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

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

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

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

Planar Technology for Silicon Detector Process

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: O2 + 0.5% TCA About 0.5µm thickness (wafer about 400µm)

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

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

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)

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

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

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

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

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

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

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

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