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 TT TT T TT TT T T T T T T T T TT T TT TT T TT TT T T T T T T T T TT T Step 1Step 2Step 3Step 4Step 5Step 6 Step 7Step 8Step 9Step 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

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

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.

TT TT T TT TT T 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.

TT TT T TT TT T Grey=Si, Blue=Silicon Dioxide, Red=Photoresist, Purple= Phosphorus Doped Glass, Lilac= Silicon with Diffused Phosphorus, Silver=Aluminum Aluminum Removal by Acid 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 T T T T T T T TT T Photoresist Removed Step 11: Photoresist Removal The wafer was placed in an organic solvent called NMP for 5 minutes to remove the remaining photoresist.

T T T T T T T TT T 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 Deposition Overhead 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 Cited