Chapter 10 ETCHING

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Chapter 10

ETCHING


CONTENTS

Introduction Basic Concepts

– Wet etching– Plasma etching

Manufacturing Methods– Plasma etching conditions and issues– Plasma etch methods for various films

Measurements Methods Models and Simulations Limits and Future Trends


Introduction

After a thin film is deposited, it is usually etched to remove unwanted materials and leave only the desired pattern on the wafer

The process is done many times(review flow chart of Chapter 2)

An overview of the process is shown in Figure 10-1 In addition to deposited films, sometimes we also need

to etch the Si wafer to create trenches (especially in MEMS)

The masking layer may be photoresist, SiO2 or Si3N4

The etch is usually done until another layer of a different material is reached


Introduction


Introduction

Etching can be done “wet” or “dry” Wet etching

– uses liquid etchants– Wafer is immersed in the liquid– Process is mostly chemical

Wet etching is not used much in VLSI wafer fab any more


Introduction

Dry etching– Uses gas phase etchants in a plasma– The process is a combination of chemical

and physical action– Process is often called “plasma etching”

This is the normal process used in most VLSI fab

The ideal etch produces vertical sidewalls as shown in 10-1

In reality, the etch occurs both vertically and laterally (Figure 10-2)


Introduction


Introduction

Note that– There is undercutting, non-vertical

sidewalls, and some etching of the Si The photoresist may have rounded tops and

non-vertical sidewalls The etch rate of the photoresist is not zero and

the mask is etched to some extent This leads to more undercutting


Introduction

Etch selectivity is the ratio of the etch rates of different materials in the process

If the etch rate of the mask and of the underlying substrate is near zero, and the etch rate of the film is high, we get high selectivity

This is the normally desired situation If the etch rate of the mask or the substrate is

high, the selectivity is poor Selectivities of 25 – 50 are reasonable Materials usually have differing etch rates due

to chemical processes rather than physical processes


Introduction

Etch directionality is a measure of the etch rate in different directions (usually vertical versus lateral)


Introduction

In isotropic etching, the etch rates are the same in all directions

Perfectly anisotropic etching occurs in only one direction

Etch directionality is often related to physical processes, such as ion bombardment and sputtering

In general, the more physical a process is, the more anisotropic the etch is and the less selective it is

Directionality is often desired in order to maintain the lithographically defined features


Introduction

Note, however, that very anisotropic structures can lead to step coverage problems in subsequent steps

Selectivity is very desirable– The etch rate of the material to be removed

should be fast compared to that of the mask and of the substrate layer

It is hard to get good directionality and good selectivity at the same time


Introduction

Other system requirements include– Ease of transporting gases/liquids to the

wafer surface– Ease of transporting reaction products away

from wafer surface– Process must be reproducible, uniform,

safe, clean, cost effective, and have low particulate production


Basic Concepts

We consider two processes– “wet” etching– “dry” etching

In the early days, wet etching was used exclusively

It is well-established, simple, and inexpensive The need for smaller feature sizes could only

be met with plasma etching Plasma etching is used almost exclusively

today


Basic Concepts

The first wet etchants were simple chemicals By immersing the wafer in these chemicals,

exposed areas could be etched and washed away

Wet etches were developed for all step For SiO2, HF was used. Wet etches work through chemical processes

to produce a water soluble byproductO2HSiFH6HFSiO 2622


Basic Concepts

In some cases, the etch works by first oxidizing the surface and then dissolving the oxide

An etch for Si involves a mixture of nitric acid and HF

The nitric acid (HNO3) decomposes to form nitrogen dioxide (NO2)

The SiO2 is removed by the previous reaction The overall reaction is

22222 2HNOHSiOO2H2NOSi

222623 HOHHNOSiFH6HFHNOSi


Basic Concepts

Buffers are often added to keep the etchants at maximum strength over use and time

Ammonium fluoride (NH4F) is often used with HF to help prevent depletion of the F ions

This is called Basic Oxide Etch (BOE) or Buffered HF (BHF)

The ammonium fluoride reduces the etch rate of photoresist and helps eliminate the lifting of the resist during oxide etching

Acetic acid (CH3COOH) is often added to the nitric acid/HF Si etch to limit the dissociation of the nitric acid


Basic Concepts

Wet etches can be very selective because they depend on chemistry

The selectivity is given by

Material “1” is the film being etched and material”2” is either the mask or the material below the film being etched

If S>>1, we say the etch has good selectivity for material 1 over material 2

2

1

r

rS


Basic Concepts

Most wet etches etch isotropically The exception is an etch that depends on the

crystallographic orientation Example—some etches etch <111> Si slower

than <100> Si Etch bias is the amount of undercutting of the

mask If we assume that the selectivity for the oxide

over both the mask and the substrate is infinite, we can define the etch depth as “d” and the bias as “b”


Basic Concepts


Basic Concepts

We often deliberately build in some overetching into the process

This is to account for the fact that – the films are not perfectly uniform– the etch is not perfectly uniform

The overetch time is usually calculated from the known uncertainties in film thickness and etch rates

It is important to be sure that no area is under-etched; we can tolerate some over-etching


Basic Concepts

This means that it is important to have as high a selectivity as possible to eliminate etching of the substrate

However, if the selectivity is too high, over-etching may produce unwanted undercutting

If the etch rate of the mask is not zero, what happens?

If m is the amount of mask removed, we get unexpected lateral etching


Basic Concepts


Basic Concepts

m is called “mask erosion” For anisotropic etching, mask erosion should

not cause much of a problem if the mask is perfectly vertical

Etching is usually neither perfectly anisotropic nor perfectly isotropic

We can define the degree of anisotropy by

vert

latf r

rA 1


Basic Concepts

Isotropic etching has an Af = 0 while anisotropic etching has Af = 1

There are several excellent examples in the text that do simple calculations of these effects

These examples should be studied carefully


Example

Consider the structure below

The oxide layer is 0.5 m. Equal structure widths and spacings, Sf, are desired. The etch anisotropy is 0.8.

If the distance between the mask edges, x, is 0.35 m, what structure spacings and widths are obtained?


Example

To obtain equal widths and spacings, Sf, the mask width, Sm, must be larger to take into account the anisotropic etching

Since

where b is the bias on each side, and Since

Thus

bSS fm

d

bAf 1

fffm AxSS 12


Example

This result makes sense

– For isotropic etching, Af=0 and Sm is a maximum

– For perfectly anisotropic etching, Af=1 and Sm=Sf and is a minimum

The distance between the mask edges (x) is the minimum feature size that can be resolved

But

Substitution and rearranging yields (note typo in text)

mF SSx 2

fff

fff

AxxS

AxSx

12

12


Example

Substituting numbers for the problem

This result shows that the structure size can approach the minimum lithographic dimension only when the film thickness gets very small OR as the anisotropy gets near 1.0

Very thin films are not always practical This means we need almost vertical etching Wet etching cannot achieve the desired results

m 55.0

8.01m 0.52m 35.0

fS


Plasma Etching

Plasma etching has (for the most part) replaced wet etching

There are two reasons:– Very reactive ion species are created in the

plasma that give rise to very active etching– Plasma etching can be very anisotropic

(because the electric field directs the ions) An early application of plasma etching (1970s)

was to etch Si3N4 (it etches very slowly in HF and HF is not very selective between the nitride and oxide)


Plasma Etching

Plasma systems can be designed so that either reactive chemical components dominate or ionic components dominate

Often, systems that mix the two are used– The etch rate of the mixed system may be

much faster than the sum of the individual etch rates

A basic plasma system is shown in the next slide


Plasma Etching


Plasma Etching

Features of this system– Low gas pressure (1mtorr – 1 torr)– High electric field ionizes some of the gas

(produces positive ions and free electrons)– Energy is supplied by 13.56 MHz RF

generator– A bias develops between the plasma and

the electrodes because the electrons are much more mobile than the ions (the plasma is biased positive with respect to the electrodes)


Plasma Etching


Plasma Etching

If the area of the electrodes is the same (symmetric system) we get the solid curve of 10-8

The sheaths are the regions near each electrode where the voltage drops occur (the dark regions of the plasma)

The sheaths form to slow down the electron loss so that it equals the ion loss per RF cycle

In this case, the average RF current is zero


Plasma Etching

The heavy ions respond to the average voltage The light electrons respond to the

instantaneous voltage The electrons cross the sheath only during a

short period in the cycle when the sheath thickness is minimum

During most of the cycle, most of the electrons are turned back at the sheath edge

The sheaths are thus deficient in electrons They are thus dark because of a lack of light-

emitting electron-ion collisions


Plasma Etching

For etching photoresist, we use O2

For other materials we use species containing halides such as Cl2, CF4, and HBr

Sometimes H2, O2, and Ar may be added The high-energy electrons cause a variety of reactions The plasma contains

– free electrons– ionized molecules– neutral molecules– ionized fragments – Free radicals


Plasma Etching


Plasma Etching

In CF4 plasmas, there are

– Free electrons

– CF4

– CF3

– CF3+

– F CF and F are free radicals and are very

reactive Typically, there will be 1015 /cc neutral species

and 108-1012 /cc ions and electrons


Plasma Etching Mechanisms

The main species involved in etching are– Reactive neutral chemical species– Ions

The reactive neutral species (free radicals in many cases) are primarily responsible for the chemical component

The ions are responsible for the physical component

The two can work independently or synergistically


Plasma Etching Mechanisms

When the reactive neutral species act alone, we have chemical etching

Ions acting by themselves give physical etching

When they work together, we have ion-enhanced etching


Chemical Etching

Chemical etching is done by free radicals Free radicals are neutral molecules that have

incomplete bonding (unpaired electrons) For example

Both F and CF3 are free radicals Both are highly reactive F wants 8 electrons rather than 7 and reacts

quickly to find a shared electron

eFCFCFe 34


Chemical Etching

The idea is to get the free radical to react with the material to be etched (Si, SiO2)

The byproduct should be gaseous so that it can be transported away (next slide)

The reaction below is such a reaction

Thus, we can etch Si with CF4

There are often several more complex intermediate states

4SiFSi4F


Chemical Etching


Chemical Etching

Gas additives can be used to increase the production of the reactive species (O2 in CF4)

The chemical component of plasma etching occurs isotropically

This is because– The arrival angles of the species is isotropic– There is a low sticking coefficient (which is

more important) The arrival angle follows what we did in

deposition and there is a cosn dependence where n=1 is isotropic


Chemical Etching

The sticking coefficient is

A high sticking coefficient means that the reaction takes place the first time the ion strikes the surface

For lower sticking coefficients, the ion can leave the surface (usually at random angles) and strikes the surface somewhere else

incident

reactedc F

FS


Chemical Etching

One would guess that the sticking coefficient for reactive ions is high

However, there are often complex reactions chained together. This complexity often means low sticking coefficients

Sc for O2/CF4 on Si is about 0.01 This additional “bouncing around” of the ions

leads to isotropic etching


Chemical Etching


Chemical Etching

Since free radicals etch by chemically reacting with the material to be etched, the process can be highly selective


Physical Etching

Due to the voltage drop between the plasma and the electrodes and the resulting electric field across the sheaths, positive ions are accelerated towards each electrode

The wafers are on one electrode Therefore, ionic species (Cl+ or Ar+) will be

accelerated towards the wafer surface These ions striking the surface result in the

physical process The process is much more directional because

the ions follow the field lines


Physical Etching


Physical Etching

This means n is very large in the cosn distribution

But, because the process is more physical than chemical, the selectivity will not be as good as in the more chemical processes

We also assume that the ion only strikes the surface once (which implies that the sticking coefficient is near 1)

Ions can also etch by physical sputtering (Chapter 9)


Ion-Enhanced Etching

The ions and the reactive neutral species do not always act independently (the observed etch rate is not the sum of the two independent etch rates)

The classic example is etching of Si with XeF2 and Ar+ ions are introduced



The shape of the etch profiles are interesting The profiles are not the linear sum of the

profiles from the two processes The profile is much more like the physical etch

alone (c)



If the chemical component is increased, the vertical etching is increased, but not the lateral etching

The etch rate is also increased The mechanisms for these effects are poorly

understood Whatever the mechanism, the enhancement

only occurs where the ions hit the surface Since the ions strike normal to the surface, the

enhancement is in this direction This increases the directionality



Possible models include – Enhancement of the etch reaction– Inhibitor removal

The reaction takes place only where the ions strike the surface

Since the ions strike normal to the surface, removal is thus only at the bottom of the well

It is assumed that etching by radicals (chemical etching) is negligible

Note that even under these assumptions, the side walls may not be perfectly vertical



Note that an inhibitor can be removed on the bottom, but not on the sidewalls

If inhibitors are deliberately deposited, we can make very anisotropic etches

If the inhibitor formation rate is large compared to the etch rate, one can get non-vertical walls (next slide)


Types of Plasma Systems

Several different types of plasma systems and modes of operation have been developed– Barrel etchers– Parallel plate systems (plasma mode)– Parallel plate systems (reactive ion mode)– High density plasma systems– Sputter etching and ion milling


Barrel Etchers

Barrel etchers were one of the earliest types of systems VT has a small one Here, the electrodes are curved and wrap around the

quartz tube The system is evacuated and then back-filled with the

etch gas The plasma is kept away from the wafers by a

perforated metal shield Reactant species (F) diffuse through the shield to the

wafers Because the ions and plasma are kept away from the

wafers, and the wafers do not sit on either electrode, there is NO ion bombardment and the etching is purely chemical


Barrel Etchers


Barrel Etchers

Because the etches are purely chemical, they can be very selective (but is almost isotropic)

The etching uniformity is not very good The systems are very simple and throughput

can be high They are used only for non-critical steps due to

the non-uniformity They are great for photoresist stripping


Parallel Plate Systems

Parallel plate systems are commonly used for etching thin films



This system is very similar to a PECVD system (Chapter 9) except that we use etch gases instead of deposition gases

These systems are much more uniform across the wafer than the barrel etcher

The wafers are bombarded with ions due to the voltage drop (Figure 10-8)

If the plates are symmetric (same size) the physical component of the etch is found to be rather small and one gets primarily chemical etching



By increasing the energy of the ions (increasing the voltage) the physical component can be increased

This can be done by decreasing the size of the electrode on which the wafers sit and changing which electrode is grounded

In this configuration, we get the reactive ion etching (RIE) mode of operation

Here, we get both chemical and physical etching

By lowering the gas pressure, the system can become even more directional


High-Density Plasma Etching

This system is becoming more popular These systems separate the plasma density

and the ion energy by using a second excitation source to control the bias voltage of the wafer electrode

A different type of source for the plasma is used instead of the usual capacitively coupled RF source

It is non-capacitively coupled and generates a very high plasma density without generating a large sheath bias



These systems still generate high ion fluxes and etch rates even though they operate at much lower pressures (1—10 mtorr) because of the higher plasma density

Etching in these systems is like RIE etching with a very large physical component combined with a chemical component involving reactive neutrals

They thus give reasonable selectivity


Summary

Chapter 10 ETCHING

Documents

Transcript of Chapter 10 ETCHING