CHARACTERIZATION OF DEEP REACTIVE ION ETCHING (DRIE) PROCESS FOR ELECTRICAL THROUGH-WAFER INTERCONNECTS FOR PIEZORESISTIVE INERTIAL SENSORS
Maria Suggs, Physics Major, Southern Polytechnic State UniversityP.I.: Prof. Beth Pruitt, Mentors: Alvin Barlian & Nahid Harjee
Dept. of Mechanical Engineering, Stanford UniversitySummer 2007
MOTIVATION:
PROBLEM: Currently, metal interconnects of piezoresistive inertial sensors are exposed to harsh environments (liquid)SOLUTION: Place metal on the unexposed side of the device’s chip. This method requires etching through the wafer.
from Chow, 2001
Electrical ThroughWafer Interconnects
PROCEDURE:
Optimize the existing recipe on STS-HRM (Surface Technology Systems)
Based on the results develop a reliable method to etch through wafer
ETCH PROCESS AND GOALSBosch Method
Etch SF6 (chemical and physical process) and Dep C4F8 (to protect the side walls)
Anisotropic etching, straight walls (90°< with photoresist)
No grass
Small “scallops”
ETCH PROCESS AND GOALSBosch Method
Etch SF6 (chemical and physical process) and Dep C4F8 (to protect the side walls)
Anisotropic etching, straight walls (90°< with photoresist)
No grass
Small “scallops”
ETCH PROCESS AND GOALSBosch Method
Etch SF6 (chemical and physical process) and Dep C4F8 (to protect the side walls)
Anisotropic etching, straight walls (90°< with photoresist)
No grass
Small “scallops”
ETCH PROCESS AND GOALSBosch Method
Etch SF6 (chemical and physical process) and Dep C4F8 (to protect the side walls)
Anisotropic etching, straight walls (90°< with photoresist)
No grass
Small “scallops”
STS-HRM: Surface Technology Systems Bosch Method
SI WAFER PROCESS
SUPPLY ROOM 4” Silicon WaferThickness: ~500 microns
SVGCOAT Spin 3 microns of positive photoresist (SPR220-3)
KARLSUSS Expose (and pattern) using a mask with 15% etch area
SVGDEVELOP Develop the resist (and hard bake for 30 minutes in 110C oven)
STS-HRM: Surface Technology Systems
Etch the wafer using a recipe based on the Smooth Shallow Template
SEM: Scanning electron microscope
Examine the samples and take manual measurements
silicon photoresist
REPRESENTATIVE SEM IMAGES
Optimized RecipeSF6 was maximizedNo grassStraight wallNegligible scallops
SI WAFER RECIPE RESULTS
1112.514 35
2
2.5
3
3.5
4
4.5
5
5.5
6
Etch Rate (micron/min)
Gate Position (%) [inv. press]
etch cycle
time (s)
Etch Response
5.5-65-5.54.5-54-4.53.5-43-3.52.5-32-2.5
3545
553
5
1.52
2.5
3
3.5
4
4.5
5
Etch Rate (micron/min)
Platen Power (W)
Etch Cycle Time (sec)
Etch Response
4.5-54-4.53.5-43-3.52.5-32-2.51.5-2
THROUGH WAFER ETCHINGSTS-HRM Challenge: Thermal Conductivity
He release and photoresist (PR mask)
Coolant at 0°C – 20°C
PR mask
Oxide 0.5umBacking wafer
2um
RF Coil P (2500 W)
Wafer
He 10 T
Chuck (Al)
40 mTPlaten P (50W)
10um
Polymer Bonding
Bonding resist
(1500 W)
POLYMER BONDING PROCESSPlace wafers on 90 C hot plate for 7 minTest the bond with the vacuum for 5 minSTS-HRM: “Smooth Shallow Template” Coil P (1500 W)Separate wafers by soaking in Acetone for 1 hour
THROUGH WAFER RESULTS
CONCLUSION AND FUTURE WORKCONCLUSION
Based on the optimization experiment in STS-HRM, reducing the thermal load by decreasing the source power was the key to bonded wafer through etching A wafer-to-wafer polymer bonding technique and a release method were developed for successful through wafer etching in the high rate STS-HRM machine.
FUTURE WORKExplore other methods for through wafer (Aluminum as an etch stop)Work on setting up the interconnects through the etched wafer and test the device in harsh environments.
WHAT I LEARNED
Trained in various equipment in the clean room such as SVGcoat, Karlsuss, STS-HRM, SEM, wetbenches, YesOven, SVGDev, Nanospec, etc
Fabrication process in MEMS
The experience of graduate work with students and professors
How to conduct my own research
ACKNOWLEDGEMENTSI would like to thank the following people & institutions:
Mike Deal and Maureen BaranProf. Beth Pruitt, Alvin Barlian, Nahid Harjee, Ed Myers, Laura RobeckREU staff at SNF and NNINNational Science Foundation (NSF)Stanford Center for Integrated Systems (CIS)Lab members and staffSpecial thank you to Eric PerozzielloDr. Patrick & Dr. Pace
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