1. Fabrication of Active Matrix (STEM) Detectors
Wei Chen
Silicon Detector Group
Instrumentation Division

2. Contents Our capabilities The concept of a diode detector
Planar-technology for silicon detector processing
New processes developed for Active Matrix detectors
Current status

3. Our Capabilities
Class 100 cleanroom equipped with oxidation furnace, spin-coating track, mask-aligner, sputtering system.
Simulation tools, mask design tools and testing equipment.
Strip, Pad, Pixel, Drift, Stripixel and Active matrix detectors
We are the only one in the US

4. Basic Concept of Diode Detector
P-type: 0.1mm
N-type: 300mm
W=(2esV/qNd)1/2
Nd=1/qmr
r =W2/(2esmV)
V~100V, W~300mm,
r ~3kWcm

5. Planar Technology for Silicon Detector Process

6. Oxidation Process Silicon Dioxide (SiO2) provides
High quality insulating barrier
Impurity-diffusion barrier
Passivation
Gettering of impurities in Si
Dry oxygen grown oxide (32hr)
Ambient: O % TCA
About 0.5µm thickness
(wafer about 400µm)

7. Photolithography Process
Produces optical images in a light sensitive film (Photoresist)
Images are a reproduction of photomask
It is an integration of steps which strongly influence one another:
Photoresist and application
Exposure
Development
Photoresist: Positive, S1811 (Shipley)
0.5mm~2.5mm
Softbake: Hotplate contact or proximity
Exposure: Proximity and Contact print, ultraviolet light
Developer: MF-312 (Shipley)
Smallest feature: 5µm

8. Etching Process (New challenge)
Wet etching
Chemical solution
Isotropic
Advantages
Low cost
Reliable
High throughput
Disadvantages
Photoresist adhesion
Non-uniformities

9. Ion Implantation Introduces dopants as ions at controlled energies
Boron: 35keV, 1x1014/cm2 (front side)
30keV, 1.2x1014/cm2 (backside)
Phos.: 50keV, 4x1014/cm2 (front side)

10. Deep Implantation Mask (New)
AZ4600 Shipley Positive photoresist
>5µm thickness
Blocks 520keV Boron or Phosphorus ions implantation
Boron: 2.6x1011/cm2
Phos.: 6.5x1011/cm2

11. Post Implant Anneal High temperature process repairs damage
(700ºC, 30min)
Electrically activates dopants
Ambient: N2

12. Metalization Process Sputtering
Provides contacts and interconnections
Requirements
Low resistance “ohmic” contacts
Low sheet resistance
Reliable interconnections

13. Polyimide Insulation Layer Between Two Metals (New)
Spin on: 1000RPM, 10sec; 4000RPM, 30sec
Combined with photoresist lithography
Thickness of Polyimide: ~2µm
Size of opening: 10µm

14. Double Aluminum (New) Advantages of Al Disadvantages Aluminum Oxide
Inexpensive
Ease of forming contacts
Excellent adherence to Si and SiO2
Low bulk resistivity (2.7 -cm)
Excellent bondability
Easy to process
Disadvantages
Spiking
Unable to sustain high temperatures (over 450 °C)
Aluminum Oxide
Need Reverse Sputtering

15. Plasma Clean (New) Ambient: Ar gas Reduces the surface contamination
After polyimide process
After second aluminum patterning

16. Current Status Total Mask steps: 14 Total processing steps: 193
Open oxide for p-n-implants
Open oxide on backside for p-implant
Photoresist mask for protecting n-region from p-implant
Oxide cut for opening up anodes
Al mask for covering p-region
Deep p-implant
Deep n-implant
Post implant anneal
Oxide step cut
Oxide step cut on backside
First metalization
First metalization on backside
Polyimide
Second metalization
Total Mask steps: 14
Total processing steps: 193
Now: at step #177 (Coating the second Al)
Remaining: Lithography for the second Al
Clean surface with plasma

17. Clean wafers
Deposit barrier layer
SiO2, metal, etc.
Coat with photoresist
Soft bake
Align masks
Exposure pattern
Develop photoresist
Hard bake
Etch windows in barrier layer
Remove photoresist
Ion-implantation or diffusion
or metal deposition
more mask steps
next processing step Y N