chapter 4: WET BULK MICROMACHINING

4.1 A 0.6µm film of silicon dioxide is to be etched with a buffered oxide etchant (BOE) with an etch rate of 750 ??/min. Process data show that the thickness may vary up to 10% and the etch rate may vary up to 15%.

(a) Specify a time for the etch process

(b) Predict how much undercut will occur at the top of the film

Professor Peter Hesketh, Georgia Tech

4.2 Mark the WRONG statements:

() The factors that affect the etching process are temperature, transport of reactant, reaction rate, diffusion rate, and so on.

( ) Acetic acid is more polar than water which helps in achieving proper wetting of slightly hydrophobic Si waters.

() Photon-pumped etching uses photogenerated electron-hole pairs to supply oxidation sites for the etching process.

() Boron doped regions can be used as an etch stop layer.

( ) KOH etching is used for MOS and CMOS processing.

Professor Karl B ?0hringer, University of Washington, Seattle.

4.3 In the isotropic etching diagram for silicon in Figure 4.29 (from H. Robbins and B. Schwartz, J. Electrochem. Soc., 107, 108-11, 1960 [5],

(a) If HF: HNO₃: HC₂H₃O₂ = 70:10:20, what is the etch rate?

(b) At this point, which component controls the etch rate?

(c) If we use water (10%) instead of HC₂H₃O₂ (10%) as our diluent, the etch rate will be more sensitive to which components?

(d) How can we significantly reduce the etch rate?

Professor Karl B ?0hringer, University of Washington, Seattle

4.4 What are the possible etching methods to make features as shown below:

Problem Figure 4.4

Professor Karl B ?0hringer, University of Washington, Seattle

4. 5 Complete the table (for silicon etching).

Professor Karl B ?0hringer, University of Washington, Seattle.

4.6 True or False.  Give an explanation.

__ Sputtering technique yields a poor quality of SiO₂

__ Etch selectivity of <110> over <111> for KOH is much higher than EDP

__ EDP does not etch Si₃N₄

__KOH does not etch SiO₂

__Doping Si with high levels of Boron provides a good etch stop for both EDP and KOH

__The lift-off technique is used mostly to pattern thick Si₃N₄ films.

4.7 You are asked to build a 600 ??m square, 25 ??m thick diaphragm in an oxidized (100) silicon wafer. Given are that the wafer thickness is 300 ??m and that you will be using an anisotropic etching solution of KOH/water which attacks the (111) crystal plane 100 times slower than the (100) crystal direction. The etch rate of (100) silicon in this specific KOH/water solution is 10 ??m/hr.

(a) Find the etching time

(b) Find the dimensions of the mask opening you would use

Professor Peter Hesketh, Georgia Tech

4.8 You are asked to make grooves 50 ??m deep in an oxidized (100) silicon wafer. Use first a BOE etch to make an opening in the oxide layer and then an anisotropic etchant like KOH/water. What are the dimensions of the mask opening you would use to make this dimension insensitive to the amount of overetch?

Professor Peter Hesketh, Georgia Tech

4.9 Write one paragraph each on the various etch stop techniques including electrochemical etch stop, P-N junction etch stop, and dopant dependent etch stop, etc.

4.10 Explain in detail the various corner compensation schemes to reduce undercutting while etching anisotropically.

4.11 Define each of the following terms:

??        Stress

??        Strain

??        Elastic modulus

??        Poisson’s ratio

??        Yield strength

??        Ultimate tensile strength

4.12 What is the Miller Index for the plane shown below?

Problem Figure 4.12

4.13 What is the number of nearest neighbors for the following crystal lattices:

(a) simple cubic

(b) face-centered cubic

(c) body-centered cubic

4.14 Describe briefly the crystal structure of silicon: include such details as structure type (how does it differ from FCC), lattice parameters, packing density, and highest packing density plane.

4.15 Which are the three orientations of silicon wafers most commonly employed in micromachining? Arrange them in order of most common usage and briefly describe their benefits and drawbacks.

4.16 Derive the relation that connects the width of an etched cavity with the mask opening used in the case of anisotropic etching of [100] silicon.

4.17 In a typical 4" wafer, of thickness 525 mm, it is proposed to create 2 distinct features. What should be the mask opening to create

(a) an orifice of width 200 mm through the wafer

(b) a V-groove through the wafer.

4.18 Compare the etching characteristics of (110) and (100) oriented silicon wafers. Which would you choose for the following applications and why?

(a) Diaphragm-based pressure sensors

(b) High-aspect-ratio comb actuators

4.19 Write a paragraph on the mechanical properties of silicon, with special emphasis on strength, plastic/elastic behavior, hardness, etc.

4.20 Why are p-type piezoresistors most commonly used?

4.21 Show the difference between convex and concave corners and the effect they produce on undercutting comparing the shape of the mask to the shape of the resulting pit? What will be the effect of an etch stop layer on undercutting?