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semiconductor

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  • The basic semiconductor material from which electronic devices are made comes in the form of round thin crystalline disks called wafers. From the statement, explain the following questions:

    a) Describe the historical development root of the semiconductor industry in the twentieth century.

    In 1950s, different types of semiconductor transistor technology were developed. The first transistor were developed from germanium. In 1954, the first manufacturable silicon transistor were made. In 1929, IC was invented to replaced vacuum tube.

    In 1960s, the semiconductor industry entered an era oriented toward solving the basic problems of producing semiconductor ICs. There was also come large captive supplier companies in the IC business such as IBM and Digital Equipment Corporation.

    The early 1970s was the period of medium-scale integration (MSI) for chip design. Microprocessors were invented separately in the early 1970s at Texas Instrument and Intel. Asia emerged as a formidable competitor with Japan as a power house of semiconductor innovation and manufacturing. By 1979, Japan had captured more than 40% of the worlds demand for memory microchips.

    By 1980s, the widespread acceptance of solid state electronics in society was established, driving the growth of chip fabrication in the semiconductor industry. The semiconductor industry formed SEMATECH in 1987 under the guidance of Department of Defence, USA. These was formed because of Japanese competition threatens the US manufacturer. To overcome the high cost of entrance into new technology with new fabs, many US semiconductor companies entered the joint venture agreements with Japanese and European chip manufacturer to share cost of constructing wafer fabs.

    By the end of 1990s, the minimum feature size was 0.18 micrometer. In 1990s, it is critical for manufacturers to produce complex, high quality chips in record time. Semiconductor equipment is highly automated. Engineers intervene to download product recipes into the equipment software database and interpret software diagnostic commands to take corrective action for equipment repair.

    b) Correlate the three major trends associated with improvement in wafer fabrication.

    There are three major trends which the first is the increase in chip performance. A common way to judge chip performance is speed. Chip speed will improve if the device are made smaller and packed close together on the chip because the electric signal moving through the circuit has less distance to travel. This is achieved by reducing the critical dimension and thus can further reduce the number of component per chip. This results in a smaller microchips that fit into small volumes, which leads to smaller commercial product and enable the development of smartphone and small laptop that contributes to the improvement in wafer fabrication. Another way to increase the speed is to use materials that improve the passage of the electrical signal through the device and circuitry on the chip surface. Besides that, the performance of chips can be judged by power consumption of the device. The semiconductors device consume much less power than the vacuum tube. Although number of transistor per chip is increasing, the chip power consumption grows at slower rate. These are important in portable devices and improve the demand in wafer fabrication.

    The second trend is the increase in chip reliability where the chip function as intended over its expected life. Advances in technology and strict quality control measures create the product with low failures during applications. This makes semiconductor devices have high demand from the market due to its reliability.

  • The third trend is the reduction in chips price. In 50 years time, it has decreased by over 100 million times. The factors such as reduction of feature size and increasing the wafer diameter to put more chips on a wafer contributes to price reduction. This managed to increase the production of the chips with the same manufacturing cost. Large demand and economic growth also introduce economic of scales for manufacturing. The introduction of manufacturing improvements to the equipments and process used to fabricate microchips helps a lot to reduce cost. This reduction in price translate into high demand of wafer fabrication.

    c) Describe the wafer, including how it is layered and the essential aspects of the five stages of wafer fabrication.

    Wafer is the basic semiconductor material from which electronic devices are made in the form of round thin crystalline disk .

    Fabrication of the semiconductor devices occurs only in the first few microns of the silicon near the wafer surface. The bulk of the silicon wafer thickness is to give the wafer adequate rigidity during processing. Once the devices are fabricated in the silicon, then layers of metal circuitry are placed on the silicon to interconnect between the devices and the various electronic signals outside the chip.

    Fabrication of microchip involves five general steps of manufacturing which are :

    i) Wafer Preparation- This is the first stage where the silicon is mined from sand and purified and it undergoes

    specialised processing to create a silicon ingot with the appropriate diameter. The silicon ingot is then sliced into thin wafers that are used to fabricate the microchips.

    Fig. 1: Layering of wafer

  • ii) Wafer Fabrication- In the second stage, the microchips is fabricated from wafers. The bare silicon wafers arrive in

    the wafer fabrication, then they go through variety of cleaning, layering, patterning, etching, and doping steps. The completed wafers have a full complement of integrated circuits permanently etched into each silicon wafer.

    iii) Wafer test/sort- At the completion of wafer fabrication, the wafer is sent to the test/sort area where every die is

    probe and electrically tested. Die that fail the wafer test will not be sent to customer while die that pass will continue through the process.

    iv) Assembly and packaging- The wafer then enters assembly and packaging to enclose the individual chips in a protective

    package. The backside of the wafer undergoes a background to reduce the thickness of the substrate. a thick membrane of acetate tape is attached to the backside of each wafer, then the front of the wafer is sliced along the scribe lines with a diamond-tip saw blade to separate the die from the wafer. The sticky acetate holds the wafer together. Metal connection are made and then the die is hermetically sealed in a plastic or ceramic package.

    v) Final Test- To ensure chips functionality, each packaged IC is tested to meet the standard. After the final

    test, the chips is delivered to the customer to be used.

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