Bulk micromachining

1. Bulk micromachining • Explain the differences between isotropic and anisotropic etching • Explain the differences between wetand dry etching techniques • Identify several common wet etchants and explain what they are commonly used for • Explain the difference between rate limited and diffusion limited reactions • Explain in general terms the different theories behind the differences in etch rate for different crystal directions in the anisotropic etching of silicon • Discern the resulting shapes of trenches (pits) resulting from the anisotropic etching of Si for different mask and wafer combinations • List and explain the most common etch stop techniques • List and describe the most common dry etching techniques • Perform basic calculations for wet etching processes

2. Bulk micromachining Silicon etched SiO2 Isotropic etch Silicon wafer Silicon etched Anisotropicetch Silicon wafer

3. Etching Etching: • Chemical reaction resulting in the removal of material etchants in liquid form • Wet etching: • Dry etching: etchants contained is gas or plasma ionized gas material removed per time (μm/min) Etch rate:

4. Selectivity and undercutting Selectivity: • etch rate of one material compared to another • etch rate of one crystalline direction compared to another SiO2 [100] [111] 54.7° Undercutting (100) Si SEM image of a SiO2 cantilever formed by undercutting (S. MohanaSundaram and A. Ghosh, Department of Physics, Indian Institute of Science, Bangalore)

5. Application and properties of different wet etchants High HF tends to etch SiO2 • Acidicetchantstend to etch Si isotropically • Basic etchantstend to etch Si anisotropically • Dependonconcentration and temperature

6. Rate versus diffusion limited etching Products Etchant Rate limited reaction Rate limited reactions are preferred  easier to control and more repeatable Products Etchant Diffusion limited reaction

7. Isotropic etching d • Estimate of etch depth • depth ≈ (D-d)/2 D undercutting • Etch rate is the same in all directions • Typically acidic • Room temperature • Isotropy is due to the fast chemical reactions • X μm/min to XX μm/min  Reaction or diffusion limited?

8. Isotropic etching • HNA: HF/HNO3/HC2H3O2 • Used in isotropic etching of silicon • Also called poly etch HNO3 (aq) + Si(s) + 6HF (aq)  H2SiF6 (aq) + HNO2 (aq) + H2O (l) + H2 (g) The etching process actually occurs in several steps. First step, nitric acid oxidizes the silicon HNO3 (aq) + H2O (l) + Si (s) SiO2 (s) + HNO2 (aq)+ H2 (g) In the second step, the newly formed silicon dioxide is etched by the hydrofluoric acid. SiO2 (s) + 6HF (aq) H2SiF6 (aq) + 2 H2O (l)

9. Isotropic etching • BOE (Buffered Oxide Etch): HF/NH4F/H2O • Used in isotropic etching of silicon dioxide and glass • Basically proceeds from the second step of etching Si: SiO2 (s) + 6HF (aq) H2SiF6 (aq) + 2 H2O (l)

10. Anisotropic etching d [111] 54.7° [100] undercutting • Etch rate is different for different crystal plane directions • Typically basic etchants • Elevated temperatures (70-120°C) • Different theories propose for anisotropy • Slower etch rates, ~ 1 μm/min • Etch depths depend on geometry • Undercutting also depends on geometry  Reaction or diffusion limited?

11. Properties of different anisotropic etchants of Si

12. Theories for anisotropic etching (111) • Siedelet al. 2 dangling bonds 1 dangling bond (100) Silicon lattice The lower reaction rate for the {111} planes is caused by the larger activation energy required to break bonds behind the etch plane. This is due to the larger bond density of silicon atoms behind the {111} plane.

13. Theories for anisotropic etching • Siedelet al. (Continued) • Reduction of water believed to be the rate determining step • OH- believed to be provided by H2O near Si surface Si + 2OH- SiOH2++ + 4 e-(oxidation step) SiOH2++ + 4 e- + 4 H2O  Si(OH)6-- +2 H2(reduction step) • Elwenspoeket al. • Suggests surface roughness is reason • {111} plane is atomically flat, no nucleation sites

14. Self-limiting etch and undercutting Concave corner [111] [111] D Convex corner exposes other planes Resulting undercutting can be used to create suspended structures D • Intersection of {111} planes can cause self-limiting etch. • Only works with concave corners

15. Anisotropic etching of (110) silicon Mask with large aspect ratio {111} {111} Mask with small aspect ratio {110} {111} {111} • Vertical sidewalls and 90° angles! • Long narrow mask openings can be used to create long narrow channels with vertical sidewalls Top view {110} planes etch about twice as fast as {100} planes in KOH

16. Anisotropic etching of (111) silicon • usually used as base (Big green Lego®) • for surface micromachining • How fast does the (111) plane etch? Sin embargo, todavíaesposbileusar lo en “bulk micromachining” pre-etched pit protected sidewalls

17. Tetoca a ti • Sketch the cross-sections resulting from anisotropically etching the silicon wafers shown with the given masks.

18. Etch stop • Etch stop: Technique to actively stop the etching process Insulator etch stop Self-limiting etch insulting layer Timed etch Etch stop via doping p-n junction

19. Etch stop via doping • Boron etch stop Si + 2OH- SiOH2++ + 4 e-(oxidation step) SiOH2++ + 4 e- + 4 H2O  Si(OH)6-- +2 H2(reduction step) n type wafer heavily doped with B (called a p+ wafer) n region p region p-n junction High level of p-type doping is not compatible with CMOS standards for integrated circuit fabrication p region Si deficient in e-

20. Etch stop via doping • Electrochemical etch stop (ECE) Si + 2OH- SiOH2++ + 4 e-(oxidation step) SiOH2++ + 4 e- + 4 H2O  Si(OH)6-- +2 H2(reduction step) e- e- p type wafer doped n-type dopant “Reverse bias” voltage applied to p-n junction keeps current from flowing - V + p region SiO2 diode n region p-n junction Very light doping compared to boron etch stop. OK with CMOS standards for integrated circuit fabrication.

21. Dry etching Etching: • Chemical reaction resulting in the removal of material electrodes • Wet etching: etchants in liquid form - - - - - - - - - excited ions • Dry etching: etchants contained is gas or plasma Accelerated to target via the electric field + + + + + + + + wafer Plasma etching: mostly chemical etching • Chemically reactive gas formed by collision of • molecules of reactive gas with • energetic electrons • Excited/ignited be RF (radio frequency) electric field ~ 10-15 MHz Reactive ion etching (RIE): In addition to the chemical etching, accelerated ions also physically etch the surface

22. Reactive ion etching • Plasma hits surface with large energy • In addition to the chemical reaction, there is physical etching (Parecetirarpiedras en la arena) • Can be very directional—can create tall, skinny channels • If there is no chemical reaction at all, the technique is called ion milling. (Intellisense Corporation)

23. Common dry etchant/material combinations

24. Deep reactive ion etching (DRIE) • Bosch process • 1st, reactive ion etching step takes place • 2nd, fluorocarbon polymer deposited to protect sidewalls “Scalloping” Kane Miller, MingxiaoLi, Kevin M Walsh and Xiao-An Fu, The effects of DRIE operational parameters on vertically aligned micropillar arrays, Journal of Micromechanics and Microengineering, 23 (3)

25. Tetoca a ti • Wet etching problems • A pattern is etched into a <100> Si wafer as described below. Answer the questions that follow. • A 300 nm thick layer of oxide is grown on the surface of the Si wafer. Photoresist is applied to the oxide surface, and patterned using standard photolithographic techniques. The pattern is etched into the oxide. The exposed Si is etched anisotropically to achieve the desired feature. • Should the photoresist be removed before the Si etching step? Justify your answer. • What etchant will you use for the oxide? • What etchant will you use for the Si? • You are asked to make a V-shaped grooves 60 μm deep in an oxidized <100> silicon wafer • How wide must the opening in the oxide mask be in order to achieve this result? • Will the degree of undercutting, due to etching into the <111> plane, be appreciable compared to the dimensions of the desired feature? Justify your answer.