Because of equipment availability, cost, and time, we will use aluminum as the top side conductor
Top Side Conductor vacuum deposition Aluminum sputter deposit in Argon plasma CVC 601-sputter deposition tool
A conductor metal is vacuum deposited on to the wafer Aluminum will be used instead of silver Sputtered aluminum
Photolithography Photolithography is the transfer of patterns, circuits, device structures, etc. to a substrate or wafer using light and a mask or stencil to stop the light. Photolithography is used extensively in the progression of microelectronics. Today, because of the sizes involved in current computer microprocessor devices, other methods like direct patterning using electron beams are used. Photolithography is still used for dimensions down to about 0.5um. The wavelength of UV light is.35-.45 um.
Top side conductor grid is created using a transparency mask The top side conductor grid is created on a transparency sheet to keep cost low Once top side conductor grid design is finalized, a chrome on glass professional mask can be made to go into mass production
Top side conductor Because of Cameron tester limitations (1.0A) cell size is limited to about 65mm x 65mm max
UV light sensitive material called photoresist is spin coated on to the aluminum conductor on the top side of the wafer (polished side) Positive photoresist - 1813 Aluminum Silicon wafer
Wafers are spin coated with Shipley 1813 UV sensitive photoresist spin coating produces a uniform coating Run a test spin without the photoresist to verify operation Spin speed is set here. Spin speed is 4K rpm Close lid and apply photoresist through the lid opening A vacuum chuck holds the wafer Light sensitive material is stored in amber dropper bottles Use 1813
Transparency is used as a photomask. Cells can be designed to various sizes Cells can be of various sizes but must line up for saw cutting
The transparency is called a photo mask. Production photo masks would be made on glass plates with high precision patterns. Transparency mask A glass plate is placed on top of transparency mask to keep it flat and prevent light to leak under the transparency Glass plate Positive photoresist - 1813 Aluminum Silicon wafer
HTG mask aligner is used to provide the UV Clear glass plates are used to make sure the transparency lays flat to the wafer light source The UV light source is a mercury vapor lamp at 436nm wavelength UV light with filter surrounding it Exposure time set on timer Wafer is held by vacuum, mask is placed on top and brought into contact with wafer
Ultraviolet light is projected down on to the photoresist coated wafer UV light Glass plate Aluminum Silicon wafer Exposed photoresist
The wafer is developed, leaving photoresist where no UV light has penetrated the mask Aluminum Silicon wafer Unexposed photoresist
Solitec automatic developer vacuum chuck holds wafer, tool develops, rinses, and dries the wafer Minimum vacuum level is 15 on vacuum gauge Vacuum switch Start switch hold down until tool starts
The wafer is immersed in the aluminum etch and the top side aluminum conductor is dissolved (etched away) Because it is a liquid etch and can undercut the photoresist it is important to remove the wafer from the etchant as soon as the pattern clears Liquid aluminum etchant PR Al PR PR PR Al Al Al Silicon wafer
Unwanted aluminum is etched away using the aluminum etchant at about 80 0 C Aluminum etch @ 80 o C Aluminum etch contains strong acids and all safety precautions are needed
After etching, the top conductor grid pattern will be left on the wafer. The photoresist is removed leaving the top side aluminum grid Completed topside conductor Al Al Al Al N- region P type silicon wafer
Once the top side conductor grid is complete, the back side conductor can be deposited, again aluminum sputtered with the CVC 601 will be used Al Al Al Al N- region P type silicon wafer Backside aluminum
Assignment Complete the photolithography worksheet