Microchip Manufacturing

Microchip manufacturing has evolved rapidly since the invention of the integrated circuit. Silicon rapidly became the material of choice for fabricating microchips because a high-quality insulating and passivating layer is easily formed on its surface by thermal oxidation. This oxide layer can be readily patterned to serve as an isolation layer, as masks for diffusion and ion-implantation, as well as for the critical transistor components.

Silicon-device technology development took off on several fronts after the 1960 demonstration of MOS field-effect transistors and the 1963 invention of CMOS (Complementary MOSFETs), which dissipate very little power in standby operation. Circuit designers used bipolar transistors for their speed, p-channel and n-channel MOSFETs for their process simplicity, and CMOS for their low power dissipation. BiCMOS combined bipolar and CMOS transistors on the same silicon chip. As their dimensions have become ever smaller over the years, breaching the micron level during the 1980s, CMOS devices steadily improved in speed while maintaining their low power dissipation. Since the early 1990s, CMOS has become the technology of choice for digital devices; bipolar and BiCMOS transistors are primarily used for analog and microwave applications.

Microchip fabrication involves the sequential application of many processing steps. For example, CMOS manufacturing employs literally hundreds of individual steps - of a few basic types. The most important is photolithography, in which a pattern is created on the chip surface by exposing a light-sensitive layer (photoresist) with an image of this pattern; the developed image in the photoresist is then used as a selective mask in removing the underlying material. The resolution of this process determines the minimum size of the transistors that can be fabricated and hence the density of components on the resulting microchip. By the end of the 20th century, individual features of CMOS transistors were about 250 nanometers across, and gigabit micro-chips had become possible.

Another basic process entails the formation of insulating or conductive films. Silicon dioxide films are readily formed by thermal oxidation or deposition, while films of other materials can be formed using various processes such as chemical vapor deposition. By employing a combination of lithography and etching, microchip manufacturers can then pattern these films as desired. Local oxide films, for example, can be formed by masked oxidation of silicon.

A third basic microchip manufacturing process involves increasing the level of certain impurities in selected areas of the silicon. This is achieved by diffusing these impurities from a source material or by implanting ions of them. Silicon-oxide or silicon-nitride films are commonly used as diffusion masks, while the layers of photoresist or other films serve as masks for ion implantation. These processes can be used to change the conductivity types of selected regions from p-type to n-type, or vice-versa, thus creating pn junctions in the silicon.

The fabrication of a microchip typically has two main parts, the front end and the back end. The former consists of forming individual devices in the silicon, while in the latter metal wires are added to interconnect them into the desired circuit and system functions. The silicon wafer is then sliced into individual microchips, which are placed on a module or board containing wires whose purpose is to make interconnections among the microchips and to other system components.

Exercise 5

Give synonyms and antonyms to the following words.

Exercise 6

Make a short outline of the text.

PART 3

P I O N E E R S

ALESSANDRO VOLTA

( 1745 – 1827 )

It is well known that Volta invented the primary battery and in so doing moved electrical science into an age of electrodynamics. What is less well known is that he proposed a fundamental unit of electric tension some years before that invention, when scientists were still deep in the age of electrostatics. It is perfectly appropriate then that the unit for electromotive force (a term he introduced) is named after him. We may be thankful, though, that his original unit was never accepted; it is roughly equal to 13 350 volts!

Volta was already an established scientist with a reputation for experimental work when he announced the invention of the "Pile", the first electric battery. The importance of the invention was instantly recognised as being of the first rank and it opened new avenues of enquiry, including electrochemistry and electrodynamics. It quickly led to experimental electric light and industrial electroplating.

Volta was born in Como in the duchy of Milan in Northern Italy on the 18th February 1745 and died there 82 years later on the 5th March 1827. On his mother's side he came from a family with a leaning towards the law1; his father's family was devoted to the church. One of his three paternal uncles was a Dominican, one a canon, and the third an archdeacon.

Alessandro was seven when his father died. When he was 12 one of his uncles took charge of his education, which began at a Jesuit college and nearly led to him becoming a Jesuit. His uncles decided they did not want that and so his education continued elsewhere. It was a wealthy friend, Giulio Cesare Gattoni, who provided the books and equipment which helped him to begin studying electricity.

The uncles had by now chosen his future career: the law. Somehow he avoided this path and continued to study what he termed his genius: electricity. Boldly, he wrote to leading scientists to discuss problems he encountered. One, Beccaria, recommended his own writings and also told Volta to experiment. So Volta began to develop his gift for making inexpensive but effective instruments.

Slowly, from the mid-1760s he learned the science and practice of electricity and in October 1774 he received his first academic appointment, at the Gymnasium in Como. The next year he was appointed professor of experimental physics. About the same time he made his first important invention, the electrophorus, and followed that with the discovery of methane.

The electrophore was probably the most significant electrical invention since the Leyden Jar capacitor2. After considerable experimentation, in June 1775, he announced his "elettroforo perpetuo". It was an inductive device for repeatedly charging a tin-foil covered shield which, in turn, was used to build up a large charge on a Leyden Jar capacitor. Whilst others had come close, only Volta produced a sturdy and usable instrument.

In 1776, he briefly turned to the study of gases and discovered a new gas which we know as methane. "Inflammable air" (hydrogen) had been isolated chemically ten years earlier and was known to exist naturally. Volta became intrigued by the "different kinds of air" and searched the countryside for the telltale bubbles until he found a new gas at Lake Maggiore. Hydrogen, however, was more explosive and it was hydrogen and air (oxygen), not methane, that Volta used in an "inflammable air pistol" which was fired by an electric spark. The pistol fired a lead ball, denting wood at 15 feet. From related experiments he concluded that about 20% of common air was oxygen. He narrowly missed synthesizing water3, but his method was successfully used later by Lavoisier, Laplace and Monge in France.

His discovery of methane obviously enhanced his scientific reputation and his reward was a travel grant, which took him to Switzerland and Alsace. The grant came from the Austrian government which then ruled Northern Italy. Then came Volta's appointment to the professorship of experimental physics at the University of Pavia; his popular professorship there ran for nearly 40 years. In 1781/82 he visited France and England, and in 1784 he went to Germany. On such state-financed trips he bought new equipment for the laboratory at Pavia. Most of the instruments he built up were destroyed in a fire in 1899.

Politically, Volta had much to be thankful for to the Austrians, but in l796 they were driven out of Northern Italy by the French, led by Napoleon. Volta was chosen in May of that year as one of a delegation to represent Como in honouring Napoleon. Later, he became an official of the new Government of Como but it was a position from which he soon resigned; his lingering loyalty to the Austrians, the damage done to his laboratory by French troops and his coldness towards the French led to his expulsion from Pavia for a while. It did him no harm, however, when the Austrians retook the country in 1799: though they closed the university, Volta remained free.

Thirteen months later the French were back. The university was reopened, Volta was once again a professor and accepted his status as a citizen of the new republic. A trip to Paris to express the university's thanks to Napoleon became a triumph for Volta. The primary battery was by then well known and its chemical power had made scientific headlines. Even Napoleon attended his demonstrations at the French Academy and Volta was awarded a gold medal.

In many ways Volta's discoveries captured Napoleon's heart and he continued to be an admirer. A prize of 60,000 francs was announced for "whoever by his experiments and discoveries makes a contribution to electricity and galvanism comparable to Franklin's and Volta's". Volta was later given a pension, and made a count and a senator 4 in the kingdom of Italy.

Volta's pile

The roots of Volta's invention go back to the discovery by his fellow Italian, Luigi Galvani, of frog legs fame. A full account was published in 1791 and caused great excitement amongst both physicists and the medical fraternity. The latter wondered if the "vital principle" had at last been found and pondered the possibilities for new treatments. Galvani was the second to report what we would now recognise as an electrochemical effect, the Swiss J.G. Sulzer having noted in 1762 that two dissimilar metals placed on the tongue gave a sensation of taste.

At first, Volta considered Galvani's reports to be unbelievable. Pressed by colleagues, he at last investigated the phenomenon and, by the April 1, 1791, had begun the series of careful step-by-step experiments which led him to the electric battery.

Galvani explained the excitation of the dead frog's legs as being caused by animal electricity, an explanation which Volta firmly rejected. Volta was led to believe that the current flow was caused by the contact of two different metals. In that he too was wrong. It was another Italian, G.V. Fabroni, who got the right explanation by pointing to a chemical action between the liquid, which always seemed to be present in both Galvani's and Volta's work, and the two different metals.

Volta repeated Sulzer's as well as Galvani's work. In one experiment he brought insulated zinc and copper discs into contact and found that they were charged on separation. By experiment he found that zinc and silver discs best suited his purpose and eventually he arranged pairs of them in a pile. Each pair was separated from neighbouring pairs by a piece of cardboard soaked in water or brine to provide, as he believed, a conducting path between the pairs. Letting all pairs touch one another, he knew, provided only the same effect as a single pair of discs.

The finished pile of discs and cardboard multiplied the effects of a single pair many times and he was able to receive a shock from his pile similar to that from a charged Leyden Jar capacitor. The vital differences were that Volta's pile did not need to be immediately recharged and could give a continuous current.

News of the invention was announced in a letter to the Royal Society in London: "The apparatus of which I speak," wrote Volta, "will doubtless astonish you." The continuous current almost appeared as perpetual motion, "but it is nonetheless true and real, and can be touched with the hands.”

Volta's pile, "as high as can hold itself without falling," consisted of 30, 40, or 60 cells. From such primitive origins grew today's huge international industry. As an alternative to the pile, Volta also used pairs of metals soldered together with each end dipping into water or brine contained in glasses; this arrangement he called the crown of cups. Again, 30 or more cells could be arranged to produce a battery of cells. The word battery had, of course, been used earlier, not only for a battery of guns but for a battery of charged Leyden Jars.

Improvements were soon made by others. For greater voltages more cells were needed in the pile, which increased the weight and squeezed out the electrolyte from the cardboard discs. In Germany, J.W. Ritter turned up the edges of his metal discs and obtained batteries which lasted for two weeks! A horizontal wooden trough provided an even better battery: zinc plates, for example, could be fixed vertically to a support and lowered into the trough between vertical plates of the other metal. This trough arrangement has been suggested as the origin of our circuit symbol for the battery.

Volta received many honours in his lifetime, including recognition by learned societies in London, Paris and Berlin. His financial rewards from his university salary were boosted in 1805 by the annuity he received from Napoleon and, in 1809, by his senatorial salary. For the last two decades of his life he had the income of a wealthy man.

Task I

Speak on Volta’s scientific interests and experiments.

Task II

Dscribe Volta’s pile and explain its work.

THOMAS ALVA EDISON

(1847-1931)

Thomas Edison, American inventor, is one of the greatest inventors of all time. Edison began to work at an early age and continued to work right up until his death. Throughout his prolific career as an inventor, he was well known for his focus and determination. During his career Edison patented more than 1,000 inventions, including the electric light, the phonograph, and the motion-picture camera. These three inventions gave rise to giant industries - electric utilities, phonograph and record companies, and the film industry - thus changing the work and leisure habits of people throughout the world. The period from 1879 to 1900, when Edison produced and perfected most of his devices, has been called the Age of Edison.

Early Life

Edison's family was part Dutch and part British. His ancestors, who supported the king in the American Revolution (1775-1783), fled to Canada with more than 30,000 others when the war ended. In 1837 Edison's father became engaged in an unsuccessful revolution against the Canadian government and was forced to flee back to the United States. Thus, Thomas was born in Milan, Ohio, in 1847.

In 1854 the family settled in Port Huron, Michigan, where Edison attended school for three months. This was his only formal public education. His mother continued his education, teaching him reading, writing, and arithmetic. She also read to him from well-known English writers, such as Edward Gibbon, William Shakespeare, and Charles Dickens.

Edison earned by selling newspapers, apples, and candy on the Detroit and Port Huron branch of the Grand Trunk Railroad when only 12 years old. Around this time his hearing began to decline, possibly due to a childhood attack of scarlet fever. Edison once said that he sometimes considered his partial deafness almost an asset, particularly when he wanted to concentrate on an experiment. However, in a poignant entry in his diary some years later, he wrote, "I haven't heard a bird sing since I was 12 years old."

When 15 years old, while still working on the railroad, Edison bought a small secondhand printing press and 136 kg of type. He installed the press in a baggage car and soon began producing a newspaper, the Weekly Herald, which he printed, edited, and sold on the Grand Trunk Railroad.

In the summer of 1862, Edison saved a boy from being run over by a boxcar. The boy, only three years old, was the son of the stationmaster in Mount Clemens, Michigan. In gratitude, the stationmaster offered to teach Edison how to operate the telegraph. Edison had already experimented with the telegraph at home and gladly accepted the offer. For five months, he learned to send and receive dispatches, and for the next four years he traveled thousands of miles as a telegrapher. During this period he spent most of his salary on various laboratory and electrical instruments, which he would take apart and rebuild.

Family Life

Edison met his first wife, Mary Stilwell, in 1871. She was 16 years old and working in one of his companies when the inventor first met her. Edison married Stilwell on Christmas Day of that year. They had a daughter, Marion, born in 1873, and two sons, Thomas, Jr., born in 1876, and William, born in 1878.

Soon after his first wife's death in 1884, Edison met and fell in love with Mina Miller, the daughter of a wealthy manufacturer. The two married in February 1886. They had a daughter, Madeleine, born in 1888, and two sons, Charles and Theodore, born in 1890 and 1898.

Edison focused on his work so much that he spent little time with his family. He avoided most social situations, and he often wore dirty shirts and shabby working clothes. Many of his associates also spoke of Edison's virtues, however, such as good humor, even disposition, honesty, and genuine affection for his family.

EARLY INVENTIONS

Edison acquired his knowledge of electricity and telegraphy (use of a telegraph system to communicate at a distance) as a teenager. In 1868, at age 21, he developed a telegraphic vote-recording machine[1], the first of his inventions to be patented. The next year, Edison invented an improved version of the stock ticker[2], which printed stock market quotations and gold prices on a paper tape. Unlike older stock tickers, Edison's was fully automatic, and it did away with the need for a special attendant to operate each machine.

These early inventions brought Edison no financial returns. The first invention to bring him money was another improvement on the stock ticker. Edison created a central mechanism by which all the receiving tickers could be put in unison with the main sending apparatus. For this invention, Edison received $40,000, which would be worth $530,000 in 2000. He and a business partner, who operated a machine shop, used the money to start a new company to manufacture Edison's improved stock ticker. For the next five years Edison spent up to 18 hours a day in his workshop in Newark, New Jersey, inventing and manufacturing a variety of electrical devices. One important device that he designed during this period was the quadruplex, a highly efficient telegraph that could send four messages at a time over a telegraph wire, instead of just one.

Date: 2015-12-24; view: 942