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浑慰慎
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1 理論 Lecture 8 (Etching) 1

2 Outline Introduction How to Control Etching Process? Isotropic Wet Etching Anisotropic Wet Etching Dry Etching 2

3 Lecture 4 Lecture 5&8 3

4 類路理兩留路兩類 4

5 Types of Etching Processes Isotropic ( ): Best to use with large geometries, when sidewall slope does not matter, and to undercut the mask Quick, easy, and cheap Anisotropic ( ): Best for making small gaps and vertical sidewalls Typically more costly 5

6 Etch Parameters Etch Rate ( 率, r): Rate of material removal (µm/min) Function of concentration, mixing, temperature, Etch Selectivity (, S=r 1 /r 2 ): Relative (ratio) of the etch rate of the film to the mask, substrate, or another film Trade off etch rate and selectivity Etch Geometry ( 狀 ): Sidewall slope 6

7 Undercutting: Isotropic Near vertical sidewalls can be obtained with isotropic etching Caveat: long over-etch is required (i.e., it is difficult to make small gaps in thick films) 7

8 Undercutting: Isotropic Etch bias (along x-axis) Overetching (along y-axis) 8

9 Example 1: Selectivity 1µm SiO 2, 率 0.40µm/min, 25 1,, Si? S = r SiO r Si Etching rate of Si 0.4 m / min 25 = rsi r Si = 0.016µ m / 2 µ min Etching 1µm of SiO 2 t SiO = 1µ m /(0.40µ m / min) = min Over etching of Si thickness Si = ( 0.016µ m / min) 0.5 min = 8nm 9

10 Etch process & Etch Rate 10

11 Mechanism of the Etching Process SLOWEST Dominates!!! 11

12 Slowest Step Dominates (What if each is slowest?) Etchant Species Generation Etch rate will be slow Diffusion to Surface Etch rate will be highly dependent on mixing but not temperature Also dependent on layout and geometry Reaction Etch will be highly dependent on temperature but not on mixing or layout and geometry Diffusion to Bulk Etch rate will be highly dependent on mixing but not temperature Also dependent on layout and geometry 12

13 Etch Rate Variation Function of : Setup Material being etched Layout and structure 13

14 1. Etch Rate Variation (Due to Setup) Temperature Loss of reactive species Loss of liquids to evaporation Mixing Etch-product blocking of chemical flow Applied potential Illumination Contamination 14

15 2. Etch Rate Variation (Due to Material Being Etched) Impurities in or on the materials Microstructure Film Stress 15

16 3. Etch Rate Variation (Due to Layout or Structure) Distribution and fraction of surface area of the exposed target layout (loading effect) Structure geometry 16

17 Wet Etching ( ) 17

18 利料利來行狀了 18

19 Wet Etching ( ) Mixtures of acids, bases, and water HF, HPO 3, H 2 SO 4, KOH, H 2 O 2, HCL,.. 10:1, 5:1, or maximum (solubility limit) Can be used to etch many materials Si, SiO 2, Si 3 N 4, PR, Al, Au, Cu,. Etch Rate: Wide range Etch Selectivity: Typically quite high Sensitive to contamination Etch Geometry: Typically isotropic Some special cases are anisotropic 19

20 Isotropic Etching Etch at equal rates in all directions Rounds sharp edges or corners Removes roughness and damaged surfaces Amorphous and polycrystalline Films Always Single Crystal Materials Possible 20

21 Isotropic Wet Etching HF for SiO 2 Etch Selective (room temperature) Etches SiO 2, but not Si Also attack Al, Si 3 N 4 Rate depends strongly on concentration Maximum: 40% HF ~ 2µm/min Controlled: 5 to 50:1 ~ <0.1µm/min (Note: different with BOE) Dangerous Not a strong acid, but penetrates skin Will target bones, and attacks slowly Etch Geometry: Completely isotropic 21

22 Isotropic Wet Etching HNA for Si Etch Mixture of nitric (HNO 3 ) and hydrofluoric (HF) acids Si etch rate can be up to 50 µm/min HNO 3 oxidizes Si, removes SiO 2, repeat High HF, but Low HNO 3 : etching limit by oxide formation => rough surface Low HF, but High HNO 3 : etching limit by oxide removal => smooth surface Dilute with water or acetic acid (CH 3 COOH) Acetic acid is preferred because it prevents HNO 3 disassociation 22

23 Isotropic Wet Etching HNA for Si Etch CH 3 COOH or H 2 O 23

24 Isotropic Wet Etching Phosphoric Acid for Si 3 N 4 Etch Selective (room temperature) Etches Si 3 N 4, but not Si or SiO 2 Also attack Al Rate is slow H 3 PO 4 at 160 C ~ 0.005µm/min Tough masking material needed PR will not survive Oxide is typically used Etch Geometry: Completely isotropic 24

25 Isotropic Wet Etching Masking materials 25

26 Anisotropic Etching 26

27 Silicon Crystal Orientation 27

28 Miller Index Indices determined by 1 / intercept of the vector from origin Within unit cell Direction [1/x, 1/y, 1/z] Normal to plane ( ), { } 28

29 Wafer Flat 29

30 Anisotropic Wet Etching (on <100> wafer) Concentration : etching rate selectivity surface roughness Temperature : etching rate selectivity surface roughness 30

31 Anisotropic wet etching (on <100> wafer) 31

32 Comparison (100) wafer <110> direction <110> (100) wafer 32

33 (Wet Bench) 龍 33

34 34

35 Dry Etching( ) 35

36 (Dry etching) 利離行理來離離例利行異量離行離率異 36

37 Mechanism of the Etching Process 37

38 Gas Phase Dry Etching 38

39 RF Plasma-Based Dry Etching 39

40 Plasma Etching Mechanism Typical reactions and species present in a plasma used for plasma etching Processes involved in chemical etching during plasma etch process 40

41 Etching of a Silicon Surface 41

42 Fluxes of Species in Plasma Etching (a) Fluxes of reactive neutral chemical species (such as free radicals), with a wide arrival angle distribution and low sticking coefficient (b) Fluxes of ionic species, with a narrow, vertical arrival angle distribution and high sticking coefficient 42

43 Ion-Enhanced Etching (a) The chemical etch reaction is enhanced by ion bombardment (RIE) (b) An inhibitor is formed which is removed by ion bombardment, allowing chemical etching to proceed (Deep RIE) 43

44 Types of Dry Etching Processes Cross Section Depends On: Energy Pressure (mean free path) Bias Directionality Crystal orientation 離離 44

45 Types of Dry Etching Processes 45

46 Dry Etching Control Parameters 46

47 Dry Etch Chemistries 47

48 Etching Si/SiO 2 in CF 4 : Selectivity 48

49 Deposition of a Nonvolatile Residue 49

50 Anisotropy in Dry Etching 50

51 Dry Etching Problems 料累離路 Flow rate effect Loading effect Aspect ratio effect Undercutting 51

52 Summary of Different Processes in Dry Etching 52

53 離 (Dielectric RIE) 料率料不參金料 53

54 度離 (HDP-RIE) 料率率冷 54

55 Deep Reactive Ion Etching Reactive Ion Etching can be used to etch Si as discussed before. However, it is often desired in MEMS and 3-D packaging applications to fabricate very narrow and tall structures or grooves which can t be done using the standard RIE process. A new process, called DRIE process, has been successfully developed. The process utilizes special gas chemistry to form a polymer on the sidewalls or the trench as it is being etched. The thin layer of polymer prevents lateral undercutting and etching of the Si on the side wall. The result is a very high aspect-ratio etch. 55

56 Cont. Ref. Gahn et al., Ref. Tith et al., Ref. BCO Tech. 56

DRIE Lag The main characteristic associated with DRIE etch is that the etch depth depends on a number of factors, most importantly the width of the feature that needed to be etched.

57 DRIE Lag The main characteristic associated with DRIE etch is that the etch depth depends on a number of factors, most importantly the width of the feature that needed to be etched. The etching depth of open areas on a silicon substrate will vary with the opening size. The larger the area is, the deeper the trench can be. That is not desirable but not much can be done about it. Ref. Tith et al., 57

58 Cont. V grooves for optical alignment Ref. Prof. Lu course handout For high Q RF passive components & thick isolation layer for RF packaging applications 58

59 (CMP) 59

60 Laser Drilling Laser assisted chemical etching (LACE) uses localized heating to drive etching reactions. Laser drilling is an ablative process. LACE can be very slow but can offer 1 µm 3 voxel resolution. Si + 2Cl SiCl 2 laser beam 4 Ref. Lexno Laser 60

wafers The process involves both mechanical polishing and chemical etching, like HNA, It is

61 Chemical Mechanical Polishing (CMP) CMP has become a very useful technique for either polishing and planarizing the surface of wafers which have tall feature and topologies, or for thinning wafers The process involves both mechanical polishing and chemical etching, like HNA, It is possible to create very flat and polished surfaces using this technique. Plummer et al., Silicon VLSI Tech. 61

62 Chemical Mechanical Polishing (CMP) The CMP materials include SiO 2, Si, W, Al, and Cu.. Etc., A good CMP should require: Low WIWNU (Within Wafer Nonuniformity) High planarization rate (~2000 to 6000Å/min) A given selectivity versus underlying layers Low defectivity of the surface after CMP Low defectivity of the stop layer surface after CMP Two examples of CMP are illustrated for the introduction : Polysilicon with amorphous silica slurry (Klebosol) Tungsten CMP with KIO 3 and H 2 O 2 Copper CMP 62

63 Polysilicon CMP Polysilicon CMP are used for polysilicon plugs, polysilicon STI, MEMS. The applications require different polishing selectivity versus the underlying layer (SiO 2 ). Several slurries have been successfully developed for different selectivity ranging from 1:1 to 1:25 Polysilicon film removal rate varies drastically with the ph. OH - anions react with Si-Si to create two products Si-H and Si(OH) 4 which dissolved into the slurry. The ph of a basic amorphous silica slurry is around 11. With the adjustment of the slurry chemistry, we can decrease significantly the oxide removal rate, keeping a high polysilicon removal rate. 63

64 Defectivity Induced by CMP CMP characterization is generally based on AFM measurement. A good CMP can provide defects with the size less than 0.01µm Ref. Joray et al., 64

65 Tungsten CMP Tungsten CMP is used for tungsten plug and damascene which have the same requirements in terms of selectivity versus the underlying layer SiO 2, removal rate and defectivity. Tungsten is a very hard material. In order to get acceptable removal rate, it is necessary to oxide the tungsten surface since three different tungsten oxides (WO 2, W 2 O 5, and WO 3 ) are softer. The most interesting oxidizer in terms of removal rate are a combination of potassium iodate (KIO 3 ) and hydrogen peroxide (H 2 O 2 ). The etching rate is as high as 4000Å/min. The removal mechanism is not an actual polishing mechanism, but rather an etching mechanism. The CMP rate depends on polishing pressure, speed, oxidizer concentration and the ph of the slurry but not on the abrasive concentration. 65

66 Copper CMP The slurry contains ethylenediaminetatra acetic acid (EDTA) or glycine (amino acid). Both kinds of additives not only ease the dissolution of Cu and provide certain level protection on Cu surface to prevent dishing. (Aksu, et al, 2001) Copper dishing The process challenges in copper CMP: Copper is a soft material and prone to surface defects. It is very difficult in finding the slurry with proper chemistry and a pad with suitable physical properties; Excessively high pad roughness results in dishing on the surface. Low pad roughness results in low polishing rate. Redeposition of copper and copper oxide particulates due to incomplete dissolution in the slurry could reduce the mechanical polishing efficiency and increase the surface density. 66

67 Topology Comparisons Plummer et al., Silicon VLSI Tech. 67

68 68

69 Thank you! ( ) (03) ~2171 tjyen@mx.nthu.edu.tw 69

2 Miller Index (hkl) (1 00) X {hkl} {100} (100),(010),(001),(100),(0 10),(00 1) [hkl] (hkl) [100] (100) <hkl> 3 Characteristics of Etching Techniques

2 Miller Index (hkl) (1 00) X {hkl} {100} (100),(010),(001),(100),(0 10),(00 1) [hkl] (hkl) [100] (100) <hkl> 3 Characteristics of Etching Techniques 1 National Kaohsiung First University of Sci. & Tech. Silicon Anisotropic Wet Etching Department of Mechanical and Automation Engineering National Kaohsiung First University of Science and Technology MicroSystem