet Etch .................................................................................................... 86 3.2.1 Oxide Etch ......................................................................................................... 88 3.2.2 Silicon and Polysilicon Etch ............................................................................. 88 3.2.3 Photoresist Etch ................................................................................................ 88 3.2.4 Silicon Nitride Etch ........................................................................................... 89 3.2.5 Metal Etch.......................................................................................................... 89

3.3 Dry Etch .................................................................................................... 89 3.3.1 Plasma Etching Mechanism ............................................................................. 94 3.3.1.1 Chemical Etching .......................................................................................................... 94 3.3.1.2 Physical Etching ............................................................................................................ 96 3.3.1.3 Ion Enhanced Etching .................................................................................................. 97

3.3.2 Type of Plasma Etching Systems ..................................................................... 98 3.3.2.1 Barrel Etching System .................................................................................................. 99 3.3.2.2 Magnetic Enhanced and Confinement Plasma Etching System ............................. 100 3.3.2.3 Electron Cyclotron Resonance Plasma Etching System .......................................... 102 3.3.2.4 Inductively Coupled and Helicon Wave RF Plasma Etching Systems ................... 103

3.3.3 Plasma Etching for Various Films................................................................. 105 3.3.3.1 Plasma Etching Silicon Dioxide ................................................................................. 108 3.3.3.2 Plasma Etching Polysilicon ........................................................................................ 108 3.3.3.3 Plasma Etching Aluminum ........................................................................................ 108

Exercises ........................................................................................................ 109 Bibliography ................................................................................................. 112

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Figure 3.1: Illustration of the different etch profile between plasma and wet etching ........... 84 Figure 3.2: Illustration of etch bias and over etch .................................................................. 86 Figure 3.3: Illustration of etching profile caused by mask erosion. The dotted line shows the outline of the mask after etch ..................................................................... 87 Figure 3.4: Illustration of the titanium, silicidation, and wet etch ..................................... 89 Figure 3.5: Schematic diagram of RF power plasma system ............................................. 91 Figure 3.6: Voltage distribution in RF powered plasma etch system................................. 92 Figure 3.7: Typical chemical reactions and species present in plasma .............................. 93 Figure 3.8: Chemical reaction of radical species with substrate material .......................... 95 Figure 3.9: Fluxes of specifies in plasma etch. (a) Neutral radical chemical etching and (b) positive ion physical etching ............................................................................ 96 Figure 3.10: Rate of etching of silicon illustrating synergic effect of ion enhanced etching .......................................................................................................................... 97 Figure 3.11: Inhibitor prohibits lateral physical etch ........................................................... 98 Figure 3.12: Comparison of ion energy and operating pressure ranges for different types of plasma systems................................................................................................. 99 Figure 3.13: Schematic of a barrel plasma etching system showing (a) side view and (b) cross sectional view ....................................................................................... 100 Figure 3.14: Schematic of magnetic-enhanced reactive ion etching system with a timeaverage magnetic field applied parallel to the cathode .................................. 100 Figure 3.15: Schematic of magnetic-confinement reactive ion etching system with a multipolar magnetic bucket surrounding the etching chamber .............................. 101 Figure 3.16: Schematic of an electron cyclotron resonance etching system ...................... 103 Figure 3.17: Illustration of an inductively coupled plasma etching system with PMT MORI® source................................................................................................ 104 Figure 3.18: Schematic of a helicon wave plasma etching system with a double-loop antenna ........................................................................................................... 105 Figure 3.19: Boiling points of some typical etch byproducts ............................................. 106 Figure 3.20: Common etchant types for plasma etching film ............................................ 107

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Chapter 3 Etching _____________________________________________ 3.0 Introduction Microelectronic devices are built from a number of different sequentially fabrication processes. After lithography process described in the earlier chapter, whereby the desired pattern is transferred, is etching process. In the era of ULSI integrated circuit manufacturing, the etching process grows more important for fabricating features with sub-micron dimensions. In the early 1970s, lithography with Novolak-based resists was used to pattern devices with dimensions less than three micrometers. Oxygen plasma was explored for stripping of resist. The needs to maintain dimensional tolerance during etching became a critical requirement that led to numerous advances in plasma etch processing and improved reactors during the 1980s. Today, there is a broad base of empirical knowledge and some qualitative understanding of etch mechanisms. These insights and systematic experiments led to the process developments needed to meet the requirements of today's submicron device fabrication. Etch techniques consist of dry and wet etch methods. Dry etch methods include plasma etching, sputtering, ion beam etching, and reactive ion beam etching. For the chapter, plasma dry etch method will be discussed in details because they are the most popular method used in semiconductor industry. The chemistry and parameters of wet etching method used in the microelectronics industry will be discussed to a certain extend. Figure 3.1 illustrates the different results of dry etch and wet etch. As shown in Fig. 3.1(a) and Fig. 3.1(c), dry etch is basically the anisotropic etch whereby the rate of vertical etch is much higher than lateral etch. Wet etch technique would result in isotropic etch in Fig. 3.1(b) and Fig. 3.1(d), whereby the rate of vertical etch and the rate of lateral etch are comparable.

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(a)

(b)

(c) (d) Figure 3.1: Illustration of the different etch profile between plasma and wet etching

3.1 Basic Theory of Etching Rate of etching is a measure of how fast the material being removed in etching process. It is an important characteristic of the process since it directly affects the throughput of the etching process. The rate of etch can be calculated by measuring the film thickness before and after the etch process and dividing the thickness difference by etch time. Thus, the rate of etch is equal to Rate of etch 

Thickness before etch - Thickness after etch Etch time

(3.1)

For pattern etch, the rate of etch can be determined by scanning electron microscope SEM measurement, which can directly measure the removed rate of film thickness. It is very important to have uniform etch rate across the wafer or good within wafer WIW uniformity, high repeatability, or good wafer-to-wafer - 84 -

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uniformity WTW. Normally the uniformity is measured with wafer thickness by mapping the thickness of certain points before and after etch process, and calculate the rates of etch at these points. The average  and standard deviation  of the rate of etch are then determined. From the data, the standard deviation non-uniformity NU is calculated using equation (3.2). NU = /x100%

(3.2)

Another parameter called max-minus-min non-uniformity NUM is determined using equation (3.3). NUM 

Maximum rate of etch - Minimum rate of etch x100% 2

(3.3)

Etching can be very selective especially wet etch because it depends on the chemistry. The limiting step in most etch process is usually the chemical reaction in which the etch species react with the film forming soluble byproduct. The selectivity S of an etch process between two material 1 and 2 is simply the ratio of their etch rates in the etchant, which follows equation (3.4). S

r1 r2

(3.4)

Material 1 is usually the film being etched and material 2 is either the masking material such as photoresist and silicon dioxide SiO2 or material below the film. Take for example, etching polysilicon of the gate, the photoresist acts as etch mask. Thus, it is necessary to have high polysilicon-to-photoresist selectivity particularly the plasma-etch process to prevent excessive etching of photoresist before the etching process is completed. Buffering agent is often added to the etching solution to keep the etchant at maximum strength over use and time. For example, ammonium fluoride NH4F is added to hydrofluoric acid HF to help preventing depletion of the fluoride ion in the oxide etching. This is called “buffered HF” or BHF or more commonly is called “buffered oxide etch” or BOE. The addition of ammonium fluoride NH4F also reduces the rate of etching for photoresist and minimizing lifting of photoresist during oxide etching. Acetic acid CH3COOH is often added to nitric acid/hydrofluoric acid during silicon etch to limit the dissociation of nitric acid.

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3.2 Wet Etch The first etchant used in the integrated circuit industry was simple wet chemical etchant. Wet etchant works as chemical process and is not physical process. By immersing the wafer into the bath of liquid chemical, the exposed film can be etched away leaving un-etched region of the film that is masked with photoresist or other films. Most chemical wet etch is isotropically. The exception is the one which is sensitive to crystallographic orientation. Some etchants etch much slower in the <111> direction in crystalline silicon as compared to the other direction. However, most of the commonly wet etch is usually equal in all directions. Figure 3.2 shows the amount of undercutting which is called the etch bias. One assumes that the selectivity of the etching with respect to both the mask and the substrate is infinity that is no etch-away at all. The etch thickness or depth is d and the bias is b. For perfect isotropic etching b is equal to d. It is true only if the etching is stopped before reaching the bottom.

(a) Etch bias (b) Over etch Figure 3.2: Illustration of etch bias and over etch

Over etch is always exist. It is due to thickness of film is not uniform. It is the standard operating procedure in microelectronic industry that over etching is allowed to ensure film is completely etch-away. The amount of over etch is usually measured in terms of time or % time. It can be determined by estimating the uncertainty in etch rate and in the non-uniformity of the thickness and then calculating the worst case etch time needed. Ten to twenty percent over etch is common. Since over etch is unavoidable, it is important that the selectivity with respect to the layer below be as high as possible or else some etching of the below layer will occur during over etch. - 86 -

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Etch rate of mask such as photoresist is always there. Figure 3.3(a) shows the isotropic etch with a rectangular shaped mask. m is the amount of mask etched in all directions. This is called mask erosion. For the case of anisotropic etch, as long the mask is a perfectly rectangular shape, etching will occur only on the top of the mask. For the case as shown in Fig. 3.3(b), mask erosion leading to extra undercut can happen even through it is completely anisotropic etch.

(a) Ideal rectangular mask isotropic etch (b) Sloped mask anisotropic etch Figure 3.3: Illustration of etching profile caused by mask erosion. The dotted line shows the outline of the mask after etch

The degree of anisotropy of a film Af is defined as A f 1

rlat rver

(3.5)

where rlat and rver are lateral etch rate and vertical etch rate respectively. The degree of anisotropy of a film Af can also be defined based on the depth of etch and thickness of undercut, which is A f 1

b d

(3.6)

Obviously for isotropic etch, the degree of anisotropy is zero since b is equal to d. For the complete etch to the bottom, d is equal to thickness of film, which is xf then degree of anisotropy is equal to A f 1

b xf

(3.7)

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3.2.1 Oxide Etch Silicon dioxide SiO2 is usually removed by aqueous hydrofluoric acid HF with or without adding of ammonium fluoride NH4F as buffered HF. SiO2 + 6HF  H2SiF6 + 2H2O

(3.8)

H2SiF6 + 2H2O  SiF4 + 2HF

(3.9)

The byproduct H2SiF6 is a water soluble complex which can be removed by cleaning. Hydrofluoric acid is normally diluted with water or ammonium fluoride to control the pH value and to slow down the oxide etching process by controlling the amount of fluoride. A 6:1 buffered solution of HF or 10:1, and 100:1 in water are commonly used.

3.2.2 Silicon and Polysilicon Etch For the case of isotropically etching silicon Si, the common etchant is the mixture of nitric acid HNO3 and hydrofluoric acid HF. The etching occurs by oxidizing the surface of the silicon to form silicon dioxide SiO2 by partially decomposed nitric acid into nitrogen dioxide NO2 following the chemical equation (3.7). Si + 2NO2 + 2H2O  SiO2 + H2 + 2HNO2

(3.10)

The hydrofluoric acid HF then dissolves the silicon dioxide SiO2 by reaction given in equation (3.11). 2SiO2 + 6HF  H2SiF6 + 2H2O

(3.11)

Mixture of potassium hydroxide KOH, isopropyl alcohol CH3H8OH and water H2O can etch selectively to different orientation of single crystal silicon. With 23.4 wt% potassium hydroxide KOH, 13.3 wt% C3H8OH, and 63.3 wt% H2O at 80 to 820C, the rate of etch along (100) plane is about 100X higher than along (111) plane.

3.2.3 Photoresist Etch The photoresist can be removed by sulfuric acid H2SO4 and hydrogen peroxide H2O2 for wafer without metal. This is because sulfuric acid will etch metal away. For wafer with metal, organic etchant is used. - 88 -

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3.2.4 Silicon Nitride Etch Silicon nitride is widely used for isolation formation process. Hot phosphoric acid H3PO4 is most commonly used to etch silicon nitride. At 1800C with 91.5% H3PO4 concentration, the silicon nitride etch rate is about 10nm/min. Silicon nitride etch process has a very good selectivity to thermally grown silicon dioxide, which greater than 10:1 and to silicon is greater than 33:1. Increasing the concentration of H3PO4 to 94.5% and temperature to 2000C, increases rate of etch to 20nm/min. However, the selectivity to silicon dioxide reduces to about 5:1, while the selectivity to silicon reduces to around 20:1.

3.2.5 Metal Etch Metal such as aluminum can be etched with a wide variety of acidic formulation. One of the most commonly used etchant is the mixture of 80% phosphoric acid H3PO4, 5% acetic acid CH3COOH, 5% nitric acid HNO3 and 10% water. The rate of etching of pure aluminum is about 30nm/min at temperature 450C. The mechanism of aluminum etching is similar to etching of silicon. Nitric acid oxidizes aluminum to alumina Al2O3 and phosphoric acid H3PO4 dissolves Al2O3. In the titanium silicide process, the extra titanium is usually wet etch away with 1:1 mixture of hydrogen peroxide and sulfuric acid. Hydrogen peroxide will oxidize titanium to titanium oxide TiO2 then titanium oxide reacts with sulfuric acid to form soluble titanium sulphate TiSO4 and water. The illustrations of the processes of titanium deposition, silicidation, and wet etch are shown in Fig. 3.4.

(a) Titanium deposition (b) Silicidation annealing (c) Titanium wet etch Figure 3.4: Illustration of the titanium, silicidation, and wet etch

3.3 Dry Etch Dry etch or plasma etch uses gaseous chemical etchant to react with the material to be etched to form volatile byproduct that will be removed from the substrate - 89 -

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surface. Plasma generates chemically reactive free radical that significantly increases the rate of chemical reaction and enhances chemical etching process. Plasma also causes ion bombardment of wafer surface. Ion bombardment of the wafer surface both physically removes materials from the surface and breaks the chemical bonds between atoms on the surface, which significantly accelerates the chemical reaction rate for the etch process. There are two major reasons why dry etch is used. In the etching of silicon nitride, the wet etch process is very slow and not selective with respect to silicon dioxide. Moreover, the hydrofluoric acid HF etchant often causes lift off problem to photoresist masking. Owing to this reason, silicon dioxide SiO2 mask has to be used. The dry etch allows directional or anisotropic etching. Directional etching is needed to minimize under etching and etching bias, which allows smaller and more tightly packed structure to be fabricated. The directional etch is due to the presence of ionic species in the plasma and electric field that directs them normal to the surface of wafer. Plasma is a partially ionized gas. It is free electron colliding with neutral atoms/molecules and through a dissociative process. It can remove one electron from the atom/molecule, which gives a net of two electrons and one ion. Depending on the energy of the incoming electron, this collision can result also in other species, such as negative ions due to electron association, excited molecules, neutral atom and ion. The light emitted by the plasma is due to the return of excited electrons to their ground state. As the energy between the electron states is well defined for each element, each gas will emit light at specific wavelength that allows analysis of plasma. Plasma etch system can be designed either to be reactive chemical components or ionic component dominant. In many cases, a combination of ionic and reactive chemical species usually acting in a synergistic manner is utilized. The net etch rate can be much faster than the sum of individual etch rates when each reacting alone. Another advantage is that it is not only directional type of etching. It can maintain an acceptable degree of selectivity. Most reactant gases for plasma etching contain halogen. Generally chlorine Cl and fluorine F, and sometimes bromine Br. Free radicals of these species can be easily produced in plasma which can efficiently etch many film types, and produce volatile etched products. - 90 -

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The exact choice of reactant gases to etch a specific film type depends on a number of factors, which are etching selectivity to underlying film, anisotropic etching, and volatility of the main etched byproducts. A basic capacitive coupled RF plasma etching system is shown in Fig. 3.5. This system is similar to plasma enhanced chemical vapor deposition PECVD and radio frequency RF sputtering systems. A low pressure about 1 mtorr to 1 torr of gas is used in the chamber. By applying a high electrical field across two electrodes, some of the gas atoms are ionized producing positive ions and free electrons and creating plasma. The energy is supplied by an RF generator usually operating at 13.56MHz. A voltage bias would develop between the plasma and the electrodes due to difference in mobility of the electrons and the ions.

Figure 3.5: Schematic diagram of RF power plasma system

At frequencies between 1.0MHz and 100.0MHz, the free electron is able to follow the variation of the applied electric field. Unless the electron suffers collision, it can gain considerable energy in the order of few hundred electron volts. On the other hand, in this frequency range, the positive ion has very little influenced by the electric field. Its energy mainly comes from the temperature of the environment and it is in the order of a few hundredths of an electron volt i.e. ~0.01eV. - 91 -

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Initially, the high mobility electrons are lost to the electrodes at a faster rate than the slower ions. This results in the plasma being biased positively with respect to the electrode. For symmetric RF plasma system, with two electrodes of equal area, the voltage distribution that develops is shown in by the solid curve in Fig. 3.6.

Figure 3.6: Voltage distribution in RF powered plasma etch system

The sheath is considered as dielectric or a capacitor formed to slow down the electron loss due to initial loss of electron at the beginning of few cycles. It has created a net negative voltage near the electrode. The RF voltage becomes superimposing on this negative dc voltage. As the result, the average current to the electrode is zero. The heavy ions respond to the average sheath voltage and the light electron responds to the instantaneous voltage. However, due to the self biasing, they only cross the sheath during a short period per cycle when sheath voltage and thickness are near their minimum. During most of RF cycle the electrons are turned back at the sheath edge, resulting in the sheath, on the average, being deficient or depleted of electrons. This depletion of electrons in the sheath results in the sheath being dark because lack of electron/atom collisions and subsequent relaxation by light emission. In the bulk of plasma, both ionization and excitation events occurred. It produces the characteristic glow of plasma. The lack of electron in sheath resulting in high impedance causes voltage drop across the sheath. If one of electrode is smaller then the voltage distribution becomes asymmetry with much larger voltage drop - 92 -

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occurring from plasma to the smaller electrode. This shall mean that the RF current density must be much higher at the smaller electrode in order to maintain current continuity throughout the system. Therefore, field must be higher at the smaller electrode. For etching materials other then photoresist whereby oxygen is used as etchant, the reactant gases in the plasma are usually halide containing species such as Freon 14 CF4, Cl2, and hydrogen bromide HBr. Sometime small amount of other gas such hydrogen, oxygen, and argon are added. The high energy electrons in the plasma can cause a variety of reactions to occur with reactant gases including electron-induced ionization, dissociative ionization, recombination, and excitation reaction as illustrated in Fig. 3.7. Take for an example, CF4 reacts with an electron to form CF3, F, and one electron. This is a continuous dissociation process. The neutral F atom becomes a reactive agent for etching process.

Figure 3.7: Typical chemical reactions and species present in plasma

Thus, for CF4 plasma, there are electrons, CF4, CF3, and other fluorinated CFx fragment, CF3- and F. Among them CF3 and F are very reactive free radical species. Typically, there will be about 1015cm-3 neutral species and about 1 to 10% may be free radicals and 108 to 1012cm-3 ions and electrons. The actual numbers of the different species will depend on the balance between generation and loss or recombination reactions. - 93 -

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3.3.1 Plasma Etching Mechanism As mentioned earlier, there are two main types of species involved in plasma etching, which are reactive neutral chemical species and the ionic species. The reactive neutral species, which are free radicals in many cases but sometimes other reactive species such as chlorine and fluorine are primarily responsible for the chemical component in plasma etch process. Ionic species are responsible for the physical component. It can work independently or it can work together to get synergistic reaction. When the reactive neutral species work by themselves, the process or mechanism is called chemical etching. Ionic species acting by themselves can be resulted in physical etching or sputtering. When the reactive neutral species and ionic species act in synergistic manner, this is called ion enhanced etching. 3.3.1.1 Chemical Etching Chemical etching of materials in plasma is commonly done by free radicals. Free radicals are electrically neutral species having incomplete bonding, in which they have unpaired electrons. The fluorine free radical F and CF3 neutral free radical can be produced by reaction of CF4 and electron. e- + CF4  CF3 + F + e-

(3.12)

e- + CF4  CF3 + F + 2e-

(3.13)

or

Owing to its incomplete octal bonding structure, free neutral radical such as fluorine is highly reactive chemical species. Free fluorine radical, with seven electrons in its outermost shell instead of eight electron stable state would like be bonded with other atom so that all electrons are paired. The ideal in plasma etching is the reaction of neutral species with the material to be etched. Take for example, the fluorine radical reacts with silicon to form volatile SiF4 gas. This gas would easily vaporize and it leaves the surface exposed with more silicon for further etch. The chemical reaction is shown in equation (3.14). 4F + Si  SiF4

(3.14)

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There are several chemical reactions happened before it arrives to final SiF 4 gas. The part of fluorine F diffuses to the surface of silicon substrate. The other part forms fluorine molecule, recombine, attract to the walls, or being pumped out etc. The diffused fluorine radical forms a covalent bond with silicon atom to form SiF due to Van der Waals force of attraction. Three additional covalent bonds would be formed and finally it obtains a stable state, which SiF4 gas. The chemical equation of neutral radical F reacts with silicon dioxide is shown in equation (3.15). 4F + SiO2  SiF4 + O2

(3.15)

The illustration of the generalized chemical reaction of free radical with substrate material is shown in Fig. 3.8.

Figure 3.8: Chemical reaction of radical species with substrate material

Gas additive can be used to increase rate of etching. Oxygen is often added to Freon 14 CF4 plasma. Oxygen reacts with dissociated CF4 species like CF3 or CF2 that reduces the rate of recombination of these species with fluorine F. However, too much oxygen added dilutes the fluorine F, oxidizes the surface of wafer resulting reduced rate of etching. Plasma chemical reaction acts isotropically like the wet etching chemical process due to the arrival angle distribution and a low sticking coefficient. Since the radical is neutral, the arrival angular distribution equal to one and it is random. The sticking coefficient is low shall mean there is no immediate reaction upon arrival. The sticking coefficient of the species is defined by equation (3.16). - 95 -

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Sc = Freacted/Fincident

(3.16)

A high sticking coefficient, which is one, shall mean that the reaction and etching occur the first time the species strikes the surface. A low sticking coefficient means that before the species reacts, it leaves the surface. One may think that the species should be highly reactive. However, due to availability of site, multiple atoms and species, competing reactions, and surface recombination, the sticking coefficient is actually low. 3.3.1.2 Physical Etching The other main species that participate in plasma etching are the ions. Owing to presence of sheath area whereby there is electric field drop across it, positive ions are accelerated toward each electrode. Since the wafer is placed at one of the electrodes, positive ionic species such as Cl+ or Ar+ in purely physical sputter etch system will be accelerated to the surface of wafer. This striking of the wafer surface results in more physical component of etching. The flux of ions toward the surface of wafer is much more directional than the flux of neural free radicals because of the directionality of the electric field from plasma to the surface of wafer. Thus, the etching will be more directional and more anisotropic. Indeed it is always assumed that the positive ion will arrive normal to the surface of wafer as shown in Fig. 3.9(b) versus the arrival angle of neutral free radical chemical etching, which are wide and low sticking coefficient as shown in Fig. 3.9(a).

(a) (b) Figure 3.9: Fluxes of specifies in plasma etch. (a) Neutral radical chemical etching and (b) positive ion physical etching - 96 -

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3.3.1.3 Ion Enhanced Etching Another way that ions participate in plasma etch processes is through ion enhanced etching. It is a known fact that ion and neutral radical do not act independently in etch process. It can be observed from rate of etching, in which it is equal to the sum of the rate of etching of ion and neutral radical. In many circumstances, the total rate of etching is higher than the sum of etch rate of the individual. A classic example of ion enhanced etching is shown in Fig. 3.10.

Figure 3.10: Rate of etching of silicon illustrating synergic effect of ion enhanced etching

It shows the rate of etching of silicon as xenon fluoride XeF2 gas and argon positive Ar+ ion are introduced to the surface of silicon. The result shows that rate of etching increases when XeF2 gas and positive argon ion are introduced together. Xenon fluoride gas alone or Ar+ ion alone has low rate of etching or practically zero rate of etching. This shows the synergistic effect whereby both chemical and physical etching processes are working simultaneously. The profile of etching for ion-enhanced etch is like the physical etch showing anisotropic etch as shown in Fig. 3.1(a) and not the profile of isotropic etch as in Fig. 3.1(b). If the chemical component is increased i.e. XeF2, the vertical etch rate is increased but not the lateral etch rate as it is expected for chemical etch. It has been found that chemical inert residue is often formed from etching or sputtering of photoresist. In additional species from plasma may deposit on the surface. These layers may prevent chemical reaction either by physically blocking the chemical etch species or by reacting with them or deplete them. This layer is called inhibitor can be removed from physical ion bombardment - 97 -

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leading to anisotropic etch since ion bombardment is directional etch. The sidewall is inhibited. Figure 3.11 illustrates the inhibitor is removed by physical etch on the surface of wafer.

Figure 3.11: Inhibitor prohibits lateral physical etch

3.3.2 Type of Plasma Etching Systems Plasma reactor technology in the electronics industry has changed dramatically since the first application of plasma processing to strip photoresist. A plasma etching system contains a vacuum chamber, pump system, power supply generators, pressure sensors, gas flow control units, and an end-point detector. Plasma etch tools may be categorized according to the etch mechanism each of them employs. There are many available etching systems available in the market. Examples are barrel etching system, downstream plasma etching system, in which both of them are chemical etching systems. Examples of etching system that can perform both physical and chemical etching are reactive ion etching RIE system, magnetic enhanced RIE, magnetic confinement triode RIE, electron cyclotron resonance ECR etching system, and Inductive coupled plasma ICP or Helicon wave RF plasma etching system. A comparison of pressure operating ranges and ion energies for a few types of systems is shown in Fig. 3.12. Each etch tool is designed empirically and uses a particular combination of pressure, electrode configuration and type, and source frequency to control the two primary etch mechanisms chemical and - 98 -

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physical. High etch rate and automation are greatly emphasized for most etching system used for microelectronic fabrication. We shall discuss a few of the etching systems here.

Figure 3.12: Comparison of ion energy and operating pressure ranges for different types of plasma systems

3.3.2.1 Barrel Etching System A barrel etching system usually consists of a cylindrical vacuum chamber with a pair of RF electrodes concentrically located inside. The cross-sectional and side views are shown in Fig. 3.13. The wafers are placed upright in a boat along the axis of the chamber so that they lie concentrically within the reactor. The process gas pressure is usually between 0.1 to 5 torr, depending on the type of material to be etched. A large batch of wafers can be processed in each run. In the barrel reactor the active radicals are formed between the two electrodes and diffuse through the holes in the electrode to the wafers. The resultant etch profile is isotropic because there is not much ion bombardment on the wafer surface. The barrel etching system is suitable for less critical process steps such as the removal of photoresist. Reactants transport to the wafer surface through gas-phase diffusion from the edge of the wafer to the center of the wafer. Therefore, etch uniformities within a wafer and among wafers are very poor, - 99 -

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and a large amount of overetch is usually needed to solve this etch uniformity problem.

(a) (b) Figure 3.13: Schematic of a barrel plasma etching system showing (a) side view and (b) cross sectional view

3.3.2.2 Magnetic Enhanced and Confinement Plasma Etching System Magnetic-enhanced reactive ion plasma etching system uses a group of permanent magnets located behind the etching wafer or pairs of direct-current electric coils to generate a magnetic field parallel to the wafer surface, as shown in Fig. 3.14.

Figure 3.14: Schematic of magnetic-enhanced reactive ion etching system with a timeaverage magnetic field applied parallel to the cathode - 100 -

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The magnetic field is perpendicular to the electric field due to the cathode dc bias and confines electrons to a circular trajectory near the cathode. Electron confinement reduces the loss of electrons to the wall of the system and increases the frequency of electron-neutral collisions. The associated higher frequency of collisions increases ion density and electron confinement reduces the mobility of electrons toward the cathode, thereby reducing the self-bias voltage. To achieve the required time-average plasma uniformity across the wafer surface, the field direction is rotated electrically with a period that is short with respect to the processing time. Magnetic fields also modify the ion-bombardment energies and can be used as another parameter for process optimization. Uniformity of the plasma and radical fluxes to the wafer are critical issues. Magnetic-confinement plasma etching system uses permanent-magnet pole pieces and they are arranged alternately around the process chamber to create a magnetic-field-free region around the wafer, as shown in Fig. 3.15.

Figure 3.15: Schematic of magnetic-confinement reactive ion etching system with a multipolar magnetic bucket surrounding the etching chamber

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A high density of ions is achieved through the reflection of electrons back into the plasma from the surface magnetic-field bucket. This greatly increases the effective path length of electrons, and higher ion density is generated from the higher frequency of electron-neutral collisions. 3.3.2.3 Electron Cyclotron Resonance Plasma Etching System Most parallel-plate plasma etching systems, except triode RIE, do not provide the ability to independently control plasma parameters, such as electron energy, ion energy, plasma density, and reactant density. As a result, ion-bombardmentinduced damage becomes serious as device dimensions shrink. ECR etching uses microwave excitation in the presence of a magnetic field to generate a high-density discharge. The Lorentz force causes the electrons to circulate around the magnetic field lines in circular orbits, with a characteristic cyclotron frequency follows equation (3.17). f Ce 

2qB me

(3.17)

where q is the electron charge, B is the magnetic field, and me is the electron mass. When this frequency equals the applied microwave frequency, a resonance coupling occurs between the electron energy and the applied electric field, which results in a high degree of dissociation and ionization (10-2 for ECR compared to 10-6 for RIE). With a microwave frequency of 2.45GHz, the required magnetic field is 875 gauss. Figure 3.16 shows one of the possible ECR plasma etching configurations. Microwave power is coupled via a waveguide through a dielectric window into the ECR source region. The magnetic field, supplied from magnetic coils, decreases as a function of distance from the coils. Owing to the gradient in the magnetic field decreases, the electrons are accelerated away from the plasma source, creating a negative potential in the direction of the wafer. Ions diffuse by ambipolar diffusion out of the source region into the wafer process chamber. The wafer is RF or dc-biased to control the energy of the ions to achieve the desired etch anisotropy. Plasma uniformity degrades because of the ambipolar diffusion and mirror magnetic field, but a multi-pole magnetic bucket can be used to improve plasma uniformity. Etch uniformity can also be improved by putting the wafer in an ECR source region surrounded by an optimized magnetic field.

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Figure 3.16: Schematic of an electron cyclotron resonance etching system

3.3.2.4 Inductively Coupled and Helicon Wave RF Plasma Etching Systems As feature sizes for integrated circuit continue to decrease, the conventional capacitive coupled RF plasma etching system has reached its limit. Only the t ECR plasma systems are suitable for etching ULSI submicron device. However, they are not popular in manufacturing because of their complexity. Other types of high-density plasma sources such as inductively coupled plasma ICP sources or helicon plasma sources may become the main plasma sources for future ULSI etching process. The PMT MORI® source is a helicon wave plasma source. The Lam TCP source is a transformer-coupled plasma source. An inductively coupled plasma source as shown in Fig. 3.17 generates high-density, low-pressure plasma that is decoupled from the wafer. It allows independent control of ion flux and ion energy. Plasma is generated by a flat spiral coil that is separated from the plasma by a dielectric plate on the top of the reactor. The wafer is located several skin depths away from the coil, so it is not affected by the electromagnetic field generated by the coil. There is little plasma density loss because plasma is generated only a few mean free paths away from the wafer surface. Therefore, a high-density plasma and high etch rates are achieved.

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Figure 3.17: Illustration of an inductively coupled plasma etching system with PMT MORI® source

A helicon RF wave plasma source can also be used to generate a high-density (>1011/cm3) discharge. A transverse electromagnetic radio-frequency wave (13.56MHz), excited by a double-loop or single-loop antenna located outside a quartz source tube, is coupled with a steady longitudinal magnetic field B0 of approximately 100 gauss generated by a solenoid coil, as shown in Fig. 3.18. The resonance condition for propagation of a helicon wave depends on the magnitude of the longitudinal field and the dimension of the reactor. If the wavelength of the helicon wave is the same as the antenna length, the coupling will be resonant. High-density plasma then diffuses into the wafer chamber. In addition, the wafer can be biased separately with a second RF generator.

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Figure 3.18: Schematic of a helicon wave plasma etching system with a double-loop antenna

3.3.3 Plasma Etching for Various Films Most reactant gases for plasma etching contain halogen. Generally they are chlorine Cl and fluorine F and sometimes bromine Br. Free radicals of these species can easily be produced in plasma which can efficiently etch many film types and volatile etch products commonly produced by these species. The - 105 -

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choice of reactant gas to etch a specified film type depends on a number of factors. The most important factors are etching selectivity to underlying film, anisotropic etching, and volatility of main etch byproducts. Volatility of the byproduct is an important factor to determine the type of reactive gas to be used for etching because byproduct has to be removed from surface of the underlying. Otherwise, it would impede the process of etching. The volatility or tendency to evaporate depends on the boiling point of the byproduct. The lower the boiling point, the less tightly bounding of the byproduct to the surface, the lower the surface binding energy, and the higher the volatility. Figure 3.19 illustrates a list of boiling points of some typical etch byproducts. Element To be etch

Chloride byproduct

Boiling Point 0 C 177.8 (sublimation)

Fluoride Byproduct

Al

AlCl3

Cu

CuCl

1490

CuF

Si

SiCl4

57.6

SiF4

Ti

TiCl3

136.4

TiF4

W

WCl6 WCl5 WOCl4

347 276 227.5

WF6 WOF4

AlF3

Boiling Point 0 C 1,291 (sublimation) 1,100 (sublimation) 86 284 (sublimation) 17.5 187.5

Figure 3.19: Boiling points of some typical etch byproducts

One can see that if aluminum is etched with fluorine reactant gas, the byproduct AlF3 has boiling point 1,2910C transforming directly from solid to gas state (sublimation). This means that the etch byproduct will be tightly bounded to the surface with very low vapor pressure. Even with ion bombardment, it is difficult to leave the surface. On the other hand, if aluminum is etched with chlorine reactant gas, the byproduct AlCl3 has boiling point 177.80C. This shows that it is loosely bonded to surface and it is a much more volatile species. Figure 3.20 lists some common etchants used in plasma etching films in integrated circuit fabrications. These etchants are chosen mostly based on issues associated with byproduct, volatility, etch selectivity, and etch profiles. The exact profiles and selectivities will depend on the specific system and plasma conditions. - 106 -

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Materials Polysilicon/Single Crystalline Si

Etchant CF4 CF4/H2 CF4/O2,SF6

Silicon dioxide SiO2

HBr, Cl2, Cl2/HBr/O2 SF6, NF3, CF4/O2, CF4

CF4/H2, CHF3/O2, C2F6, C3F8 CHF3/C4F8/CO Silicon nitride Si3N4

CF4/O2 CF4/H2 CHF3/O2, CH2F2

Aluminum Al

Cl2 Cl2/CHCl3, Cl2/N2

Tungsten W

CF4, SF6 Cl2

Ti/TiN TiSi2 Photoresist

Comments Isotropic or near isotropic (significant undercutting); fair to no selectivity over SiO2. Very anisotropic; non selectivity over SiO2. Isotropic or near isotropic; good selectivity over SiO2. Very anisotropic; most selectivity over silicon dioxide SiO2. Can be near isotropic (significant undercutting); anisotropy can be improved with higher ion energy and lower pressure; poor or not selective over silicon. Very anisotropic; selective over silicon Anisotropic; selective over silicon nitride Isotropic; selective over silicon dioxide SiO2 but not silicon Si. Very anisotropic; selective over Si but not silicon dioxide SiO2. Very anisotropic; selective over silicon Si and silicon dioxide SiO2. Near isotropic (significant undercutting). Very isotropic; BCl3 often added to scavenge oxygen. High etch rate; non selective over silicon dioxide SiO2. Selective over silicon dioxide SiO2.

Cl2, Cl2/CHCl3, CF4 Cl2, Cl2/CHCl3, CF4/O2 O2 Very selective over other film.

Figure 3.20: Common etchant types for plasma etching film - 107 -

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3.3.3.1 Plasma Etching Silicon Dioxide Fluorine based reactant gas is usually used to etch silicon dioxide. The volatility of the main etch byproduct SiF4 is very high as indicated by low boiling as shown in Fig. 3.17. Thus, SiF4 can easily leave the surface via thermal desorption. Fluorine can etch silicon dioxide by reaction of the fluorine radical with the overall reactions given by equation (3.15). CF2 and CF3 free radicals may also be directly involved in the case of fluoricarbon etchant in which CO containing byproduct would also be produced. 3.3.3.2 Plasma Etching Polysilicon The choice of etchant for polysilicon depends on the same factors as etching silicon dioxide, which byproduct volatility, selectivity, and anisotropy. Another issue is the presence of ubiquitous native oxide layer on the surface of the film that requiring initial breakthrough etch step to remove it. Fluorine based etchant can be used for polysilicon etching because of the volatility of SiF4. However, etching polysilicon with SF6, CF4, or CF4/O2 has relatively large isotropic component due to high concentration of fluorine radical produced in plasma combined with little polymer formation. Isotropic etch can be enhanced by adding hydrogen gas H2. By doing this, it also enhances the etch rate of silicon dioxide over silicon Si by forming polymer on silicon. If anisotropy is not important then adding oxygen to CF4 allows etching polysilicon with good selectivity than silicon dioxide. 3.3.3.3 Plasma Etching Aluminum Plasma etching of aluminum interconnect line has been a challenge for process engineer but successful strategies have been developed to overcome the many problems associated with this film. Besides those mentioned criteria earlier, native oxide and potential corrosion problem as well as copper doping in the film lead to more difficulties. Owing to the presence of native alumina Al2O3 on the aluminum film surface, an initiation or breakthrough etch step must be done before the main aluminum etch step. Ion bombardment or sputtering is usually done to remove this film in conjunction with chemical scavenging. Argon sputtering can be used but gases such as boron trichloride BCl3, tetrachlorosilane SiCl4, carbon - 108 -

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tetrachloride CCl4, or boron tribromide BBr3 are often used to scavenge the oxygen O2, and water H2O and remove aluminum oxide Al2O3 on the aluminum Al surface by both physical and chemical means. A load lock system is required to control the water level. The byproduct of etching aluminum with fluorine based radical aluminum fluoride AlF3 is not very volatile so chlorine based radical is most often used for main aluminum etch step. Chlorine gas is commonly used and the neutral chlorine radical can directly etch aluminum. However, chlorine neutral radical etches isotropically and produces an undercut. Ion bombardment has little effect on the etch rate. To suppress lateral etching of aluminum and obtain vertical profile, sidewall inhibitor formation is needed. Therefore, inhibitor source or promoter is often added to the gas mixture. These include chloroform freon-11 CHCl3, CFCl3, CCl4, and nitrogen N2. Aluminum interconnect line usually contains silicon and copper, which can complicate the etching process. While silicon is readily etched by chlorine Cl but copper Cu is not. Note that the byproduct from copper etch with halogen etching has quite low volatility. Ion bombardment and often higher temperature are needed for etching copper. An over etch step is especially important for aluminum etching to remove the residues both from inhibitor layer and the copper precipitate. The removal of copper precipitate and residue is difficulty. It depends on the aluminum deposition conditions. With a higher deposition temperature, it gives larger precipitates and more residues.

Exercises 3.1.

o

The thermal oxide film has thickness 5,000 A . After 30s of plasma etch, o

the thickness is reduced to 2,500 A . Calculate the rate of etching. 3.2.

o

Before etching the five point thickness mesaurements are 3,500 A , o

o

o

o

3,510 A , 3,499 A , 3,501 A , and 3,493 A respectively. After 60s etching the o

o

o

o

measurments are 1,500 A , 1,455 A , 1,524 A , 1,451 A , and o

1,563 A respectively. Calculate the average etch rate and max-minu-min non-uniformity. 3.3.

Define isotropic etch. - 109 -

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3.4.

State a reason why selectivity of a wet etch process between two materials is important for etching process.

3.5.

Polysilicon is to be wet etched using photoresist as a mask. What should be ratio of etch rate of polysilicon as compared with etch rate of photoresist in order to prevent over etch?

3.6.

State the reason why wet etch is not suitable for VLSI device fabrication.

3.7.

1.2m of silicon dioxide is to be etched way on top of doped silicon substrate. The rate of etching is 0.4m/min and etch selectivity of the silicon dioxide with respect to doped silicon is 25-to-1. Calculate the time taken to etch way this thickness of oxide.

3.8.

A 0.5m layer of silicon oxide on silicon substrate needs to be etched away. Assume that the nominal oxide etch rate is r ox m per minute. There is a 5% variation in the thickness of oxide and a 5% variation in oxide etch rate. (i). How much overetch is required in % time in order to ensure that all oxide is etched? (ii). What is the selectivity of the oxide etch rate to the silicon etch rate is required that a maximum of 5.0nm of silicon is etched?

3.9.

Consider the structure shown in the figure below. A 0.5m thick oxide layer is etched to achieve equal structure width and spacing Sf. The etch process produces a degree of anisotropy of 0.8. If the distance between the mask edge x is 0.35m, determine the spacing and width of the structure. You may assume no over etch.

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3.10. Silicon dioxide of thickness 1.5m is to be etched way on top of doped silicon substrate. The etching time is 4.0mins and selectivity of the silicon dioxide with respect to doped silicon is 25 to 1. Calculate the etching rate of silicon dioxide if there is 0.003m of silicon substrate is being etched away during the etching process.

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Bibliography 1.

JD Pummer, MD Del, and Peter Griffin, “Silicon VLSI Technology” Fundamentals, Practices, and Modeling”, Prentice Hall, 2000.

2.

Hong Xiao, “Introduction to Semiconductor Manufacturing Technology”, Pearson Prentice Hall, 2001.

3.

SM Sze, “VLSI Technology”, second edition, McGraw-Hill, 1988.

4.

CY Chang and SM Sze, “ULSI Technology”, McGraw-Hill, 1996.

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Index Hydrogen ........................................................... 108 Hydrogen bromide ............................................... 93 Hydrogen peroxide ........................................ 88, 89

A Acetic acid...................................................... 85, 89 Alumina................................................ 89, 108, 109 Aluminum .......................................... 106, 108, 109 Aluminum fluoride ............................................. 109 Ammonium fluoride ...................................... 85, 88 Argon ................................................................... 93

I ICP ......................... See Inductively coupled plasma Inductive coupled plasma etching system ... 98, 103 Inductively coupled plasma ......................... 98, 103 Ion enhanced etching .................................... 94, 97 Isopropyl alcohol .................................................. 88

B Barrel etching system .................................... 98, 99 BHF .................................................See Buffered HF Boron tribromide ............................................... 109 Boron trichloride ............................................... 108 Bromine ............................................................. 105 Buffered HF .......................................................... 85

L Lorentz force ...................................................... 102 M Magnetic confinement plasma etching system . 101 Magnetic enhanced RIE etching system ...... 98, 100 Mask erosion ........................................................ 87 Max-minus-min non-uniformity .......................... 85

C Capacitive coupled RF plasma etching system ... 91, 103 Carbon tetrachloride ......................................... 109 Chemical etching ................................................. 94 Chlorine ..........................................90, 94, 105, 109 Chloroform ........................................................ 109 Copper ............................................................... 109

N Nitric acid ................................................. 85, 88, 89 Nitrogen ............................................................. 109 Nitrogen dioxide .................................................. 88 O

D

Oxygen ........................................... 93, 95, 108, 109

Degree of anisotropy ........................................... 87 Downstream plasma etching ............................... 98 Dry etch ............................................................... 89

P Phosphoric acid .................................................... 89 Photoresist ........................................................... 85 Physical etching.................................................... 96 Plasma enhanced chemical vapor deposition ...... 91 Plasma etch .................................. 83, 89, 90, 94, 96 ® PMT MORI ........................................................ 103 Polysilicon ............................................ 85, 107, 108 Potassium hydroxide............................................ 88

E ECR .....................See Electron cyclotron resonance Electron cyclotron resonance ...................... 98, 103 Electron cyclotron resonance etching system .... 98, 102 Etching ................................................................. 83 Etching Method Chemical etching ............................................. 94 Ion beam etching ............................................. 83 Ion enhanced etching ................................ 94, 97 Plasma etching ........................................... 83, 94 Reactive etching .............................................. 83 Sputtering etching ........................................... 83

R Rate of etching ..................................................... 84 Reactive ion etching system ................................ 98 RF plasma system................................................. 92 RF sputtering system ........................................... 91 S

F

Scanning electron microscope ............................. 84 Selectivity ............................................................. 85 SEM ................... See Scanning electron microscope Semiconductor Silicon ................................................. 85, 94, 109 Silicon dioxide .................... 85, 88, 90, 95, 107, 108 Silicon nitride ............................................... 89, 107 Standard deviation non-uniformity ..................... 85 Sulfuric acid .......................................................... 88

Fluorine .....................................90, 94, 95, 105, 108 Fluorine radical .................................................... 94 Freon .................................................................... 95 Freon-11 ............................................................ 109 H Helicon wave RF plasma etching system ..... 98, 104 Hydrofluoric acid ..................................... 85, 88, 90 - 113 -

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Magnetic enhanced RIE etching system .. 98, 100

Tetrachlorosilane ............................................... 108 Titanium oxide ..................................................... 89 Titanium silicide ................................................... 89 Titanium sulphate ................................................ 89 Type of Etching System Barrel etching system ...................................... 98 Barrel plasma etching system .......................... 99 Capacitive coupled RF plasma etching system 91, 103 Electron cyclotron resonance etching system 98, 102 Helicon RF wave plasma etching system . 98, 104 Inductive coupled plasma etching system 98, 103 Magnetic confinent plasma etching system .. 101

U Ultra large scale integration................................. 83 V Van der Waals force ............................................. 95 W Wet etch .............................................................. 86 X Xenon fluoride ..................................................... 97

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