Lecture 15 Etching. Chapters 15 & 16 Wolf and Tauber. ECE611 / CHE611 Electronic Materials Processing Fall John Labram 1/76

Transcription

1 Lecture 15 Etching Chapters 15 & 16 Wolf and Tauber 1/76

2 Announcements Term Paper: You are expected to produce a 4-5 page term paper on a selected topic (from a list). Term paper contributes 25% of course grade. You should have all been assigned your first-choice topic. The term paper should be handed in at the start of class on Tuesday 21 st November. Details / regulations are on the course website. The term paper will be returned to you in class on Tuesday 28 th November. 2/76

3 Announcements Homework 4/4: Will be set on Tuesday 21 st. It will be a normal length homework (like HW 1 & 2). 25 marks. Due Tuesday 28 th November. I will return it after the final exam. I will post the solutions immediately after I receive all the homeworks. 3/76

4 Announcements Final Exam: Gleeson 100. Tuesday December 5 th Scheduled: 14:00 to 15: minutes (1 hour 50 minutes). 3 out 4 questions. Otherwise format will be the same as midterm. Closed book and closed notes. All constants and most formulae will be given. Review lecture November 28 th will go through examples. No lecture November 30 th I will upload some sample questions. 4/76

5 Useful Links MIT: cture12.pdf Berkeley: Georgia Tech: Etching%20especially%20Plasma%20Etching.pdf 5/76

6 Lecture 15 Etching Overview / Review. Figures of Merit for Etching. Etching Non-Uniform Features. Wet Etching. Dry Etching. 6/76

7 Etching Overview 7/76

8 Etching Etching is the removal of regions of deposited films or substrates. The overall goal of the etch process for VLSI fabrication is to be able to reproduce the features on the mask with fidelity We want to be able to control the slope of the features we etch. Anisotropic etching Isotropic etching Photoresist SiO 2 Substrate: Si Photoresist SiO 2 Substrate: Si 8/76

9 Dry Etch vs Wet Etch Wet etch: E.g. HF etch (see Lecture 4): SiO 2 + 6HF L H 2 SiF 6 L + 2H 2 O(L) Photoresist SiO 2 Substrate: Si Typically isotropic. Poor control over feature size. High selectivity. 9/76

10 Dry Etch vs Wet Etch Dry etch: Ions in plasma are applied directionally. Photoresist SiO 2 Substrate: Si Can achieve anisotropic etching. Necessary for small features. Can have poor selectivity. 10/76

11 Figures of Merit for Etching 11/76

12 Selectivity Etch selectivity is defined as follows: Selectivity of A on B Pre-Etch S AB = R A R B = Etch rate of Material to be removed Etch rate of Material to remain Low Selectivity Vertical etch rate of material B Vertical etch rate of material A High Selectivity Photoresist Photoresist Photoresist Film Film Film Substrate Substrate Substrate 12/76

13 Selectivity In wet etching selectivity is controlled by: Chemicals employed. Concentration. Temperature. In dry etching selectivity is controlled by: Plasma chemistry. Gas Pressure. Flow rate. Applied voltage. Temperature. In dry etching S ~ 10. In wet etching S can be ~. 13/76

14 Selectivity We often want to etch all the way to the substrate: But we don t want to etch the substrate itself. Photoresist Film Photoresist Substrate Film Photoresist Substrate Film Substrate 14/76

15 Selectivity So we need to define two types of selectivity: Film-mask selectivity S fm = R f R m Photoresist Photoresist Film Substrate Film-substrate selectivity S fs = R f R s Film Where: Substrate R f = Vertical etch rate in film. R f = Vertical etch rate in mask (photoresist). R s = Vertical etch rate in substrate. 15/76

16 Selectivity of Si/SiO 2 Si/SiO 2 etched by HF solution: S SiO2,Si ~ Si/SiO 2 etched by reactive ion-etching (RIE): S SiO2,Si ~10 SiO 2 Si 16/76

17 Example Consider a simple etch of SiO 2 on Si, where S fs = 25, and the thickness of SiO 2 is 1μm. Carry out an etch for 3 minutes with R f = R SiO2 = 4000 Å/min. SiO 2 1 μm How much Si is etched? First, determine the amount of time to etch the SiO 2. Si t f = 10, = 2.5 mins After the 2.5 minutes, the Si is exposed. So, the silicon is exposed for t s = = 0.5 minutes. 17/76

18 Example The silicon is exposed for 0.5 minutes. Now determine the etch rate in the Si. R Si = R SiO 2 S fs = 4000 Ås 1 25 = 160 Ås 1 The thickness etched is then: Si x = t s R Si = = 80Å 18/76

19 Bias Bias is a way of quantifying etch anisotropy. d m Bias Photoresist Film B = d f d m Substrate Largest length etched Target etch length d f Bias has dimensions of length. In this example, the bias is positive. 19/76

20 Bias We can have negative bias: d m Bias Photoresist Film B = d f d m Substrate Smallest length of film remaining Target etch length d f In this case d f is the smallest length of the film remaining. Bias quantifies how far off our target etch we are. 20/76

21 Anisotropy Dimensionless variable used to quantify how anisotropic an etch is: Photoresist Film R v Substrate R l Anisotropy A = 1 R l R v 0 A 1 Lateral etch rate Vertical etch rate Completely isotropic (wet etch) Completely anisotropic (ions) 21/76

22 Mask Erosion The mask (photoresist) will also etch laterally as well. We quantify this just by the change size in mask laterally. This is important if we wish to do multiple process steps with one mask. E.g. etch then ion implant. Photoresist Film Substrate m Photoresist Film Substrate 22/76

23 Example Let s say we want to etch 100nm SiO 2 from the surface of an Si substrate, then immediately implant the Si with a dopant. Say the feature size is 50nm. If the etch which has an anisotropy of 0.5 and a film-mask selectivity of 10, what size will be the region we implant ions over? Assume the filmsubstrate selectivity is ~, and the photoresist is 1μm thick. Photoresist SiO 2 Si 50 nm 1 μm 100 nm 23/76

24 Example Since the film-substrate (R fs ) selectivity is ~, we know the etch rate in the silicon will be zero, and hence don t have to worry about etching the Si unintentionally. Also, since the photoresist is 1μm thick, we can easily show that the photoresist will not be depleted when etching all the SiO 2 (100nm): Film-mask selectivity S fm = R f R m = 10 If in time t 100nm of SiO 2 is etched, then the etched PR will be only: 100nm = 10nm S fm 24/76

25 Example Now look at the anisotropy: Anisotropy So we can say: A = 1 R l R v R l = 0.5R v Lateral etch rate = 0.5 Vertical etch rate I.e. the lateral etch rate is half the vertical etch rate. We can assume that A is the same in the resist and the film. 25/76

26 Example Now look at the selectivity: Film-mask selectivity S fm = R f R m = 10 R f = 10R m I.e. the film (SiO 2 ) we be etched 10 as fast as the mask. Combined with the anisotropy (R l = 0.5R v ), we can hence say the film will be etched vertically 20 as fast as the mask is etched laterally. Hence if it takes a time t to etch 100nm of SiO 2, then the mask erosion will be: m = = 5nm 26/76

27 Example The mask erosion is m = 5nm. Recall, the erosion takes place on either side. Hence the gap is now 50nm + 2 5nm = 60nm. So when we dope the wafer, it will now be over a length of 60nm. This is important, when we anneal the wafer, then apply the diffusion equation to determine the final dopant distribution. 50 nm m Photoresist Film Substrate 27/76

28 Etching Non-Uniform Features 28/76

29 Etching a Step In multi-layer VLSI, we are often etching non-uniform features. Film R v Vertical etch rate R l Lateral etch rate As we mentioned previously, the etch rate can be different in the vertical and lateral directions. The relative etch rates are given by the anisotropy parameter: Substrate A = 1 R l R v 0 A 1 29/76

30 Steps With a Slope Often the features have some sort of slope. How do we determine the film dimensions after a certain amount of time (t)? The thickness change will Vertical etch rate R still just be determined by v R v. Film R v t For the lateral etch, we have two contributions. Substrate x = x 1 + x 2 30/76

31 Steps With a Slope For the lateral etch, we have two contributions. Film θ R v Vertical etch rate x = x 1 + x 2 R l R l t Lateral etch rate There is the contribution from the lateral etch rate: x 1 = R l t Substrate The total etched distance in time t is then: x = R v cot θ + R l t The contribution from the vertical etch rate is: x 2 = R vt tan θ = R vt cot θ 31/76

32 Uniformity The etch rate is not always uniform across the wafer. Film Substrate This is quantified with the uniformity parameter: Maximum etch rate U = R max R min R max + R min If R max = R min, i.e. the etch is completely uniform: U = 0 If we have a high film-substrate selectivity (S fs ), then U > 0 isn t necessarily a problem. Minimum etch rate 32/76

33 Uniformity The starting film is also likely to have some non-uniformity. Film h f This is quantified by the thickness variation parameter: δ 0 δ 1 Substrate If the mean film thickness is h f, the maximum thickness is given approximated by: h f max = h f 1 + δ Run-to-run variation of thickness data are recorded. Once the deposition process is under control, the maximum / minimum values will be used to define δ. 33/76

34 Uniformity The variation in etch rate can also be quantified by the rate variation parameter: φ 0 φ 1 Film If the mean etch rate of the film is R f, the minimum etch rate is then: Substrate R f min = R f 1 φ In order to ensure that we etch everything (worst case scenario), we then know the time is therefore: t max = h f max R f min = h f 1 + δ R f 1 φ 34/76

35 Example Consider again an etch of SiO 2 on Si, where: S fs = 25. S fs = 25. h f = 1μm. R f = R SiO2 = 4000 Å/min. But now consider finite variations in film and etch rate: δ = φ = 0.1. SiO 2 If we carry out an etch to ensure that all SiO 2 is removed, what is the maximum depth of Si that will be etch? Si 1 μm 35/76

36 Example First, determine the etch time to ensure that all the SiO 2 is removed (accounting for variations in rate and film thickness): Average film thickness Average SiO 2 etch rate Work in Å/min: t max = t max = h f 1 + δ R f 1 φ = 2.92 min Thickness variation parameter Etch rate variation parameter This is the time required under worst case scenario (maximum thickness and slowest rate). Hence we must carry out the etch for 2.92 min. 36/76

37 Example t max = 2.92 min Now, to determine the maximum amount of silicon removed, we must consider best-case conditions. I.e. we must assume that at certain positions the rate is higher than average, and the SiO 2 thickness is lower than average. The maximum time to etch the SiO 2 was given by: t max = h f 1 + δ R f 1 φ The minimum time to etch the SiO 2 is hence: t min = h f 1 δ R f 1 + φ 37/76

38 Example t min = h f 1 δ R f 1 + φ Use this equation to determine the time to reach Si in the best-case scenario (thinnest part of film and fastest etch rate): t min = = 2.16 min Now determine the amount of time this bare Si is exposed during the etch: t Si = t max t min = = 0.76 min So we just need to the maximum amount of Si is etched in this amount of time. 38/76

39 Example t Si = 0.76 min This is the amount of time Si is exposed in the position where Si is first exposed. Recall, we are after the worst case scenario (i.e. maximum Si etched). So we again must consider the fastest etch rate: R Si max = R Si 1 + φ We can get the etch rate of silicon from the selectivity: R Si = R SiO 2 S fs = = 160Å/min R Si max = R Si 1 + φ = = 176Å/min 39/76

40 Example R Si max = 176Å/min Now we can work how much Si is etched in 0.76: x = = 134 Å 40/76

41 Wet Etching 41/76

42 Wet Etching The solution etches the film but not the mask: Diffusion PR Film Reactants Substrate Reaction Reaction products Boundary layer Diffusion Mass transport of reactants through boundary layer. Reaction between reactants and the film. Mass transport of reaction products away from the surface through the boundary layer. Wafer is just immersed in a solution containing the reactants. 42/76

43 Wet Etching Advantages: High selectivity due to the chemical reactions. Cheap. Bulk processing possible. Disadvantages: Usually isotropic. Poor process control transport or reaction rate limited. Stong T-dependence. Large amounts of chemical waste. 43/76

44 Wet Etching of Silicon Common silicon wet etch involves hydrofluoric acid: Si S + HNO 3 L + 6HF L H 2 SiF 6 L + 2H 2 O L + HNO 2 L + H 2 Nitric acid Hydrofluoric acid Hexafluorosilicic acid Nitrous acid Buffered HF etch: Use acetic acid (HC 2 H 3 O 3 ) instead of water. NH 4 F added to prevent depletion of F and retard etch of photoresist. Ammonium fluoride 44/76

45 Wet Etching of Silicon The rate of etch depends on the relative concentration of acids: The lines indicate etch rates for a given composition. The percentages refer to the concentration of the acids in water. HF(50%) = HF:H 2 O (1:1 volume %) HNO 3 (70%) = HNO 3 :H 2 O (7:3 volume %) 45/76

46 Example Determine the etch rate of a solution consisting of 2 parts HNO3 (70%), 6 parts HF (49%) and 2 parts HC 2 H 3 O 3. Etch rate = 165μm/min. 46/76

47 Wet Etching of Si 3 N 4 Silicon nitride (Si 3 N 4 ) is etched very slowly by HF solutions at room temperature. For example, using a buffered oxide etch with a volume ratio of 20:1 (NH 4 F:HF): R SiO2 = 300 Å/min. R Si3 N 4 = 5-15 Å/min. S SiO2,Si 3 N 4 = At room temperature is just HF (49%): R Si3 N 4 = 500 Å/min. 47/76

48 Wet Etching of Si 3 N 4 Silicon nitride (Si 3 N 4 ) is typically etched with phosphoric acid: R Si3 N 4 = 100 Å/min. R SiO2 = 10 Å/min. R Si = 3 Å/min. Selectivity: S Si3 N 4,SiO 2 = 10. S Si3 N 4,Si = /76

49 Wet Etching of Aluminum Aluminum etches in a mixture of phosphoric acid, water, nitric acid, and acetic acid: 50H 3 PO 4 : 20H 2 O : 1HNO 3 : 1HC 2 H 3 O 3 Phosphoric acid Water Nitric acid Acetic acid Al converts to Al 2 O 3. Dissolve Al 2 O 3. in phosphoric acid. Gas evolution leading to bubbles. Local etch rate is reduced in vicinity of bubbles. Non-uniformity. 49/76

50 Wet Etching of Photoresist Photoresist are generally formed of organic polymers (C,H containing molecules). Piranha solution is typically used: H 2 SO 4 and H 2 O 2. Wafers Solution of reactants Sulphuric acid Hydrogen peroxide H 2 SO 4 dissolves the polymer. H 2 O 2 oxidized the polymer: CH polymer S + H 2 O 2 L CO 2 G + H 2 O L 50/76

51 Common Wet Etches 51/76

52 Dry Etching 52/76

53 Plasma Etching Tetrafluoromethane We covered plasma pretty extensively in lecture 6. Overview of way to plasma etch: Use a molecular gas (often inert) e.g. CF 4. Establish a glow discharge and create reactive gas species by dissociation. E.g.: CF 4 + e CF 3 + F + e Choose chemistry so that the reactive species reacts with the solid to form a volatile product Si + 4F SiF 4 Pump away volatile product. 53/76

54 Plasma Etching Plasma energy makes ions and free radicals. Energy Ion Free Radicals Energy V V(x) V f sheath bulk V P, plasma potential X 54/76

55 Etching: Free Radicals Chemical Process Free radicals react with substrate to form products 55/76

56 Etching: Ion Bombardment Ions bombard the surface. The surface gets damaged (weakened). Physical Process Surface damage Sidewall passivation 56/76

57 Etching: Gas Products The gas products then escape from the surface. Etch chemistries are carefully selected to form volatile products. Chemical Process Material removed from under exposed surface Gas products High Vapor Pressure: Low Vapor Pressure: SiF 4 AlCl 3 CuCl BP at 1 atm -86 o C Gas products leave surface with atoms from the substrate. The desired shape is obtained. 57/76

58 Etch Chemistry of Si Carbon-fluorine gases are commonly employed in dry etching. H 2 Addition H + F HF O 2 Addition CF x + 2O CO 2 + xf CF x forms polymer F etches Si + 4F SiF 4 CF x + F CF x+1 Isotropic Etching Kay et al., Topics in Current Chemistry, Springer-Verlag, NY (1980). 58/76

59 Sidewall Passivation Depending on the process gas composition, SiO x F x functional groups can condense into polymers. These polymers forms on all surfaces. Ions preferentially remove polymer from bottom. A film on the sidewall protects them from free radicals. A strategy to enable high selectivity and high anisotropy. Photoresist SiO 2 Substrate: Si 59/76

60 Neutral CF 4 CF 4 is not hugely selective. Tetrafluoromethane Photoresist etches too quickly. Photoresist SiO 2 Substrate Substrate etches too quickly. Photoresist SiO 2 Substrate Can tune rate, by mixing with H 2. 60/76

61 Etching with CF 4 Silicon can be removed from selected regions of a substrate by etching using a CF 4 plasma. CF 4+ ions bombard and damage the surface. The plasma dissociates CF 4. F free radicals are produced. The free radicals react with the weakened silicon atoms on the surface. SiF 4 and SiF 2 are the reaction products. These molecules escape from the surface. F does not react with the photoresist. 61/76

62 Ionization of CF 4 High Speed Electron e - + CF 4 CF 4+ + e - + e - Neutral Molecule ( CF 4 ) Ion (CF 4+ ) 62/76

63 Ion Bombardment Ion bombardment damages surface (Physical process) Free radical does not react with photoresist Bulk Sheath Silicon Polymer Free radical ( F ) moving around Reaction product escapes from surface Free radical reacts with weakened silicon (Chemical process) 63/76

64 Dissociation of CF 4 High speed Electron e - + CF 4 CF 2 + F + F + e - 64/76

65 Selectivity of CF 2 Free radicals react selectively with the films Photoresist: CF 2 does not make a gas product. Oxide: CF 2 (G) + SiO 2 (S) CO 2 (G) + SiF 2 G Two gas products are made, so the film etches. Substrate: CF 2 (G) + Si (S) C(S) + SiF 2 G One product is solid, so the film does not etch (but will be contaminated). Photoresist SiO 2 Si 65/76

66 End Point Detection At the end of an etch the underlying film or substrate becomes exposed. This is known as the end point. End Point Detection: Products from the reaction of free radicals and the etched film (SiO 2 ) give color to the plasma. Products from the reaction of free radicals and the underlying film (Si) give a different color to the plasma. This color change indicates the end point. PR SiO 2 Si PR SiO 2 Si 66/76

67 End Point: Overetch When the end point is reached, the etch process is often continued for a set time. This is called overetch. Overetch makes sure that the etch is complete at every feature and no residue is left. PR RIE lag: Variation in depth with feature size SiO 2 Si (1) (2) 67/76

68 Materials Etched Various materials that are etched are shown below: Etch Silicon Insulators Metals Polymers poly-si (conductor) Si- wafer (semiconductor) Oxide (SiO 2 / BPSG) Nitride (Si 3 N 4 ) TiN / Al-Cu / TiN Stack Photoresist BARC 68/76

69 Silicon: poly-si Gases used: Cl 2 /HBr/O 2 Role of gases: Hydrogen bromide Cl 2 makes Cl free radicals which etch Si: Si S + 2Cl(G) SiCl 2 (G) HBr helps sidewall passivation (anisotropy). O 2 helps plasma generate Cl poly-si (conductor) Si- wafer (semiconductor) Etch Silicon Insulators Metals Polymers Oxide (SiO 2 / BPSG) Nitride (Si 3 N 4 ) T in / Al-Cu / T in Stack Photoresist BARC 69/76

70 Silicon: Si-wafer (Crystalline) Gases used: CF 4 /CHF 3 /O 2 Role of gases: Fluoroform CF 4 makes F free radicals which etch Si: Si S + 2F(G) SiF 2 (G) F is more reactive than Cl (used for poly-si). O 2 helps plasma generate Cl poly-si (conductor) Si- wafer (semiconductor) Etch Silicon Insulators Metals Polymers Oxide (SiO 2 / BPSG) Nitride (Si 3 N 4 ) T in / Al-Cu / T in Stack Photoresist BARC More isotropic higher etch rate, decreased anisotropy CHF 3 increases film formation. Sidewall passivation (anisotropy), Selectivity. 70/76

71 Insulators: Oxide Gases used: CF 4 /CHF 3 /CO/Ar or CF 4 /CHF 3 /Ar Role of gases: If CO is used: poly-si (conductor) Si- wafer (semiconductor) Etch Silicon Insulators Metals Polymers Oxide (SiO 2 / BPSG) Nitride (Si 3 N 4 ) T in / Al-Cu / T in Stack Photoresist BARC CF 4 makes CF 2 free radicals which etch SiO 2 SiO 2 S + 2F 2 G SiF 2 G + CO 2 If CO is not used: CF 4 makes F free radicals which etch SiO 2 Si S + 2F G SiF 2 G + O 2 G 71/76

72 Insulators: Nitride Etch process of nitrides (e.g. Si 3 N 4 ) is very similar to Oxides Gases used: CF 4 /CHF 3 /Ar Role of gases: poly-si (conductor) Si- wafer (semiconductor) Etch Silicon Insulators Metals Polymers Oxide (SiO 2 / BPSG) Nitride (Si 3 N 4 ) T in / Al-Cu / T in Stack Photoresist BARC CF 4 makes CF 2 and F free radicals which etch Si 3 N 4 Si 3 N 4 S + 3CF 2 G 3SiF 2 G + N 2 G + CN Si 3 N 4 S + 6F G 3SiF 2 G + 2N 2 G Roles of other gases are the same as for SiO 2. 72/76

73 Metal: TiN/Al-Cu/ TiN Stack Gases used: Boron trichloride Etch BCl 3 /Cl 2 Role of gases: Cl 2 makes Cl free radicals which etch Al (AlF 3 is not volatile): poly-si (conductor) Si- wafer (semiconductor) Silicon Insulators Metals Polymers Oxide (SiO 2 / BPSG) Nitride (Si 3 N 4 ) T in / Al-Cu / T in Stack Photoresist BARC Al S + 3Cl(G) AlCl 3 (G) BCl 3 provides large ions for ion bombardment. Increases anisotropy. Bombards photoresist, makes polymer film. Removes film at bottom Sputters CuCl which is not a gas. 73/76

74 Photoresist & BARC Dry etching: Plasma ashing Gases Used: O 2 Role of gases: Bottom Anti- Reflective Coating poly-si (conductor) Si- wafer (semiconductor) O 2 makes O free radicals which etch the C-H polymer Etch Silicon Insulators Metals Polymers Oxide (SiO 2 / BPSG) Nitride (Si 3 N 4 ) CH polymer S + O G CO 2 G + H 2 O G T in / Al-Cu / T in Stack Photoresist BARC 74/76

75 Gas-Solid Systems Solid Etch Gas Etch Product Si CF 4, Cl 2, NF 3, HBr SiF 4, SiCl 4, SiCl 2, SiBr 4 SiO 2 CF 4, SF 6, NF 3 SiF 4 Al BCl 3 /Cl 2 Al 2 Cl 6, AlCl 3 W, Ta, Nb, Mo SF 6, CF 4 WF 6, TaF 6 Ti, TiN Cl 2 TiCl 4 Organics, C O 2, O 2 /CF 4 CO, CO 2, HF, H 2, H 2 O GaAs Cl 2, BCl 3 Ga 2 Cl 6, AsCl 3 HgCdTe, InP CH 4 /H 2 In(CH 3 ) 3, PH 3, Cd(CH 3 ) 2 Cr O 2 /Cl 2 CrO 2 Cl 2 75/76