1. Chapter 10 ETCHING

2. 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

3. 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

4. Introduction

5. 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

6. 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)

7. Introduction

8. 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

9. 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

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

11. 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

12. 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

13. 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

14. 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

15. 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 byproduct

16. 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

17. 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

18. 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

19. 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”

20. Basic Concepts

21. 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

22. 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

23. Basic Concepts

24. 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

25. 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

26. 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?

27. 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

28. 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)

29. 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

30. 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)

31. 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

32. Plasma Etching

33. 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 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)

34. Plasma Etching

35. 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

36. 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

37. 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

38. Plasma Etching

39. 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 /cc ions and electrons

40. 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

41. 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

42. 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

43. 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

44. Chemical Etching

45. 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

46. 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

47. 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

48. Chemical Etching

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

50. 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

51. Physical Etching

52. 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)

53. 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

54. Ion-Enhanced Etching

55. Ion-Enhanced Etching 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)

56. Ion-Enhanced Etching
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

57. Ion-Enhanced Etching

58. Ion-Enhanced Etching 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

59. Ion-Enhanced Etching
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)

60. Ion-Enhanced Etching

61. 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

62. 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

63. Barrel Etchers

64. 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

65. Parallel Plate Systems
Parallel plate systems are commonly used for etching thin films

66. Parallel Plate Systems
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

67. Parallel Plate Systems
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

68. 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

69. High-Density Plasma Etching

70. High-Density Plasma Etching
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

73. Summary

74. Summary