EE C245 ME C218 Introduction to MEMS Design

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1 EE C245 ME C218 Introduction to MEMS Desin Fall 2008 Prof. Clark T.-C. Nuyen Dept. of Electrical Enineerin & Computer Sciences University of California at Berkeley Berkeley, CA Lecture 5: Process Modules II: Film Deposition, Lithoraphy EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 1

2 Announcements Discussion Section: now on Mondays, 5:30-6:30 p.m., in 3109 Etcheverry Reminder: TA office hours are bein held in the Moore Room in the courtyard of Cory Hall. You miht want to update this in your course information sheet. EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 2

3 Lecture Outline Readin: Senturia, Chpt. 3; Jaeer, Chpt. 2, 3, 6 Example MEMS fabrication processes Film Deposition Evaporation Sputter deposition Chemical vapor deposition (CVD) Plasma enhanced chemical vapor deposition (PECVD) Epitaxy Atomic layer deposition (ALD) Electroplatin Lithoraphy EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 3

4 Thin Film Deposition EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 4

5 Thin Film Deposition Methods for film deposition: Evaporation Sputter deposition i Chemical vapor deposition (CVD) Plasma enhanced chemical vapor deposition (PECVD) Epitaxy Electroplatin Atomic layer deposition (ALD) Evaporation: Heat a metal (Al,Au) to the point of vaporization Evaporate to form a thin film coverin the surface of the Si wafer Done under vacuum for better control of film composition EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 5

6 Evaporation Filament Evaporation System: W filament + - wafer Al staples 1. Pump down to vacuum reduces film contamination and allows better thickness control 2. Heat W filament melt Al, wet filament 3. Raise temperature evaporate Al mean free path Vacuum Pump = λ = kt 2π Pd k = Boltzmann Constant T = temperature P = pressure d = diameter of as molecule 2 EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 6

7 Evaporation (cont.) λ can be ~60m for a 4Å particle at 10-4 Pa (-0.75 μtorr) thus, at 0.75 μtorr, et straiht line path from Al staple filament to wafer Problem: Shadowin & Step Coverae Get an open Source Problem: line of siht deposition Solns: i. Rotate water durin Source evaporation ii. Etch more radual sidewalls Better Solution: foret evaporation sputter deposit the film! EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 7

8 Sputter Deposition Use an eneretic plasma to dislode atoms from a material taret, allowin the atoms to settle on the wafer surface Not as low a Taret vacuum as evaporation Ar+ Ar+ (~100 Pa) (750 mtorr) plasma (Al, SiO 2, Si 2 N 4, ZnO, Ti, ) Vacuum Pump wafer EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 8

9 Step-by-step procedure: 1. Pump down to vacuum Sputter Deposition Process (~ 100 Pa) 1Pa = mtorr 760 Torr atm = Torr atm 2. Flow as (e.., Ar) 3. Fire up plasma (create Ar+ ions) apply dc-bias (or RF for non-conductive tarets) t 4. Ar+ ions bombard taret (dislode atoms) 5. Atoms make their way to the wafer in a more random fashion, since at this hiher pressure, λ ~60μm for a 4Å particle; plus, the taret is much bier Result: better step coverae! EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 9

10 Problems With Sputterin 1. Get some Ar in the film 2. Substrate can heat up up to ~350 o C, causin nonuniformity across the wafer but it still is more uniform than evaporation! 3. Stress can be controlled by chanin parameters (e.., flow rate, plasma power) from pass to pass, but repeatability is an issue Solution: use Chemical Vapor Deposition (CVD) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 10

11 Chemical Vapor Deposition (CVD) Even better conformity than sputterin Form thin films on the surface of the substrate by thermal decomposition and/or reaction of aseous compounds Desired material is deposited directly from the as phase onto the surface of the substrate Can be performed at pressures for which h λ (i.e., the mean free path) for as molecules is small This, combined with relatively hih temperature leads to Excellent Conformal Step Coverae! Types of films: polysilicon, SiO 2, silicon nitride, SiGe, Tunsten (W), Molybdenum (M), Tantalum (Ta), Titanium (Ti), EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 11

12 The CVD Process Reactant as (+ inert dilutin ases) are introduced into the reaction chamber (a) Gas species move to the substrate Gas Flow Gas Stream Atoms mirate and react chemically to form films This determines the ultimate t conformality of the film (i.e., determines step coverae) (b) (e) Reactants adsorb onto the substrate (e) (d) (c)(d) ) Wafer Reaction by-products desorbed d from surface Enery required to drive reactions supplied by several methods: Thermal (i.e., heat), photons, electrons (i.e., plasma) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 12

13 Ga pr as phase rocesses The CVD Process (cont.) Step-by-Step CVD Sequence: a) Reactant ases (+ inert dilutin ases) are introduced d into reaction chamber b) Gas species move to the substrate c) Reactants adsorbed onto the substrate d) Atoms mirate and react chemically to form films This determines to a lare extent whether or not a film is conformal (i.e.. better step coverae) Surf face proce esses Not Conformal Conformal low T Hih T not enouh adatom miration Plenty of adatom miration e) Reaction by-products desorbed and removed from reaction chamber EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 13

14 CVD Modelin Simplified Schematic: N J surface δ J s N s Governin Equations: J J s = k N k s D s N = conc. of reactant molecules in the as stream N s = conc. of reactant molecules at the surface J s = flux of as molecules at the surface J = flux of molecules diffusin in from the as stream Effective diffusion const. for the as molecule [ s = surface reaction rate const.] = ( N ) ( ) N s = h N N Vapor phase mass δ s Vapor phase masstransfer coefficient EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 14

15 CVD Modelin (cont.) J h [ ] = = = s s s s k J N J J J, s s k J h N h k J N h J = = s Otherwise reactants will build up somewhere! ( ) s N h k N h k J N h h J 1 p ( ) s s s s N h k N h k J N h k J = + = = 1+ N J = = unit volume # molecules incorporated flux rowth rate N unit volume # molecules incorporated ( ) rate rowth = = = = N h k N h k J s EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 15 ( ) rate rowth = = + = = N h k N h k N s s

16 CVD Modelin (cont.) Case: k s >> h surface reaction rate >> mass transfer rate rowth rate = h N N (mass-transfer-limited) Case: h >> k s mass transfer rate >> surface reaction rate rowth rate = k s N N (surface-reaction-limited) reaction E a R o kt ~ (Arrhenius character) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 16

17 Temperature Dependence of CVD lo (rowth rate) Mass Transport Limited Reime Reaction Rate Limited Reime Slope = E a Dep. Rate less dependent on T, here for better control, better to operate here (@ hiher T) Arrhenius behavior 1 T temperature EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 17

Atmospheric Pressure Reactor (APCVD) Once used for

Substrates fed continuously Lare diameter wafers

(hih h pressure, so touher for as to et to the wafer

incorporate forein particles Poor step coverae EE

18 Atmospheric Pressure Reactor (APCVD) Once used for silicon dioxide passivation in interated circuits Substrates fed continuously Lare diameter wafers Need hih as flow rates Mass transport-limited reime (hih h pressure, so touher for as to et to the wafer surface) Problems/Issues: Wafers lay flat, and thus, incorporate forein particles Poor step coverae EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 18

19 Low Pressure Reactor (LPCVD) Many films available: polysilicon, SiGe, Si 3 N 4, SiO 2, phosphosilicate lass (PSG), BPSG, W Temp.: C Press.: Pa (200mTorr 2Torr) Reaction rate limited; reduced pressure ives as molecular hih dff diffusivity; can supply reactants very fast! Can handle several Problems: hundred wafers at a Low dep. rate (compared to atm.) time Hiher T (than atmospheric) Excellent uniformity In hot wall reactors, et deposition on tube walls (must clean) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 19

Plasma-Enhanced CVD Reactor (PECVD) RF-induced

allows lower temperature deposition with decent

Pin-holes Non-stoichiometric films Incorporation

to outassin or bubblin in later steps EE C245:

20 Plasma-Enhanced CVD Reactor (PECVD) RF-induced low dischare + thermal enery to drive reactions allows lower temperature deposition with decent conformability Still low pressure Problems: Pin-holes Non-stoichiometric films Incorporation of H 2, N 2, O 2 contaminants in film; can lead to outassin or bubblin in later steps EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 20

21 Polysilicon CVD Polysilicon Deposition: Fairly hih h temperature t conformal 600 C SiH 4 Si + 2H 2 (thermal decomposition of silane) LPCVD (25 to 150 Pa) Å/min (conformal hih T) In situ dopin of polysilicon: n-type: add PH 3 (phosphine) p or Arsine ases (but reatly reduces dep. rate) p-type: add diborane as (reatly increases dep. Rate) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 21

22 Silicon Oxide CVD Silicon Dioxide Deposition: After metallization (e.., over aluminum) Temperature cannot exceed the Si-Al eutectic pt.: 577 o C Actually, need lower than this (<500 o C) to prevent hillocks from rowin on Al surfaces Similar issues for copper (Cu) metallization Low temperature reactions: SiH 4 + O 2 SiO 2 + 2H 2 LPCVD (silane) C LTO Reactions 4PH 3 + 5O 2 2P 2 O 5 + 6H 2 (phosphine) p C Phosphosilicate h lass (PSG) Above reactions: not very conformal step coverae need hiher T for this EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 22

23 Silicon Oxide CVD (cont.) Phosphosilicate lass can be reflown 6-8 wt. % allows o C Very useful to achieve smoother toporaphy Lower concentration won t reflow Hiher concentration corrodes Al if moisture is present 5-15% P can be used as a diffusion source to dope Si Before metallization: Can use hiher temperature better uniformity and step coverae HTO SiCl 2 H 2 + 2N 2 O SiO 2 + 2N 2 + 2HCl ~900 C (dichlorosilane) (Nitrous (nice conformal oxide) step coverae) or EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 23

24 Silicon Oxide CVD (cont.) Si(OC 2 H 5 ) 4 SiO 2 + by-products C (Tetraethylorthosilicate) (excellent uniformity & (TEOS) conformal step coverae) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 24

25 Silicon Nitride CVD Silicon Nitride Deposition: First, note that thermal rowth is possible: Si in NH o C But very slow rowth rate, thus, impractical LPCVD reactions: C Silane reaction: 3SiH 4 + 4NH 3 Si 3 N H 2 Dichlorosilane reaction: (Atm. Press.) C 3SiCl 2 H 2 + 4NH 3 Si 3 N 4 + 6HCl + 6H 2 (LPCVD) Increase and T = 835 C Si rich nitride low stress Problem: Clobbers your pumps! Expensive to maintain! EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 25

26 Silicon Nitride CVD (cont.) Comments on LPCVD nitride films: Hydroen rich: ~8% H 2 Hih internal tensile stresses: films >1000Å crack and peel due to excessive stress Can et 2μm films with Si-rich nitride LPCVD ives hih resistivity (10 16 Ω-cm) and dielectric strenth (10 MV/cm) PECVD Nitride: PECVD films: Non-stoichiometric nitride or 20-25% H 2 content Can control stress 6 Ar plasma (10 Ω cm ) resistivity Nitroen dischare SiH 4 + N 2 2SiNH + 3H 2 SiH 4 + NH 3 SiNH + 3H 3 EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 26

27 Metal CVD CVD Metal Deposition: Tunston n (W) deposited d by thermal, plasma or opticallyassisted decomposition WF 6 W + 3F 2 or via reaction with H 2 : WF 6 + 3H 2 W + 6HF Other Metals Molybdenum (Mo), Tantalum (Ta), and Titanium (Ti) 2MCl 5 + 5H 2 2M + 10HCl, where M = Mo, Ta, or Ti (Even Al can be CVD ed with tri-isobutyl Al but other methods are better.) (Cu is normally electroplated) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 27

28 Epitaxy Epitaxy: Use CVD to deposit Si on the surface of a Si wafer Si wafer acts as a seed crystal Can row a sinle-crystal Si film (as opposed to poly-si) Modelin similar to CVD in fact, the model discussed so far for CVD is more relevant to epitaxy than CVD! Lo (rowth rate) et similar curve: Mass transport limited Reaction rate limited Reactions can use SiCl 4, SiH 4, SiH 2 Cl 2 for vapor phase epitaxy. 1 T SiCl 4 : Silicon tetrachloride SiH 4 : silane SiH 4 Cl 2 : dichlorosilane EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 28

29 Epitaxy (cont.) Most popular: (Note that this is reversible!) ibl!) 1200 C SiCl 4 (as) + 2H 2 (as) Si (solid) + 4HCl (as) Also et a competin reaction. Reverse reaction (i.e., etchin) if have excessive HCl sometimes used before deposition to clean the Si wafer surface. SiCl 4 (as) + Si (solid) 2SiCl 2 (as) Too much SiCl 4 etchin rather than rowth takes place! Growth rate too fast et polysilicon instead of Si. (> 2μm/min.) Important that the riht conc. of SiCl 4 is used! See Fiure EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 29

30 Epitaxial Growth Rate Dependencies Fiure 4.2 EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 30

31 Epitaxy (cont.) Alternative reaction: pyrolytic decomposition of silane: 650 C SiH 4 Si + 2H 2 not reversible, low T, no HCl formation however, requires careful control of the reaction to prevent formation of poly-si also, the presence of an oxidizin species Dopin of Epitaxial Layers: causes s silica formation on 1. Just add impurities durin rowth: Arsine, diborane, Phosphine Control resistivity by varyin partial pressure of dopant species i. Arsine, Phosphine slow down the rowth rate ii. Diborane enhances rowth rate EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 31

32 Dopin of Epitaxial Layers 2. Use autodopin when rowin own heavily-doped substrates Impurity evaporates from wafer (or liberated by Cl etchin of surface durin dep.) Incorporates into as stream Impurities dope new layer Examples of autodopin: Bipolar Processin: n+ P-Si epitoxy MOS: n+ Buried collector n n+ to reduce n- P-Si collection R n+ epitoxy Dopant radient helps to prevent latch up and punch throuh EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 32

33 Atomic Layer Deposition (ALD) EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 33

34 Atomic Layer Deposition (ALD) Precursor A Precursor B Fundamental Components: Self-limitin limitin surface reactions of suitable precursor compounds A & B A & B then form the desired product S in a binary reaction cycle consistin of two sequential half-reactions EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 34

35 Atomic Layer Deposition (ALD) Remarks: Both half-reactions must be complete and self-limitin at the monolayer level l The total film thickness d(tot) can be diitally controlled by the number of applied deposition cycles N(A/B): d(tot) = d(mono) N(A/B) The reaents A & B in the half reactions are normally chemical reactions But they don t need to be They can also represent a physical process, e.., heatin, irradiation, electrochemical conversion EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 35

36 Advantaes of ALD Surface limited reaction excellent step coverae and refillin Self-limitin mechanism Monolayer deposition Composition control Thickness control ( # of cycles) Less sensitive to flow rate & temperature Note, thouh, that there s still rowth a temperature cycle window: monolayer Incomplete Reaction Condensation ALD Window Decomposition Re-evaporation temperature EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 36

37 ALD Reactor 200 o C to 400 o C needed Must pure completely before the next pulse Usually mixed w/ an inert as to achieve lower effective vapor pressures slows reaction, but needed to allow rapid pulsin & purin EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 37

38 Al 2 O 3 ALD In air H 2 O vapor is adsorbed on most surfaces, formin a hydroxyl roup With silicon this forms :Si-O-H (s) Place the substrate in the reactor Pulse TrimethylAluminum (TMA) into the reaction chamber EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 38

39 Al 2 O 3 ALD TrimethylAluminum (TMA) reacts with the adsorbed hydroxyl roups, producin methane as the reaction product Al(CH 3 ) 3 () + :Si-O-H (s) ô :Si-O-Al(CH 3 ) 2 (s) + CH 4 EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 39

40 Al 2 O 3 ALD TrimethylAluminum (TMA) reacts with the adsorbed hydroxyl roups, until the surface is passivated TMA does not react with itself, so terminates the reaction to one layer This leads to the perfect uniformity of ALD. The excess TMA and methane reaction product is pumped away EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 40

41 Al 2 O 3 ALD After the TMA and methane reaction product is pumped away, water vapor (H 2 O) is pulsed into the reaction chamber. EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 41

42 Al 2 O 3 ALD H 2 O reacts with the danlin methyl roups on the new surface formin aluminum-oxyen (Al-O) brides and hydroxyl surface roups, waitin for a new TMA pulse Aain methane is the reaction product 2 H 2 O () + :Si-O-Al(CH 3 ) 2 (s) ô :Si-O-Al(OH) 2 (s) + 2 CH 4 EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 42

43 Al 2 O 3 ALD The reaction product methane is pumped p away Excess H 2 O vapor does not react with the hydroxyl surface roups Aain, et perfect passivation to one atomic layer EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 43

44 Al 2 O 3 ALD One TMA and one H 2 O vapor pulse form one cycle Here, three cycles are shown, with approximately 1 Å per cycle Each cycle includin pulsin and pumpin takes, e.., 3 sec Al(CH 3 3) 3 () + :Si-O-H (s) ô :Si-O-Al(CH 3 3) 2 (s) + CH 4 2 H 2 O () + :Si-O-Al(CH 3 ) 2 (s) ô :Si-O-Al(OH) 2 (s) + 2 CH 4 EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 44

45 ALD Video Run the ALD video EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 45

46 ALD Capability Excellent conformality, even at the bottom of the trench! (aspect ratio ~60:1) Al 2 O 3 EE C245: Introduction to MEMS Desin Lecture 5 C. Nuyen 9/11/08 46

EE C245 ME C218 Introduction to MEMS Design Fall 2007

EE C245 ME C218 Introduction to MEMS Design Fall 2007 Prof. Clark T.-C. Nguyen Dept. of Electrical Engineering & Computer Sciences University of California at Berkeley Berkeley, CA 94720 Lecture 4: Film