Process Flow : Overhead and Cross Section Views (Diagrams courtesy of Mr. Bryant Colwill)

Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

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Step 1 Step 2 Step 3 Step 4 Step 5 Step 6

Step 7 Step 8 Step 9 Step 10 Step 11 Step 12

Step 1: Cleaning

Color Code: Grey=Si, Blue=Silicon Dioxide, Red=Photoresist

Starting Si Wafer&

Sulfuric Acid / Peroxide Clean

Cross-section View of Wafer

First, we started with a positively charged, P-type, silicon wafer. We cleaned it with a mixture of sulfuric acid and hydrogen dioxide to remove surface impurities and reduce errors.

Overhead View of Wafer

Si Wafer

Step 2: Deposition of

Color Code: Grey=Si, Blue=Silicon Dioxide, Red=Photoresist

Plasma Enhanced Chemical Vapor Deposition

Of Silicon Dioxide

Overhead View of Wafer

Cross-section View of Wafer

The PECVD (Plasma Enhanced Vapor Deposition) Machine was used to heat the wafer to 400 degrees and react ionized gas with the wafer’s surface to add a film of silicon dioxide. PECVD Machine

Color Code: Grey=Si, Blue=Silicon Dioxide, Red=Photoresist

Lithography Patterning

Overhead View of Wafer

Cross-section View of Wafer

Step 3: Lithography Patterning

Lithography comes from the Latin roots “lithos” and “graphy” meaning “writing on stone.” We used photolithography which involved using photosensitive chemicals and UV light to pattern the wafer.

Photolithography Machine

Color Code: Grey=Si, Blue=Silicon Dioxide, Red=Photoresist

Lithography Patterning

Overhead View of Wafer

Cross-section View of Wafer

Step 3: Lithography Patterning (cont.)

The Spin Coater

First, we applied HMDS to help adhere the photoresist, a photosensitive chemical. We then used spin coating at 4000 RPM to cover the surface of the wafer with a thin layer of photoresist. Next, we baked the covered wafer on a hot plate for a minute to harden the photoresist.

Color Code: Grey=Si, Blue=Silicon Dioxide, Red=Photoresist

Lithography Patterning

Overhead View of Wafer

Cross-section View of Wafer

Step 3: Lithography Patterning (cont.)

UV Contact Aligner Machine

The Rinsing Station

We covered the wafer with a mask that left some area exposed and protected others. Then, the wafer was exposed to UV light, which broke the cross-linked polymer chains in the uncovered photoresist.Next, we rinsed off the weakened photoresist with a weak base and then a water bath with bubbling nitrogen. This left the surface of the wafer patterned.

Step 4: Acid Wash

Color Code: Grey=Si, Blue=Silicon Dioxide, Red=Photoresist

Acid Washing

Overhead View of Wafer

Cross-section View of Wafer

We used hydrogen fluoric acid to remove the exposed silicon dioxide layer. This revealed the base silicon of the wafer in regions not protected by cross-linked photoresist.

Acid Etching and Rinse Station

Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Photoresist Removed

Overhead View of Wafer

Cross-section View of Wafer

Step 5: Removal of Photoresist

The wafer was cleaned of the remaining photoresist and impurities with hydroxide and sulfuric acid. Next, it was washed in water and placed in a rack to be spun dry in nitrogen gas.

Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Overhead View of Wafer

Cross-section View of Wafer

Phosphorous Doping

Step 6: Phosphorous Doping

The tube furnace was used to heat the wafer to 1100 degrees Celsius and expose it to , which grew a layer of phosphorous-doped glass on the regions not covered by silicon dioxide. This was in order to create the N-type well of the P-N junction. The Tube Furnace

Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Diffusion of Phosphorous

Overhead View of Wafer

Cross-section View of Wafer

Step 7: Phosphorous Diffusion

The phosphorous glass was removed, leaving a layer of negatively doped silicon. This is an essential feature of the P-N Junction.

Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Aluminum Coating

Cross-section View of Wafer

Step 8: Aluminum Deposition

Overhead View of Wafer

Sputtering Machine

The wafer was attached to a larger carrier wafer and placed in the sputtering machine to be coated with aluminum.

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Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Photolithography Patterning

Step 9: Photolithography

Cross-section View of Wafer

Overhead View of Wafer

Photolithography was used again to pattern the wafer. The mask had to be aligned precisely with the patterning already on the wafer. The wafer was developed, rinsed, and baked to imprint the design. The covered areas will become the conductive circuitry of the final product. In the end, only select areas of the aluminum remained covered with photoresist.

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Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Aluminum Removal byAcid Etch

Overhead View of Wafer

Cross-section View of Wafer

Step 10: Wet Etching

The wafer was submerged in a mixture of acetic acid, phosphoric acid, nitric acid, and water to remove all of the exposed aluminum.

Rinse Tank

Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Overhead View of Wafer

Cross-section View of Wafer

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Photoresist Removed

Step 11: Photoresist Removal

The wafer was placed in an organic solvent called NMP for 5 minutes to remove the remaining photoresist.

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Grey=Si, Blue=Silicon Dioxide, Red=Photoresist , Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum

Bottom Coated with Aluminum

Cross-section View of Wafer

Step 12: Aluminum DepositionOverhead View of Wafer

The silicon dioxide on the bottom of the wafer was removed using hydrofluoric acid. The wafer was again put into the sputtering machine, this time to coat the bottom of the wafer with aluminum. This allowed for conductivity and a low resistance.

Works Citedhttp://cnx.org/content/m25495/latest/