Beschreibung:

A collection of Early English Silicon and Gallium Arsedide wafers, including a case of 25 blank silicon wafers in a plastic case, a chrome coatedglass lithography mask (used to pro by Microfab for Marconi (c.late 1960's) and another smaller lithography mask; a transparent experimental Gallium Arsenide printed wafer; a slicon printed wafer, and a number of single crystal silicon blanks for making alpha particle detectors
The Photolithography Process in the 1960s
1. Silicon Wafer Preparation - the silicon wafer (a thin slice of pure silicon) was first meticulously cleaned to remove any impurities or particles. Any contamination could disrupt the microfabrication process.
2. Oxidation Layer - The wafer was coated with a thin layer of silicon dioxide (SiO₂) by heating it in an oxygen-rich environment. This layer acted as an insulating and protective layer.
3. Photoresist Application - A light-sensitive material called 'photoresist' was evenly applied to the wafer's surface. This material would react to ultraviolet (UV) light, becoming either soluble or insoluble, depending on whether a positive or negative photoresist was used.
4. Aligning the Mask - The 'lithography mask'—a glass or quartz plate with intricate patterns of the circuit or device to be fabricated—was carefully aligned over the wafer. These masks were highly precise and contained the negative or positive image of the circuit layer.
5. Exposure to UV Light - The wafer and mask were placed into a **mask aligner or stepper**, which directed UV light through the transparent parts of the mask onto the photoresist. The UV light changed the solubility of the photoresist in the exposed areas.
6. Developing the Photoresist - The wafer was then immersed in a chemical developer solution, which washed away the exposed (or unexposed) photoresist, depending on whether it was a positive or negative resist. This left a patterned photoresist layer on the wafer.
7. Etching - The exposed areas of the silicon dioxide (not protected by the photoresist) were etched away using either a **chemical etchant** (wet etching) or **plasma etching** (dry etching). This created a pattern in the oxide layer that matched the mask design.
8. Doping or Metallization - The patterned wafer was subjected to further processes - Doping: Introducing impurities into the exposed silicon areas to modify its electrical properties: Metallization - Depositing metal layers (like aluminum or gold) to form connections and contacts.
9. Photoresist Removal - After etching or doping, the remaining photoresist was stripped away using solvents or plasma. This left behind the desired pattern on the silicon wafer.
10. Layer Repetition - For complex circuits, the process was repeated multiple times with different masks for each layer. Each mask corresponded to a specific circuit layer, and the layers had to align perfectly (a process called 'registration') to create functional devices.
Role of the Lithography Mask - The lithography masks offered here played a vital role in defining the geometric patterns of the circuit. In the late 1960s, these masks were typically created using chrome-coated glass plates, where patterns were etched into the chrome layer using high-precision tools. These patterns represented transistor arrangements, resistors, capacitors, and wiring, which would eventually become part of the microcircuit.
Significance in History - In the 1960s, this process was cutting-edge and foundational for the rapid development of integrated circuits (ICs). Companies like Marconi would have used lithography masks to fabricate transistors, diodes, or other early semiconductor components for telecommunications, defense systems, and other technologies.
The mask from MicroFab is a piece of technological history, showcasing the craftsmanship and innovation that laid the groundwork for the modern computing revolution. Early photolithography techniques like this evolved into the extremely sophisticated processes used today, enabling the miniaturization and complexity of modern chips.
Footnote: These silicon wafers were purchased from one of the world's leading suppliers of silicon wafers namely, Wacker Chemie in Germany. The wafers were processed at various laboratories including e2v in Chelmsford, SemiFab in Scotland and Southampton University. The end product was to make a Field Effect Transistor used as the first element in the amplifying section of an X-Ray spectrometer. The Field Effect Transistor (JFET) was directly connected to the output of the sensor and matched to the sensor's capacity. They are called JFETs to signify that these were Junction Field Effect Transistors and the original design is credited to Wrangy Kandiah, a Physicist working at the UKAEA facility in Harwell.
The work started in the late 1970s and carried on into the early 2000s. These devices have now been superseded by Silicon Drift Detectors where the JFET has been replaced with a CMOS FET embedded into the anode of the sensor.
e2v in Lincoln was the first semiconductor foundry in Europe and they specialized in Gallium Arsenide for applications in Radar.
The wafers went through various stages in the processing laboratory. The first step was to oxidise the wafer and then to create the JFET structure using Lithography and finally coating the wafers to reveal the electrodes to the transistor. The final step was to saw the wafers and release individual JFETs for use in the spectrometers.The footprint of each JFET was 1mm by 1 mm and the gate width on the Fet was 1 micron.