MEMS LAB SESSION 2
Undercut Silicon Nitride using KOH Etching
OVERVIEW OF LAB SESSION,
This lab session utilizes potassium hydroxide (KOH) wet etching to undercut and release the
silicon nitride cantilevers and fixed-fixed beams,In Step 2.1,KOH etching is used to
anisotropically etch the silicon,and undercut the silicon nitride,The wafers undergo a series of
solvent cleans and then are allowed to dry in the air in Step 2.2,
LAB OBJECTIVES,
This lab session has the following primary objectives,
? Introduction to wet,anisotropic KOH etching,
? Complete and release silicon nitride beams,
? Complete electron-beam evaporation of chromium and gold,
? Initial instruction on the following major pieces of lab equipment,
KOH Wet Station
Acidhood 2 Wet Station
Electron-Beam Evaporative Deposition System
Before the beginning of this lab,make sure to read the corresponding Standard-Operating-
Procedures (SOP) for these major pieces of equipment,The SOP for each of the equipments can
be accessed at the following site,
http://www-mtl.mit.edu/mtlhome/3Mfab/sop.html
LAB PROCEDURES,
MEMS Lab Session 2 has 2 major steps,
2.1 RELEASE CANTILEVER
2.1.1 Tool,KOH Wet Station
2.1.2 The silicon nitride beams are undercut using a solution of potassium hydroxide
(KOH),This strong-base solution preferentially etches the <100> and <110> planes,
relative to the <111>,in single-crystal silicon,thereby creating an anisotropic etch,
Pour 3 liters of DI water into the quartzware,Use the scale to measure out a
sufficient amount of KOH pellets to create a 20% by weight solution,Set the water
bath temperature to 85
o
C,which will give a KOH bath temperature of about 80
o
C,
The etch rate is exponentially dependent on temperature,and therefore maintaining
good temperature uniformity in the bath is important,
2.2 EVAPORATED SOLVENT DRY
2.2.1 Tool,KOH Wet Station
2.2.2 Due to the small scale of microfabricated devices,surface forces become
considerable compared to body forces,To try to avoid any stiction of the released
Page 1 of 3
cantilevers to the sloped <111> sidewall,the wet wafers will be gently sprayed with
isopropyl acohol,The alcohol will then be allowed to dry in the air,
PRE-LAB QUESTIONS,
1,What chemical is used to etch the silicon? Is this an isotropic or anisotropic etch?
2,Given that the density of water is 1 gram/ml,how much KOH should be mixed
with 3 liters of de-ionized water to create a 20% by weight solution?
3,Referring to Figure 2.1,what is the KOH etch rate in the <100> direction for the
given process parameters?
4,Using the mask shown below,(Figure 2.2),sketch how the cross-sections A-A and
B-B would look after Step 2.1,
Reference,H,Seidel et al.,“Anisotropic Etching of Crystalline Silicon in Alkaline Solutions”,J,
Electrochem,Soc.,137(11):3612-32,1990,
Figure 2.1
Page 2 of 3
AA
B
B
Figure 2.2
Page 3 of 3
Undercut Silicon Nitride using KOH Etching
OVERVIEW OF LAB SESSION,
This lab session utilizes potassium hydroxide (KOH) wet etching to undercut and release the
silicon nitride cantilevers and fixed-fixed beams,In Step 2.1,KOH etching is used to
anisotropically etch the silicon,and undercut the silicon nitride,The wafers undergo a series of
solvent cleans and then are allowed to dry in the air in Step 2.2,
LAB OBJECTIVES,
This lab session has the following primary objectives,
? Introduction to wet,anisotropic KOH etching,
? Complete and release silicon nitride beams,
? Complete electron-beam evaporation of chromium and gold,
? Initial instruction on the following major pieces of lab equipment,
KOH Wet Station
Acidhood 2 Wet Station
Electron-Beam Evaporative Deposition System
Before the beginning of this lab,make sure to read the corresponding Standard-Operating-
Procedures (SOP) for these major pieces of equipment,The SOP for each of the equipments can
be accessed at the following site,
http://www-mtl.mit.edu/mtlhome/3Mfab/sop.html
LAB PROCEDURES,
MEMS Lab Session 2 has 2 major steps,
2.1 RELEASE CANTILEVER
2.1.1 Tool,KOH Wet Station
2.1.2 The silicon nitride beams are undercut using a solution of potassium hydroxide
(KOH),This strong-base solution preferentially etches the <100> and <110> planes,
relative to the <111>,in single-crystal silicon,thereby creating an anisotropic etch,
Pour 3 liters of DI water into the quartzware,Use the scale to measure out a
sufficient amount of KOH pellets to create a 20% by weight solution,Set the water
bath temperature to 85
o
C,which will give a KOH bath temperature of about 80
o
C,
The etch rate is exponentially dependent on temperature,and therefore maintaining
good temperature uniformity in the bath is important,
2.2 EVAPORATED SOLVENT DRY
2.2.1 Tool,KOH Wet Station
2.2.2 Due to the small scale of microfabricated devices,surface forces become
considerable compared to body forces,To try to avoid any stiction of the released
Page 1 of 3
cantilevers to the sloped <111> sidewall,the wet wafers will be gently sprayed with
isopropyl acohol,The alcohol will then be allowed to dry in the air,
PRE-LAB QUESTIONS,
1,What chemical is used to etch the silicon? Is this an isotropic or anisotropic etch?
2,Given that the density of water is 1 gram/ml,how much KOH should be mixed
with 3 liters of de-ionized water to create a 20% by weight solution?
3,Referring to Figure 2.1,what is the KOH etch rate in the <100> direction for the
given process parameters?
4,Using the mask shown below,(Figure 2.2),sketch how the cross-sections A-A and
B-B would look after Step 2.1,
Reference,H,Seidel et al.,“Anisotropic Etching of Crystalline Silicon in Alkaline Solutions”,J,
Electrochem,Soc.,137(11):3612-32,1990,
Figure 2.1
Page 2 of 3
AA
B
B
Figure 2.2
Page 3 of 3
