Lecture 12 – Anisotropic Etching

1. Lecture 12 – Anisotropic Etching • Anisotropic Etchants & Characteristics • Temperature Dependence & Anisotropy Ratio • Surface Roughness & Etch Uniformity • Masking and Dopant dependence • Vertical Sidewalls • Corner Compensation Techniques • Crystallographic Alignment Patterns

2. Anisotropic Etching Characteristics • Anisotropic etching etches different planes of silicon at different rates • Patterns are bound by the slowest etching plane • Provided more control than that of isotropic wet etching • Pattern transfer with accuracy and reproducibility of 0.5 µm can be achieved • These types of etchants are generally slower than isotropic etchants • class wafers etched around 0.35 µm/min at 63oC • Need elevated temperatures (60-115oC) • Not as sensitive to agitation (although we did some stirring in lab)

3. Reactant diffuses to the surface Surface reaction Reaction product diffuses away from the surface 1 2 3 Anisotropic Etchants • A wide variety of isotropic etchants have been used • Alkaline aqueous based etchants include KOH, NaOH, LiOH, CsOH, RbOH, NH4OH, and TMAH • Alkaline organics include ethylenediamine (EDP), choline, hydrazine, and sodium silicates • No external voltages are needed for etching • Etching is dopant insensitive over several orders of magnitude • Alcohols (propanol and isopropanol butanol) slow the etching process • The etch rate is reaction rate controlled 1 3 2

4. Arrhenius Plot for Anisotropic Etching • Etch rate of <100>, <110>, <111> planes of silicon is shown for EDP: • Large temperature dependence • Slope is different for different planes • For EDP, the anisotropy ratio, AR, for (110)/(100)/(111) is smaller than KOH 400/200/1 • Adding organics (IPA) can change the etch rates (hkl)1 etch rate AR = anisotropy ratio =_____________ (hkl)2 etch rate Etch rates for EDP as a function of 1/T

5. Choosing Anisotropic Etchants Of the main three (KOH, EDP, and TMAH), consider: • Ease of handling • Toxicitiy • Etch rate • Desired topology of the etched bottom surface • Etch stop • Etch selectivity over other materials • Mask material and thickness of mask

6. Common Anisotropic Etchants KOH (most widely used) • Advantages: Fastest, greatest selectivity, only one that makes vertical sidewalls • Disadvantages: Not CMOS compatible (etches many metals and oxides), alkali ion contamination EDP • Advantages: lots of masks, CMOS compatible, lowest Boron doping etch stop • Disadvantages: toxic corrosive, optically dense (hard to see sample), ages quickly, low AR, and precipitates form during cooling TMAH • Advantages: CMOS compatible, smooth surface • Disadvantages: slow etch rate, poor selectivity (low AR)

7. Si Surface Roughness • Anisotropic etchants can leave surface roughness behind • Isotropic etchants can be used to touch up the surfaces after anisotropic etching • Macroscopic roughness (notching or pillowing) generally increases with depth but microscopic roughness does not • Average roughness is influenced strongly by fluid agitation, • stirring can reduce roughness magnitudes • ultrasonic agitation can reduce or eliminate it • Average surface roughness (macro and micro) decrease with increasing KOH concentration Macroscopic RoughnessMicroscopic Roughness (variations across 10’s of microns) (variations within a micron) “hillocks” notching

8. KOH Etch Rate{100}and Uniformity vs Concentration What concentration is etch rate maximal? Where is etch uniformity optimal? Madou, pg 216

9. KOH Surface Roughness{100}vs Concentration Smoother surfaces are produced by higher or lower concentrations of KOH? Madou pg 216

10. Etch Rate and Dopant Concentration • High boron concentration drastically reduces the etch rates of silicon • Other impurities also reduce the etch rate but at much higher concentrations • Ion implanted dopants can be used as etch stops • EDP is even more sensitive than KOH to dopant concentrations

11. Vertical Sidewall Anisotropic Etching • Openings along the <100> direction in {100} silicon with anisotropic etching, etches down at the same rate (equidistant) as it etches under the mask: <110> <100>

12. Vertical Sidewall – Example 2 <110> <100>

13. Vertical Sidewall – Example 2 cont

14. How to Make a Proof Mass • “Interior corners” are not stable when etching with KOH • Pyramidal structures are necessary for proof masses in cantilever pressure sensors and elsewhere. • A decent numerical simulation can be made by assuming only three planes are exposed by KOH etching: {111}, {100}, and {411}

15. Corner Compensation

16. {111} & {411} Intersection

17. Corner Compensation

18. Corner Compensation

19. Actual Corner Compensation

20. Double Square Corner Compensation

21. Actual Double Square Corner Compensation

22. <110> Crystallographic Alignment Patterns • “Wagon-wheel” etch mask when anisotropically etched produces a “flower” pattern in {100} Si that • Delineates the {110} directions to better than 0.1° and • Visually depicts the etch ratios between the various crystal planes.

23. Quotes • Never test the depth of the water with both feet. • Silence is often misinterpreted, but never misquoted. • Laugh every day, it's like inner jogging. • There is no key to happiness. The door is always open. • A grudge is a heavy thing to carry.