CHARACTERIZATION OF DEEP REACTIVE ION ETCHING (DRIE ... · CHARACTERIZATION OF DEEP REACTIVE ION...

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CHARACTERIZATION OF DEEP REACTIVE ION ETCHING (DRIE) PROCESS FOR ELECTRICAL THROUGH-WAFER INTERCONNECTS FOR PIEZORESISTIVE INERTIAL SENSORS Maria Suggs, Physics Major, Southern Polytechnic State University P.I.: Prof. Beth Pruitt, Mentors: Alvin Barlian & Nahid Harjee Dept. of Mechanical Engineering, Stanford University Summer 2007

Transcript of CHARACTERIZATION OF DEEP REACTIVE ION ETCHING (DRIE ... · CHARACTERIZATION OF DEEP REACTIVE ION...

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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

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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

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PROCEDURE:

Optimize the existing recipe on STS-HRM (Surface Technology Systems)

Based on the results develop a reliable method to etch through wafer

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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”

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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”

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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”

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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”

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STS-HRM: Surface Technology Systems Bosch Method

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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

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REPRESENTATIVE SEM IMAGES

Optimized RecipeSF6 was maximizedNo grassStraight wallNegligible scallops

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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

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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)

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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

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THROUGH WAFER RESULTS

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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.

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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

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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