1. Chapter 5-1.
Chemcal Etching

2. Outline Terminology Wet Etching Dry Etching DRIE
Si / SiO2/ SiN/ Metal Dry etching

3. Etching terminology

4. Etching terminology

5. Etching terminology

6. Etching terminology

7. Etching terminology

8. Mechanism of wet etching

9. Mechanism of wet etching

10. Mechanism of wet etching

11. Mechanism of wet etching

12. Mechanism of wet etching

13. Experimental conditions for Si wafer etching
Pattern size 100㎛
a=0
a= 100/1.414=70.72
Etch time
70/Etch rate of KOH per a hour
=70/31=2.26hour
Pattern size 400㎛
a=0
a= 400/1.414=282.89
282.89/Etch rate of KOH per a hour
=282.89/31=9.13hour
<100> Wafer

14. 100um patterns by Si wafer etching

15. Wet etching VS Dry etching
- 식각용액을 사용, 화학적인 반응만으로 박막 식각
- Selectivity 가 좋음
- 고가 장비 필요 없음, low cost
한꺼번에 많은 기판 처리, productivity ↑
undercut 발생, 용액의 측면 침식으로
미세 pattern 구현이 어려움 (3um 이하 어려움)
- 화학약품의 과다 사용으로 환경문제 대두
- 비등방성 식각 가능
- 측면 침식이 거의 없음
- 식각 가공 resolution이 좋음 (1um 이하 가능)
- Gas 사용으로 습식식각에 비해 상대적으로
깨끗하고 안전
- 물리적 충돌에 의한 식각도 일어나므로
완벽하게 특정 물질의 선택적 식각이 어려움

16. Dry etching # Advantage # Disadvantage
- Uses small amounts of chemical
- Isotropic or Anisotropic etch profile
- Directional etching without using the crystal orientation of Si
- High resolution and cleanliness
- Less Undercutting
# Disadvantage
- Some gases are quite toxic
- Need for specialized (expensive) equipment
- Re-deposition of non-volatile compounds

17. Atom elastic collision Excitation & Relaxation
Mechanism of dry etching
Vacuum Chamber
Electrode
: 전자의 충분한 가속
Mean free path ↑
Electrode
Plasma
DC or RF
- 반응가스 도입 후, 용기 내 압력을 10-3~1 torr 유지한 상태에서 plasma 이용 식각
- 글로우 방전(Glow discharge)에 의한 plasma 형성
압력이 매우 낮은 용기의 양쪽 전극에 전력을 인가, 한쪽 전극에서 전자 방출
- 방출된 전자는 가스 입자와 충돌하여 plasma 형성
- plasma 중에 형성된 이온과 라디칼이 식각하고자 하는 박막과 반응하여 식각
Atom elastic collision
Excitation & Relaxation
Ionization

18. Mechanism of dry etching
What is Plasma?
The 4th state of material (e.g., Solid, Liquid, Gas, Plasma)
Quasi Neutral State with Collectively behavior
(‘Plasma’; 1927’ Nomenclature by Langmuir)
Hot Plasma (e.g., ICP-AES for analysis), Cold Plasma (e.g., ICP, etc for etching)
The Plasma Universe (99.9%) Ar O2
Ar+
O2+ e-
Breakdown
Gas state
Plasma state

19. Plasma Surface Interactions
Equivalent flux density (/cm2sec)
1022
10W/cm2
1020
Ion beam modification
(implantation, surface treatment)
1018
Film deposition
1016
1014
Etching
1012
accelerator
Plasma chemistry
1010
10-2
100
102
104
106
108
Kinetic energy (eV)
Thermal activation of
atom migration
Sputtering
Electronic excitation
Displacement of lattice atoms
Increased
sticking
Implanatation
Desorption

20. Plasma Surface Interactions
General Plasma Generation
Nonconductive material → Dielectric Breakdown → Conductive material
High frequency electric field generate following reactions
Precursors
Evacuation
Surface reactions
Electrode -
Ionization
Dissociation R +
Recombination
Drift/Accelerating
Diffusion
Ion-molecule reactions
Radical-molecule reactions
Optical emission
Main Plasma Reactions
Excitation
Dissociation
Ionization
Recombination
Absorption
Sputtering
Polymerization

21. Plasma enhanced chemical etching
Bulk Plasma +
reactive ion reactive species electron PR particle + e- e- + + + e-
mask (mask erosion) +
volatile product
sidewall passivation + +
Sheath
λD ~
<1 mm
Substrate e- e- e- e- e- e- e- e- x
Electrode
plasma
presheath
sheath s
Sheath
edge

22. Various Plasma Etching Reactors
Schematic configuration of several dry etching reactors;
(a) chemically assisted ion beam etching (CAIBE) reactor, (b) reactive ion etching (RIE) chamber,
(c) inductively coupled plasma (ICP) reactor, (d) electron cyclotron resonance (ECR) reactor.

23. ICP (Inductively Coupled Plasma)
# ICP Equipment
Gas Inlet
Ceramic Process Chamber
Process Height
Wafer/Sample
Helium Cooling
Pumping Port
RF Matching
Unit ~
Plasma
- 반응용기 외부에 coil을 감아 RF 전원 을 인가하면 패러데이의 전자유도법칙에 의해
coil에 유도자장이 발생하게 되고, 이에 따른 유도전자기장이 반응용기 내부에 형성
고밀도 plasma 가 생성
- Platen 의 RF 전원 은 plasma 를 substrate 로 당기는 역할

24. ICP (Inductively Coupled Plasma)
# ICP Equipment – remote plasma
Processing
Height ~
Matching
Unit
Gas Inlet
Coil
Helium Cooling Gas Inlet

25. Silicon plasma etching
Conventional Plasma Etching System
High Density Plasma Source : ICP(DRIE), ECR, etc
Relatively Low Density Plasma Source : RIE, Ion Milling, Sputter
Advantages : HAR st., Relatively contamination free process, etc
For MEMS HAR devices and structures, Optical applications, etc
ICP(DRIE) - STS
RIE
ECR-asher
RIE-PECVD

26. Silicon etching – RIE system
High rate Isotropic Oxide/Nitride/Si etch with good uniformity:
Thermal oxide etch rate ~ 818Å/min ± 5% across 6” wafer, sel to AZ6615 PR ~ 0.5:1
Single-crystal Si etch rate ~ 870 Å/min, sel to AZ6615 ~ 0.53:1
Adjust pressure or power for uniformity.
CF4 35sccm (or CHF3)
O2 3sccm
150mT
300W (on 240mm diameter electrode)
-579V
20C, graphite cover plate
Silicon etch using an SiO2 mask (150mT, 300W)
Silicon wet etching result
Si (100) for convex cornor
Henning Schröder, Ernst Obermeier, Anton Horn, and Gerhard
K. M. Wachutka, J. Micros. Sys., 2001, 10, 88

27. DRIE etch Principle # Bosch process - 1st step : Etch (SF6 gas)
- 2nd step : Deposit passivation layer (C4F8 → CFx 계열로 분해)
- 3rd step : Wallside polymer etch << Bottom side polymer etch
- 1st step again : polymer and Silicon etch (SF6 gas)

28. < Deposition step>
DRIE etch Principle
# Passivation step and Etch step
< Deposition step>
< Etch step >
- Coil power ↑ ▶ passivation rate ↑ - Coil power ↑ , Wafer Temperature ↑ ▶
passivation rate ↓ ∴ He back side cooling
- Coil power, Gas flow ↑ ▶ SF6 로부터 F•
etchant species 많이 발생 ▶ etch rate ↑ - High pressure ▶ 높은 F• etchant species로
etch rate ↑

29. DRIE etch Principle
# repeat the etch and passivation steps

30. High Aspect Ratio Etch
# High Aspect Ratio Etch

31. < Fluidic Chennels >
High Aspect Ratio Etch
< Gas Turbine >
< Gyroscope >
< Fluidic Chennels >
< Optic switch >

32. < scallop effect >
# Bosch process and scallop effect
< sidewall >
< scallop effect >

33. Scallop effect # Bosch process and scallop effect < Top >
Mask undercut : 0.11um
Top Scallop : 47.6nm
< Top >
Bottom Scallop : 47.6nm
< bottom >

34. Scallop effect # removal of the scallop effect
< before thermal oxidation >
< thermal oxidation after DRIE process >
< after oxidation removal >

35. Etch rate & Selectivity & Uniformity
In same Plasma Condition EA = Etch Rate of Layer A EB = Etch Rate of Layer B SA/B : Selectivity of A to B
# Uniformity
Ei : Etch Rate at Several Points Emax : Maximum Etch Rate Emin : Minimum Etch Rate

36. Profile after DRIE 90-α 90+α t = (A-B)/2 θ = tan-1 (t/d) A B t
d (depth) θ
t = (A-B)/2
θ = tan-1 (t/d)

37. Range of Profile
← negative
anisotropic
positive →

38. Profile faults # Bowing < Bowing > * Reason
- too high a platen power and too high pressure
- poor ion directionality
- secondary ion etching with ions that have bounced off the bottom of trench
(increasing deposition time, increasing the polymer gas flow, reducing the etch time)
※ usual solution
- to drop the platen power
- pressure reduction

39. < Undercut by chemical isotropic etching >
Profile faults
# Undercut
Undercut
< Undercut by chemical isotropic etching >
※ solution
- reducing the total cycle time to as low as possible
(maintaining the same etch/deposition time ratio)
- etch rate가 높으면 undercut의 주된 원인이 됨.
but, undercut을 줄이려다 보면 etch rate가 낮아짐.

40. Microloading effect * reason
- Pattern의 open된 area 차이에 따라 etch rate이 달라짐.
- 압력이 높고, Open Size가 아주 적을 때는 상대적으로 반응 부산물이 Wafer의 표면에서
머물게 될 확률 ↑ ▶ 식각 공정의 활성도 저해
* solution
- 반응 부산물 생성 억제 or 반응 부산물을 Wafer의 표면에서 제거
- 반응 부산물의 생성을 억제하는 방법은 selectivity 저하 등 또 다른 문제점 초래
- 후자의 방법 선택, 압력을 낮게 유지하는 방법을 사용

41. Notch (footing effect)
OXIDE
NOTCH
MASK
SILICON
1. Build-up of negative charge on side
walls and top of trench due to
isotropic electron flux
2. Reduced electron flux to base of
trench due to repulsion at top of
trench
3. Accumulation of positive ions on
insulator surface
4. Further positive ions deflected to
sidewalls =>Sidewall Notch.
※ solution
- Increasing deposition characteristic : polymer gas flow ↑
  - Increasing dep time, decreasing etch time, reducing platen power

42. Notch (footing effect)
* Footing 현상의 mechanism
- 식각 gas의 양이온이 바닥에 충돌 후, 방전되지 못하고 남아있음
- 뒤따르는 양이온들과 척력을 발생, 원하지 않는 방향으로 발생 식각됨.

43. Notch (footing effect)
* 기판의 관통 공정시 최종적으로 드러나는 바닥면이 절연막이거나 유리기판인 경우
바닥 면에 전하의 charging이 일어나 식각 pattern 안쪽으로 식각이 확장
* 정확한 공정시간의 조절로 최소화해야 함.
* Microloading 현상으로 큰 pattern의 경우 필연적으로 나타남.

44. Back scattering effect
* DRIE etch through
공정에서의 footing 현상

45. Etching gas

46. Etching parameters

47. Etching parameters

48. Etching parameters

49. Etching parameters

50. Dielectric plasma etching
Low Density Plasma : very well established process, C/O control
High Density Plasma : HAR st., High etch rate, surface morphology
Advantages : Anisotropic etching, HAR st., Simple process, etc
than Dielectric wet etching process
For MEMS and Micro Optical parts devices and structures, etc

51. Requirements

52. Silicon Oxide etching mechanism (ICP)

53. - Energy transfer to surface
Silicon Oxide etching mechanism (ICP)
Chemical dominant reaction
Ion Bombardment
- Energy transfer to surface
SiF4 CO
CFx adsorption layer
SiO2
( O )
Etching reaction products
Ion driven etching reaction
Blocking film
(eg. C deposition)
Low density plasma
3/4 SiO2 + CF3 = 3/4 SiF4 + CO + 2 O Excess Oxygen atoms -> low selectivity to PR
Medium density plasma
1/2 SiO2 + CF2 = 1/2 SiF4 + CO Optimum etching mode
High density plasma
1/4 SiO2 + CF = 1/4 SiF4 + 1/2 CO + 1/2 C -- Carbon deposition -> etch stop

54. Courtesy of Wavesplitter Technologies Inc
SiNx etch, (PR mask)
Courtesy of Cambridge Univ.
20 um Core etch, (Silicon mask)
Courtesy of Wavesplitter Technologies Inc
6 um Core etch, (PR mask)
25 um Quartz Lens etch

55. Dielectric etching – RIE
Etchrate : A/min
Selectivity : 7.2 : 1
Uniformity : 1.12%
Profile : 84.7deg
CHF3 35sccm
Ar 15sccm
30mT
200W (on 240mm diameter electrode)

56. Metal plasma etching Chlorine and Inert gas based plasma used
Advantages : Anisotropic etching, Simple process, etc
than Metal wet etching process
For well defined metal pattern and reducing surface
contamination

57. Al - ICP etching 2.2um deep Al etch, Vertical Profile, No corrosion
Etch rate : 1500A/min
Selectivity : 2.5 : 1
Uniformity : <<9.66%

58. Cr - RIE etching Cl2 : 60sccm Etch rate:393A/min O2 : 3sccm
Pressure : 200mtorr
RF : 75Watts
Etch rate:393A/min
Selectivity:6.6 : 1
Uniformity:3.48%

59. Conclusion
1. Plasma etching is quite useful tool for fabricating ultra
high precision machining with few micron scale or below.
2. Various materials (e.g., Si, SiO2, Si3N4 and some metals) are
etched with vertical profile and quick and simple procedure.
3. For nano-scale fabrication one’s needs will require an unique
solution as an advanced plasma etching method.