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The first operation while producing the chip is the design. When tens of millions of transistors are to be built on a square of silicon about the size of a child’s fingernail, the placing and interconnections of the transistors must be meticulously worked out. Because of the complexity of today’s chips, the design work is done by computer, although engineers often print out an enlarged diagram of a chip’s structure to examine it in detail.
The base material for building an integrated circuit is a silicon crystal. To make a silicon crystal, raw silicon obtained from quartz rock is treated with chemicals that remove contaminants until what remains is almost 100 percent silicon. This purified silicon is melted and then formed into cylindrical single crystals called ingots. The ingots are sliced into wafers about 0.725 millimeter thick. In a step called planarization they are polished with a slurry until they have a flawless, mirror-smooth surface.
Making the transistors and their interconnections entails several different basic steps that are repeated many times. The most complex chips made today consist of 20 or more layers and may require several hundred separate processing steps to build them up one by one.
The first layer is silicon dioxide, an insulator. It is created by putting the wafers into a diffusion furnace – essentially an oven at high temperature where a thin layer of oxide is grown on the wafer surface.
Removed from the furnace, the wafer is ready for its first patterning, or photolithographic, step. A coating of a fairly viscous polymeric liquid called photoresist, which becomes soluble when it is exposed to ultraviolet light, is applied to the surface. A spigot deposits a precise amount of photoresist on the wafer surface. Then the wafer is spun so that centrifugal force spreads the liquid over the surface at an even thickness. This operation takes place on every layer that is modified by a photolithographic procedure called masking.
A mask is the device through which ultraviolet light shines to define the circuit pattern on each layer of a chip. The mask image is transferred to the wafer using a computer-controlled machine known as a stepper. It has a sophisticated lens system with a resolution as small as 0.25 micron. Ultraviolet light from an arc lamp or a laser shines through the clear spaces of the mask’s intricate pattern onto the photoresist layer of a single chip. The stepper table then moves the wafer the precise distance required to position another chip under the light. On each chip, the parts of the photoresist layer that were struck by the light become soluble and can be developed, much like photographic film, using organic solvents.
1) How many transistors may be squeezed onto computer chip?
2) What is the base of microchip?
3) What are the main stages of microchip production?
4) Why is ultraviolet, but not visible light used during treating the surface?
5) Are there any limits of size of electronic components on the surface of semiconducting single crystal?
6) Why must the single crystal of semiconductor be extremely pure?
Date: 2015-12-11; view: 177