Lecture 6 Plasmas. Chapters 10 &16 Wolf and Tauber. ECE611 / CHE611 Electronic Materials Processing Fall John Labram 1/68

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1 Lecture 6 Plasmas Chapters 10 &16 Wolf and Tauber 1/68

2 Announcements Homework: Homework will be returned to you on Thursday (12 th October). Solutions will be also posted online on Thursday (12 th October) at ~noon. Homework number 2 will be also be set Thursday (12 th October). 2/68

3 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. Details are on the course website now. Topics will be listed on Tuesday 17 th October. You need to list your favorite topics by Monday 23 rd October. You will be assigned a topic on Friday 27 th October. The term paper should be handed in at the start of class on Tuesday 21 st November. The term paper will be returned to you in class on Thursday 30 th November. 3/68

4 Announcements Mid-Term Exam: The exam will take place on Thursday (26 th October) at 10:00 am in Gleeson 100. You will have 90 minutes. Closed book and no notes. Will cover material covered in lectures 2-9. There will be a review lecture one week before (Thursday 19 th October). There will be no lecture on Tuesday 24 th October (exam preparation). 4/68

5 Useful Links Stanford lecture series on dry etching (~15 minutes each): Introduction to Dry Etching: Basics of Plasmas and Types of Dry Etching Tools: Dry Etching Mechanisms: Choosing a Dry Etching Process Tool: 5/68

6 Lecture 6 Dry Etching in CMOS. Etching Overview. Physics of Plasmas. Plasma Reactor Chamber. Sputtering and Etching. 6/68

7 Dry Etching in CMOS 7/68

8 Dry Etching Processes Start with your (doped) semiconductor wafer. n + p + p + n + n + p + substrate 8/68

9 Dry Etching Processes Oxide is grown over entire substrate (Lecture 7) oxide n + p + p + n + n + p + substrate 9/68

10 Dry Etching Processes Oxide is etched in certain regions to allow contact with doped regions (Lecture 16). oxide n + p + p + n + n + p + substrate 10/68

11 Dry Etching Processes Metal is deposited everywhere on wafer (Lecture 5). oxide n + p + p + n + n + p + substrate 11/68

12 Dry Etching Processes Metal is selectively etched (Lecture 14-16). oxide n + p + p + n + n + p + substrate 12/68

13 Dry Etching Processes Wafer is covered everywhere with SiN 3 (Lecture 14-15). SiN 3 oxide n + p + p + n + n + p + substrate 13/68

14 Dry Etching Processes Wafer is covered everywhere with SiN 3 (Lecture 14-15). SiN 3 oxide n + p + p + n + n + p + substrate 14/68

15 Dry Etching Processes Photoresist is applied (Lecture 14-15). Photoresist SiN 3 oxide n + p + p + n + n + p + substrate 15/68

16 Dry Etching Processes Photoresist is treated (Lecture 14-15). Photoresist SiN 3 oxide n + p + p + n + n + p + substrate 16/68

17 Dry Etching Processes SiN 3 and photoresist is selectively etched (this lecture). Photoresist SiN 3 oxide n + p + p + n + n + p + substrate 17/68

18 Dry Etching Processes Photoresist is removed etched (Lecture 14-15). SiN 3 oxide n + p + p + n + n + p + substrate 18/68

19 Dry Etching Processes Metal is deposited everywhere on wafer (Lecture 5). SiN 3 oxide n + p + p + n + n + p + substrate 19/68

20 Dry Etching Processes Metal is selectively etched. SiN 3 oxide n + p + p + n + n + p + substrate 20/68

21 Etching Overview 21/68

22 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 22/68

23 Selectivity Etch selectivity is defined as follows: Selectivity = Etch rate of Material to be removed Etch rate of material to remain Pre-Etch Low Selectivity High Selectivity Photoresist Photoresist Photoresist Film Film Film Substrate Substrate Substrate 23/68

24 Etching What we want from our etch: High selectivity against etching the mask layer material. High selectivity against etching the material under the film being etched. The etch rate should be fast enough to be practical, but slow enough to be controllable. The etch should be uniform across the wafer. The etch process should be safe (by products should be easily and safely removed). The etch process should cause minimal damage to the substrate. The mask should be easy to remove after the process is complete. The process should be clean (low concentration of contaminants). The process should be automated. 24/68

25 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. 25/68

26 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. 26/68

27 Physics of Plasmas 27/68

28 Plasmas So called 4 th State of Matter. An ionized gas of electrons and ions: Commonly observed in neon signs. 28/68

29 Plasmas Used to crack molecules. Use to create and accelerate ions. Composition of partially ionized gas: e + AB A + B + e e + A A + + e + e e + AB AB + + e + e e + A A + e e + AB AB + e * Denotes energy greater than ground state. = Radical (possesses unpaired electron). = Ions 29/68

30 Ionization A high speed electron hits an atom hard enough to knock out an electron. This forms an ion and another free electron. This is an elastic collision. High speed Electron Neutral Atom (Ar) Ion (Ar + ) e - + Ar Ar + + e - + e - 30/68

31 Molecules: Dissociation When a molecule undergoes dissociation we can get two reactive atoms. These atoms are called free radicals. High speed Electron Stable Molecule Reactive Free Radical e - + O 2 O + O + e - 31/68

32 energy Excitation and Relaxation An electron hits and excites a neutral atom. Which then relaxes and gives light. This is what makes a plasma glow. e - hν Characteristic Light High speed Electron Neutral Atom (Ar) Excited Atom (Ar*) Relaxed Atom (Ar) e - + Ar e - + Ar* e - + Ar + hn 32/68

33 Glow Discharge 33/68

34 Glow Discharge Color Different gasses, when excited, will glow with different colors. Nitrogen glows purple, Helium glows blue, Sodium glows yellow, Boron glows green, Neon glows red. The color of the glow (the wavelength) is related to the energy lost during relaxation by: E = hc λ This property enables the use of spectral analysis to obtain detailed information about the nature of the gasses that make up the plasma. 34/68

35 Energy vs Electron Density Plasmas used for plasma etching and other materials processing Dry Etching for VLSI, A.J. van Roosmalen, J.A.G. Baggerman, S.J.H. Brader, 1991, p.13 35/68

36 Physics of Plasmas The physics of plasmas is an incredibly rich area of study (see magnetohydrodynamics for example). We are only interested in a few properties for our needs. Elastic Collisions: Inelastic Collisions: 36/68

37 Elastic Collisions Defined as a collision between two bodies in which the kinetic energy and momentum are conserved during the interaction. Consider 1-dimensional example: Before: u 1 u 2 m 1 m 2 After: v 1 v 2 m 1 m 2 37/68

38 Elastic Collisions Before: Conservation of momentum: After: u 1 u 2 m 1 m 2 v 1 v 2 m 1 m 2 m 1 u 1 + m 2 u 2 = m 1 v 1 + m 2 v 2 Conservation of energy: m 1 u m 2u = m 1v m 2v Solving these equations: v 1 = u 1 m 1 m 2 + 2m 2 u 2 m 1 + m 2 v 2 = u 2 m 2 m 1 + 2m 1 u 1 m 1 + m 2 38/68

39 Example Consider an electron moving at 100 ms -1, collides with a stationary argon atom. High speed Electron Neutral Atom (Ar) In one dimension (assume electron hits the center of the atom), what is the velocity of the two products? We need to know: Mass of electron: m e = kg. Mass of argon atom: m A = kg. 39/68

40 Example Before: u 1 u 2 = 0 m 1 m 2 Enter values into equations: 100 ms kg kg v 1 = u 1 m 1 m 2 + 2m 2 u 2 m 1 + m 2 v 1 = ms 1 0 ms kg 0 ms kg v 2 = u 2 m 2 m 1 + 2m 1 u 1 m 1 + m 2 v 2 = ms ms -1 Electrons have significantly higher velocities than ions! 40/68

41 Collision Cross Section Collision rate can be quantified by collision cross-section: No Collision Collision r A + σ = πr 2 r = Φσρ Where: r = reaction rate (m -3 s -1 ). Φ = Beam flux (m -2 s -1 ). ρ = Density of target atoms (m -3 ). Reaction cross-section 41/68

42 Energy Distribution in Plasmas The energy of electrons in a plasma is distributed. The Druyvestian relationship is used to describe the distribution: 0.14 Maxwellian Distribution 0.12 f E = 4 A p e 2 I e = A pe 4 m ev p 2e 2 f E න m e ev p d 2 I e (E) dv p e E + ev p E de f(e) Druyvestein Distribution Energy (ev) 42/68

43 Rate of Plasma Processes The reaction rate, r, is given by: Where: r = kn e n ions k = ee m න 0 f E σ E de f (E) (E) Where: E = Electron energy f E = Electron energy distribution function (eedf). σ E = Cross section of reaction process. E 43/68

44 Plasma Reactor Chamber 44/68

45 Plasma Chamber Basically a vacuum chamber. Consider Ar as an example. Electrodes top and bottom of chamber. With no bias, just atoms flow through chamber. P Gas in 1 mtorr - 1 torr Electrodes Gas out Watts Power Supply Switch Atoms 45/68

46 Plasma Chamber When biased, electrons are accelerated between the electrodes. Electrons ionize gas atoms. Plasma glows. Gas in P 1 mtorr - 1 torr Gas out Watts Power Supply Switch Ion Electron 46/68

47 Plasma Ignition Ignition takes place when a gas breaks down, due to an electrical discharge (arc). The voltage required for ignition is described by the Paschen law: Bpd V = ln Apd ln ln γ se B = Constant related to excitation an ionization energies. A = Saturation ionization in the gas at a particular E/p. V = Breakdown voltage. p = Pressure. d = distance between electrodes. γ se = secondary-electron-emission coefficient (the number of secondary electrons produced per incident positive ion). 47/68

48 Plasma Ignition Sweet Spot 48/68

49 Plasma Chamber The plasma in the chamber is charged balanced: n + e ~n ion The gas is only partially ionized (typically ~ 0.01%). Radicals only make up about 1% of gas. (at 20 mtorr). 49/68

50 The RF Field A DC bias cannot be used if we want to deposit onto an insulator. What happens when we apply a DC bias? First consider a conductor on the cathode V Ar + Conductor 50/68

51 DC Bias - Conductor -V 1 Ar Ar Ar Ar Ar + Ar + Ar + Ar + Positive ions are attracted to the cathode (negatively charged). They obtain an electron from the surface and become neutral. Since the surface is conducting, the electrons are replenished. The voltage remains as set. The process can carry on indefinitely. 51/68

52 DC Bias - Insulator -V 1 Ar Ar Ar Ar + Ar + Ar + Positive ions are attracted to the cathode (negatively charged). They obtain an electron from the surface and become neutral. Electrons in the surface are not replenished. Ar Ar + Surface gets positively charged. Eventually field between terminals drops below level to maintain plasma. 52/68

53 The RF Field In general a MHz frequency oscillating field is applied (as defined by communication authorities). Now what happens to the components of the plasma. Positive and negative charges are alternatively accelerated to each electrode. 1 2 V b V a 53/68

54 The RF Field The mass of our ions is much larger (typically ) than electrons. So velocity of electrons is much higher in general than ions. 1 2 V b V a 54/68

55 Plasma in RF Field 1 Electrons have higher general velocities: Ar + Ar + J e J ion On average, more electrons will hit surfaces than ions per unit time. Surface will get more negatively charged initially. Ar + High speed Electron 55/68

56 Plasma in RF Field 1 Ar + Ar + At some point the charge will be sufficient to repel electrons. Eventually steady will be established. Ar + Ar + J e = J ion Ar + Ar + High speed Electron 56/68

57 Sheath Region Regions near electrodes have a greater concentration of ions than electrons. No plasma near electrodes (sheath region). sheath region sheath region 1 2 V b V a 57/68

58 Ion Bombardment Ions move around slowly in the bulk of a Plasma. When they reach the sheath, they are strongly attracted to the negative surface. They hit the surface at a very high speed. c c Bulk Sheath c Negative Surface 58/68

59 Sputtering and Etching 59/68

60 Sputtering Argon ion bombards the aluminum surface Aluminum Aluminum Target P Power Supply Displaced aluminum Atoms Sheath Bulk Bulk Gas in Gas out Sheath Silicon Wafer Aluminum atom moving around Aluminum atoms deposit on the wafer and form a film Silicon Wafer A target material, such as aluminum, is bombarded with Argon ions. The displaced atoms of the target material move across the Plasma. They are then deposited on a silicon wafer. 60/68

61 Chemical Vapor Deposition (CVD) Free radicals deposit on the wafer surface and chemically combine to form a layer of material. Silane (SiH 4 ) Molecule Oxygen (O 2 ) Molecule Bulk Sheath (SiH 2 )Free Radical (O) Free Radical Silicon dioxide Silicon (SiO 2 )Formed on surface Unreacted (O) Free Radical on surface 61/68

62 Oxygen Plasma Cleaning Recall from piranha cleaning (Lecture 4), that elemental oxygen is extremely effective at removing organic contaminants: O + O + C CO 2 O + H 2 H 2 O Oxygen plasmas are routinely employed to clean surfaces. 62/68

63 Etch Process: CF 4 Plasma Tetrafluoromethane Ion bombardment damages surface (Physical process) Bulk Free radical ( F ) moving around Free radical does not react with photoresist Sheath Polymer Silicon Reaction product escapes from surface Free radical reacts with weakened silicon (Chemical process) 63/68

64 Etch Profile: Types The shape of the feature that is etched is called the etch profile. Isotropic etch profile Etched equally in all directions. All wet etches give isotropic profiles Some dry etches give isotropic profiles Photoresist SiO 2 Substrate Anisotropic etch profile Etched in the preferred direction only. Only dry etch gives anisotropic profiles. Anisotropic profiles are needed for smaller features Photoresist SiO 2 Substrate 64/68

65 Etch Profile: Anisotropy Free Radicals Ions Free radicals, which provide the chemistry that removes material, act in all directions (isotropic). More free radicals lead to more isotropic etching. Photoresist SiO 2 Substrate: Si Ions, which physically damage the surface act in one direction(anisotropic). Strong ion bombardment leads to more etching in the preferred direction. Photoresist SiO 2 Substrate: Si 65/68

66 Sidewall Passivation Sidewall passivation (ions and free radicals) Polymer forms on all surfaces during etch process Ions preferentially remove polymers from bottom A film on the sidewall protects them from free radicals Etch recipes can try to promote a sidewall film Photoresist SiO 2 Substrate: Si 66/68

67 Aspect Ratio Aspect Ratio (AR) = h / w SiO 2 h w Substrate High Aspect Ratio w = 0.18 µm h > 1 µm 67/68

68 Aspect Ratio Dependence High AR loss of anisotropy can be due to: 1. Charge Buildup Electrons preferentially charge the upper portions of the etched features. Ions bend towards sidewalls. 2. Neutral Shadowing Collisions of free radicals with other particles or the feature. 3. Ion Shadowing Scattering and charge exchange of ions in sheath. 4. Neutral Product transport Etch products transporting out collide with particles or feature leading to redeposition. 68/68