Bulk micromachining

Question 1

Silicon mechanical material properties

Answer 1

• Hard, brittle
• Tensile yield strength 7 Gpa
• Youngs modulus ~ near stainless steel
• Mechanical properties uniform, no intrinsic stress
• melting point 1415C
• Good thermal conductor 156 W/mK
• Low thermal expansion coefficient
• High pizeresistivity

Question 2

How is bulk etching of silicon reaction limited?

Answer 2

etch rate depends on temperature

Question 3

How is bulk etching of silicon diffusion limited?

Answer 3

Etch rate is dependent on mixing
- also dependent on layout geometry and “loading”

Question 4

What factors influence the choice of etch method?

Answer 4

• desired shapes
• etch depth and uniformity
• surface roughness
• process compatibility
• safety, cost, availability
• environmental impact

Question 5

What chemicals are isotropic wet etchant HNA made from?

Answer 5

• hydrofluoric acid
• nitric acid
• acetic acid

Question 6

What is HNA etchant highly dependent on?

Answer 6

• mixing

Question 7

What are sources of etch rate variation due to wet etch set up?

Answer 7

• loss of reactive species through consumption
• evaporation of liquids
• poor mixing
• contamination
• applied potential
• illumination
• etch rate variation due to material being etched
• impurities / dopants
• etch rate variation due to layout
• distribution of area, loading

Question 8

What is the ratio of entrance: depth for anisotropic wet etching of silicon?

Answer 8

0.707a depth : a entrance

Question 9

what does micromasking from H2 bubbles do and how to fix it?

Answer 9

• leads to roughness
• stirring to displace bubbles,
• oxidizer and surfactant additives prevent bubbles

Question 10

What does undercutting do to shapes?

Answer 10

• Exposes the 111 plane which etches slower, so that everything turns into a rectangle

Question 11

What is corner compensation

Answer 11

A structure added to the corner of an object to delay attack of convex corners during etching. This aims to produce upward pyramidal shapes, but must have perfect etch timing to work

Question 12

Boron etch stop can be used to

Answer 12

Precisely control etch depths by doping with boron (p++)
- at a high dopant level, injected e- recombine with holes in valence band and are unavailable to give OH-
[B] >10^20 com^-3, reduces KOH etch rate by 20x

Question 13

Results of boron etch stop

Answer 13

• beams, suspended films
• 1-20um layers possible
• P++ is not compatible with CMOS, buried p++ is compatible

Question 14

What is an electrochemical etch stop?

Answer 14

• an n-type epitaxial layer grown on a p=type wafer forms a p-n diode
• Can use lower doping than a p++ etch stop, which allows:
• > easier integration of electronic
• > lower intrinsic stress
• > better piezoresistors

Question 15

How is an electrochemical etch stop performed?

Answer 15

• P-n diode is reverse biased
• p-substrate below passivation potential is etched
• n-layer above passivation potential is not etched
• monitoring the current can tell you when you have reached the n-layer

Question 16

problem does boron etch stop solve in piezoresistive pressure sensors?

Answer 16

Etch rate variation means that each batch may have different etch depth. By using boron etch stop, the depth of etch can be standardized