A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers, their large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, a tendency to form glasses and semicrystalline structures rather than crystals. The terms polymer and resin are synonymous with plastic; the term "polymer" derives from the Greek word πολύς and μέρος, refers to a molecule whose structure is composed of multiple repeating units, from which originates a characteristic of high relative molecular mass and attendant properties. The units composing polymers derive or conceptually, from molecules of low relative molecular mass.
The term was coined in 1833 by Jöns Jacob Berzelius, though with a definition distinct from the modern IUPAC definition. The modern concept of polymers as covalently bonded macromolecular structures was proposed in 1920 by Hermann Staudinger, who spent the next decade finding experimental evidence for this hypothesis. Polymers are studied in the fields of biophysics and macromolecular science, polymer science. Products arising from the linkage of repeating units by covalent chemical bonds have been the primary focus of polymer science. Polyisoprene of latex rubber is an example of a natural/biological polymer, the polystyrene of styrofoam is an example of a synthetic polymer. In biological contexts all biological macromolecules—i.e. Proteins, nucleic acids, polysaccharides—are purely polymeric, or are composed in large part of polymeric components—e.g. Isoprenylated/lipid-modified glycoproteins, where small lipidic molecules and oligosaccharide modifications occur on the polyamide backbone of the protein.
The simplest theoretical models for polymers are ideal chains. Polymers are of two types: occurring and synthetic or man made. Natural polymeric materials such as hemp, amber, wool and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose, the main constituent of wood and paper; the list of synthetic polymers in order of worldwide demand, includes polyethylene, polystyrene, polyvinyl chloride, synthetic rubber, phenol formaldehyde resin, nylon, polyacrylonitrile, PVB, many more. More than 330 million tons of these polymers are made every year. Most the continuously linked backbone of a polymer used for the preparation of plastics consists of carbon atoms. A simple example is polyethylene. Many other structures do exist. Oxygen is commonly present in polymer backbones, such as those of polyethylene glycol, DNA. Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network. During the polymerization process, some chemical groups may be lost from each monomer.
This happens in the polymerization of PET polyester. The monomers are terephthalic acid and ethylene glycol but the repeating unit is —OC—C6H4—COO—CH2—CH2—O—, which corresponds to the combination of the two monomers with the loss of two water molecules; the distinct piece of each monomer, incorporated into the polymer is known as a repeat unit or monomer residue. Laboratory synthetic methods are divided into two categories, step-growth polymerization and chain-growth polymerization; the essential difference between the two is that in chain growth polymerization, monomers are added to the chain one at a time only, such as in polyethylene, whereas in step-growth polymerization chains of monomers may combine with one another directly, such as in polyester. Newer methods, such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out without a catalyst. Laboratory synthesis of biopolymers of proteins, is an area of intensive research. There are three main classes of biopolymers: polysaccharides and polynucleotides.
In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA to RNA and subsequently translate that information to synthesize the specified protein from amino acids; the protein may be modified further following translation in order to provide appropriate structure and functioning. There are other biopolymers such as rubber, suberin and lignin. Occurring polymers such as cotton and rubber were familiar materials for years before synthetic polymers such as polyethene and perspex appeared on the market. Many commercially important polymers are synthesized by chemical modification of occurring polymers. Prominent examples inclu
An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material, silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller and faster than those constructed of discrete electronic components; the IC's mass production capability and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs. Integrated circuits were made practical by mid-20th-century technology advancements in semiconductor device fabrication. Since their origins in the 1960s, the size and capacity of chips have progressed enormously, driven by technical advances that fit more and more transistors on chips of the same size – a modern chip may have many billions of transistors in an area the size of a human fingernail.
These advances following Moore's law, make computer chips of today possess millions of times the capacity and thousands of times the speed of the computer chips of the early 1970s. ICs have two main advantages over discrete circuits: performance. Cost is low because the chips, with all their components, are printed as a unit by photolithography rather than being constructed one transistor at a time. Furthermore, packaged ICs use much less material than discrete circuits. Performance is high because the IC's components switch and consume comparatively little power because of their small size and close proximity; the main disadvantage of ICs is the high cost to fabricate the required photomasks. This high initial cost means. An integrated circuit is defined as: A circuit in which all or some of the circuit elements are inseparably associated and electrically interconnected so that it is considered to be indivisible for the purposes of construction and commerce. Circuits meeting this definition can be constructed using many different technologies, including thin-film transistors, thick-film technologies, or hybrid integrated circuits.
However, in general usage integrated circuit has come to refer to the single-piece circuit construction known as a monolithic integrated circuit. Arguably, the first examples of integrated circuits would include the Loewe 3NF. Although far from a monolithic construction, it meets the definition given above. Early developments of the integrated circuit go back to 1949, when German engineer Werner Jacobi filed a patent for an integrated-circuit-like semiconductor amplifying device showing five transistors on a common substrate in a 3-stage amplifier arrangement. Jacobi disclosed cheap hearing aids as typical industrial applications of his patent. An immediate commercial use of his patent has not been reported; the idea of the integrated circuit was conceived by Geoffrey Dummer, a radar scientist working for the Royal Radar Establishment of the British Ministry of Defence. Dummer presented the idea to the public at the Symposium on Progress in Quality Electronic Components in Washington, D. C. on 7 May 1952.
He gave many symposia publicly to propagate his ideas and unsuccessfully attempted to build such a circuit in 1956. A precursor idea to the IC was to create small ceramic squares, each containing a single miniaturized component. Components could be integrated and wired into a bidimensional or tridimensional compact grid; this idea, which seemed promising in 1957, was proposed to the US Army by Jack Kilby and led to the short-lived Micromodule Program. However, as the project was gaining momentum, Kilby came up with a new, revolutionary design: the IC. Newly employed by Texas Instruments, Kilby recorded his initial ideas concerning the integrated circuit in July 1958 demonstrating the first working integrated example on 12 September 1958. In his patent application of 6 February 1959, Kilby described his new device as "a body of semiconductor material … wherein all the components of the electronic circuit are integrated." The first customer for the new invention was the US Air Force. Kilby won the 2000 Nobel Prize in Physics for his part in the invention of the integrated circuit.
His work was named an IEEE Milestone in 2009. Half a year after Kilby, Robert Noyce at Fairchild Semiconductor developed a new variety of integrated circuit, more practical than Kilby's implementation. Noyce's design was made of silicon. Noyce credited Kurt Lehovec of Sprague Electric for the principle of p–n junction isolation, a key concept behind the IC; this isolation allows each transistor to operate independently despite being part of the same piece of silicon. Fairchild Semiconductor was home of the first silicon-gate IC technology with self-aligned gates, the basis of all modern CMOS integrated circuits; the technology was developed by Italian physicist Federico Faggin in 1968. In 1970, he joined Intel in order to develop the first single-chip central processing unit microprocessor, the Intel 4004, for which he received the National Medal of Technology and Innovation in 2010; the 4004 was designed by Busicom's Masatoshi Shima and Intel's Ted Hoff in 1969, but it was Faggin's improved design in 1970 that made it a reality.
Advances in IC technology smaller features and la
The iPod is a line of portable media players and multi-purpose pocket computers designed and marketed by Apple Inc. The first version was released on October 23, 2001, about 8 1⁄2 months after the Macintosh version of iTunes was released; as of July 27, 2017, only the iPod Touch remains in production. Like other digital music players, iPods can serve as external data storage devices. Apple's iTunes software can be used to transfer music, videos, contact information, e-mail settings, Web bookmarks, calendars, to the devices supporting these features from computers using certain versions of Apple macOS and Microsoft Windows operating systems. Before the release of iOS 5, the iPod branding was used for the media player included with the iPhone and iPad, a combination of the Music and Videos apps on the iPod Touch; as of iOS 5, separate apps named "Music" and "Videos" are standardized across all iOS-powered products. While the iPhone and iPad have the same media player capabilities as the iPod line, they are treated as separate products.
During the middle of 2010, iPhone sales overtook those of the iPod. The iPod was released in late 2001; the iPod line came from Apple's "digital hub" category, when the company began creating software for the growing market of personal digital devices. Digital cameras and organizers had well-established mainstream markets, but the company found existing digital music players "big and clunky or small and useless" with user interfaces that were "unbelievably awful," so Apple decided to develop its own; as ordered by CEO Steve Jobs, Apple's hardware engineering chief Jon Rubinstein assembled a team of engineers to design the iPod line, including hardware engineers Tony Fadell and Michael Dhuey, design engineer Sir Jonathan Ive. Rubinstein had discovered the Toshiba hard disk drive while meeting with an Apple supplier in Japan, purchased the rights to it for Apple, had already worked out how the screen and other key elements would work; the aesthetic was inspired by the 1958 Braun T3 transistor radio designed by Dieter Rams, while the wheel-based user interface was prompted by Bang & Olufsen's BeoCom 6000 telephone.
The product was developed in less than one year and unveiled on October 23, 2001. Jobs announced it as a Mac-compatible product with a 5 GB hard drive that put "1,000 songs in your pocket."Apple did not develop the iPod software in-house, instead using PortalPlayer's reference platform based on two ARM cores. The platform had rudimentary software running on a commercial microkernel embedded operating system. PortalPlayer had been working on an IBM-branded MP3 player with Bluetooth headphones. Apple contracted another company, Pixo, to help design and implement the user interface under the direct supervision of Steve Jobs; as development progressed, Apple continued to feel. Starting with the iPod Mini, the Chicago font was replaced with Espy Sans. IPods switched fonts again to Podium Sans—a font similar to Apple's corporate font, Myriad. Color display iPods adopted some Mac OS X themes like Aqua progress bars, brushed metal meant to evoke a combination lock. In 2007, Apple modified the iPod interface again with the introduction of the sixth-generation iPod Classic and third-generation iPod Nano by changing the font to Helvetica and, in most cases, splitting the screen in half by displaying the menus on the left and album artwork, photos, or videos on the right.
In 2006 Apple presented a special edition for iPod 5G of Irish rock band U2. Like its predecessor, this iPod has engraved the signatures of the four members of the band on its back, but this one was the first time the company changed the colour of the metal; this iPod was only available with 30GB of storage capacity. The special edition entitled purchasers to an exclusive video with 33 minutes of interviews and performance by U2, downloadable from the iTunes Store. In September 2007, during a lawsuit with patent holding company Burst.com, Apple drew attention to a patent for a similar device, developed in 1979. Kane Kramer applied for a UK patent for his design of a "plastic music box" in 1981, which he called the IXI, he was unable to secure funding to renew the US$120,000 worldwide patent, so it lapsed and Kramer never profited from his idea. The name iPod was proposed by Vinnie Chieco, a freelance copywriter, called by Apple to figure out how to introduce the new player to the public. After Chieco saw a prototype, he thought of the movie 2001: A Space Odyssey and the phrase "Open the pod bay doors, Hal", which refers to the white EVA Pods of the Discovery One spaceship.
Chieco saw an analogy to the relationship between the spaceship and the smaller independent pods in the relationship between a personal computer and the music player. Apple researched the trademark and found that it was in use. Joseph N. Grasso of New Jersey had listed an "iPod" trademark with the U. S. Patent and Trademark Office in July 2000 for Internet kiosks; the first iPod kiosks had been demonstrated to the public in New Jersey in March 1998, commercial use began in January 2000, but had been discontinued by 2001. The trademark was registered by the USPTO in November 2003, Grasso assigned it to Apple Computer, Inc. in 2005. The earliest recorded use in commerce of an "iPod" trademark was in 1991 by Chrysalis Corp. of Sturgis, styled "iPOD". In mid-2015, several new color schemes for all of the current iPod models were spotted in the latest version of iTunes, 12.2. Belgian website Belgium iPhone found the images
The Digital Revolution known as the Third Industrial Revolution, is the shift from mechanical and analogue electronic technology to digital electronics which began anywhere from the late 1950s to the late 1970s with the adoption and proliferation of digital computers and digital record keeping that continues to the present day. Implicitly, the term refers to the sweeping changes brought about by digital computing and communication technology during the latter half of the 20th century. Analogous to the Agricultural Revolution and Industrial Revolution, the Digital Revolution marked the beginning of the Information Age. Central to this revolution is the mass production and widespread use of digital logic circuits, its derived technologies, including the computer, digital cellular phone, the Internet; these technological innovations have transformed traditional business techniques. The underlying technology was invented in the half of the 19th century, including Babbage's analytical engine and the telegraph.
Digital communication became economical for widespread adoption after the invention of the personal computer. Claude Shannon, a Bell Labs mathematician, is credited for having laid out the foundations of digitalization in his pioneering 1948 article, A Mathematical Theory of Communication; the digital revolution converted technology, analog into a digital format. By doing this, it became possible to make copies. In digital communications, for example, repeating hardware was able to amplify the digital signal and pass it on with no loss of information in the signal. Of equal importance to the revolution was the ability to move the digital information between media, to access or distribute it remotely; the turning point of the revolution was the change from analogue to digitally recorded music. During the 1980s the digital format of optical compact discs replaced analog formats, such as vinyl records and cassette tapes, as the popular medium of choice. In 1947, the transistor was invented. From the late 1940s, the Military, Business developed computer systems, to digitally replicate and automate manually performed mathematical calculations, with the LEO being the first commercially available general purpose computer.
A key step toward mass commercialization was the advent of the planar process in 1959 by Jean Hoerni, an employee of Fairchild Semiconductor. This enabled integrated circuits to be mass produced using the techniques of photolithography. From 1969 to 1971, Intel developed the Intel 4004, an early microprocessor that laid the foundations for the microcomputer revolution that began in the 1970s; the public was first introduced to the concepts that would lead to the Internet when a message was sent over the ARPANET in 1969. Packet switched networks such as ARPANET, Mark I, CYCLADES, Merit Network and Telenet, were developed in the late 1960s and early 1970s using a variety of protocols; the ARPANET in particular led to the development of protocols for internetworking, in which multiple separate networks could be joined together into a network of networks. The Whole Earth movement of the 1960s advocated the use of new technology. In the 1970s, the home computer was introduced, time-sharing computers, the video game console, the first coin-op video games, the golden age of arcade video games began with Space Invaders.
As digital technology proliferated, the switch from analog to digital record keeping became the new standard in business, a new job description was popularized, the data entry clerk. Culled from the ranks of secretaries and typists from earlier decades, the data entry clerk's job was to convert analog data into digital data. In developed nations, computers achieved semi-ubiquity during the 1980s as they made their way into schools, homes and industry. Automated teller machines, industrial robots, CGI in film and television, electronic music, bulletin board systems, video games all fueled what became the zeitgeist of the 1980s. Millions of people purchased home computers, making household names of early personal computer manufacturers such as Apple and Tandy. To this day the Commodore 64 is cited as the best selling computer of all time, having sold 17 million units between 1982 and 1994. In 1984, the U. S. Census Bureau began collecting data on Internet use in the United States. S. households owned a personal computer in 1984, that households with children under the age of 18 were nearly twice as to own one at 15.3%.
By 1989, 15% of all U. S. households owned a computer, nearly 30% of households with children under the age of 18 owned one. By the late 1980s, many businesses were dependent on digital technology. Motorola created the first mobile phone, Motorola DynaTac, in 1983. However, this device used analog communication - digital cell phones were not sold commercially until 1991 when the 2G network started to be opened in Finland to accommodate the unexpected demand for cell phones, becoming apparent in the late 1980s. Compute! magazine predicted that CD-ROM would be the centerpiece of the revolution, with multiple household devices reading the discs. The first true digital camera was created in 1988, the first were marketed in December 1989 in Japan and in 1990 in the United States. By the mid-2000s, they would eclipse traditional film in popularity. Digital ink was invented in the late 1980s. Disney's CAPS system was used for a
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals; because the controlled power can be higher than the controlling power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits; the transistor is the fundamental building block of modern electronic devices, is ubiquitous in modern electronic systems. Julius Edgar Lilienfeld patented a field-effect transistor in 1926 but it was not possible to construct a working device at that time; the first implemented device was a point-contact transistor invented in 1947 by American physicists John Bardeen, Walter Brattain, William Shockley. The transistor revolutionized the field of electronics, paved the way for smaller and cheaper radios and computers, among other things.
The transistor is on the list of IEEE milestones in electronics, Bardeen and Shockley shared the 1956 Nobel Prize in Physics for their achievement. Most transistors are made from pure silicon or germanium, but certain other semiconductor materials can be used. A transistor may have only one kind of charge carrier, in a field effect transistor, or may have two kinds of charge carriers in bipolar junction transistor devices. Compared with the vacuum tube, transistors are smaller, require less power to operate. Certain vacuum tubes have advantages over transistors at high operating frequencies or high operating voltages. Many types of transistors are made to standardized specifications by multiple manufacturers; the thermionic triode, a vacuum tube invented in 1907, enabled amplified radio technology and long-distance telephony. The triode, was a fragile device that consumed a substantial amount of power. In 1909 physicist William Eccles discovered the crystal diode oscillator. Physicist Julius Edgar Lilienfeld filed a patent for a field-effect transistor in Canada in 1925, intended to be a solid-state replacement for the triode.
Lilienfeld filed identical patents in the United States in 1926 and 1928. However, Lilienfeld did not publish any research articles about his devices nor did his patents cite any specific examples of a working prototype; because the production of high-quality semiconductor materials was still decades away, Lilienfeld's solid-state amplifier ideas would not have found practical use in the 1920s and 1930s if such a device had been built. In 1934, German inventor Oskar Heil patented a similar device in Europe. From November 17, 1947, to December 23, 1947, John Bardeen and Walter Brattain at AT&T's Bell Labs in Murray Hill, New Jersey of the United States performed experiments and observed that when two gold point contacts were applied to a crystal of germanium, a signal was produced with the output power greater than the input. Solid State Physics Group leader William Shockley saw the potential in this, over the next few months worked to expand the knowledge of semiconductors; the term transistor was coined by John R. Pierce as a contraction of the term transresistance.
According to Lillian Hoddeson and Vicki Daitch, authors of a biography of John Bardeen, Shockley had proposed that Bell Labs' first patent for a transistor should be based on the field-effect and that he be named as the inventor. Having unearthed Lilienfeld’s patents that went into obscurity years earlier, lawyers at Bell Labs advised against Shockley's proposal because the idea of a field-effect transistor that used an electric field as a "grid" was not new. Instead, what Bardeen and Shockley invented in 1947 was the first point-contact transistor. In acknowledgement of this accomplishment, Shockley and Brattain were jointly awarded the 1956 Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect". In 1948, the point-contact transistor was independently invented by German physicists Herbert Mataré and Heinrich Welker while working at the Compagnie des Freins et Signaux, a Westinghouse subsidiary located in Paris. Mataré had previous experience in developing crystal rectifiers from silicon and germanium in the German radar effort during World War II.
Using this knowledge, he began researching the phenomenon of "interference" in 1947. By June 1948, witnessing currents flowing through point-contacts, Mataré produced consistent results using samples of germanium produced by Welker, similar to what Bardeen and Brattain had accomplished earlier in December 1947. Realizing that Bell Labs' scientists had invented the transistor before them, the company rushed to get its "transistron" into production for amplified use in France's telephone network; the first bipolar junction transistors were invented by Bell Labs' William Shockley, which applied for patent on June 26, 1948. On April 12, 1950, Bell Labs chemists Gordon Teal and Morgan Sparks had produced a working bipolar NPN junction amplifying germanium transistor. Bell Labs had announced the discovery of this new "sandwich" transistor in a press release on July 4, 1951; the first high-frequency transistor was the surface-barrier germanium transistor developed by Philco in 1953, capable of operating up to 60 MHz.
These were made by etching depressions into an N-type germanium base from both sides with jets of Indium sulfate until it was a few ten-thousandths of an inch thick. Indium electroplated into the depressions formed the emitter; the first "prototype" pocket transistor radio was shown by I
A Radio-controlled helicopter is model aircraft, distinct from a RC airplane because of the differences in construction and flight training. Several basic designs of RC helicopters exist, of; the more maneuverable designs are harder to fly, but benefit from greater aerobatic capabilities. Flight controls allow pilots to control the collective, the cyclic controls, the tail rotor. Controlling these in unison enables the helicopter to perform the same maneuvers as full-sized helicopters, such as hovering and backwards flight, many other maneuvers that full-sized helicopters cannot, such as inverted flight; the various helicopter controls are effected by means of small servo motors known as servos. A solid-state gyroscope sensor is used on the tail rotor control to counter wind- and torque-reaction-induced tail movement. Most newer helicopters have gyro-stabilization on the other 2 axes of rotation as well; such 3-axis gyro is called a flybarless controller, so-called because it eliminates the need for a mechanical flybar.
The engines used to be methanol-powered two-stroke motors, but electric brushless motors combined with a high-performance lithium polymer battery are now more common and provide improved efficiency and lifespan compared to brushed motors, while decreasing prices bring them within reach of hobbyists. Gasoline and jet turbine engines are used. Just like full sized helicopters, model helicopter rotors turn at high speeds and can cause severe injuries. Several deaths have occurred as as 2013. Common power sources of remote control helicopters are glow fuel, electric batteries and turbine engines. For the first 40 years, glow fuel helicopters were the most common type produced. However, in the last 10 years, electric powered helicopters have matured to a point where power and flight times are better, but not as long as glow fuel helicopters. There have been two main types of systems to control the main rotors, mechanical mixing and cyclic/collective pitch mixing. Most earlier helicopters used mechanical mixing.
Today, nearly all R/C helicopter use CCPM. Practical electric helicopters are a recent development but have developed and become more common, overtaking glow fuel helicopters in common use. Turbine helicopters are increasing in popularity, although the high cost puts them out of reach of most people; the first RC helicopters have been powered by combustion engines. Original helicopter "classes" were based on the engine size. For example, a helicopter with a 0.30 cu in engine was a 30 class and a helicopter with a 0.90 cu in engine was referred to as a 90 class helicopter. The bigger and more powerful the engine, the larger the main rotor blade that it can turn and hence the bigger the aircraft overall. Typical flight time for nitro helicopters is 7 -- 15 minutes tuning. Two small electric helicopters emerged in the mid-1990s; these were the Kalt Whisper and the Kyosho EP Concept, flying on 7–8 × 1.2 Ah NiCad batteries with brushed motors. However, the 540-sized brushed-motors were on the limit of current draw 20–25 amps on the more powerful motors, hence brush and commutator problems were common.
Recent advancements in battery technology are making electric flying more feasible in terms of flying time. Lithium polymer batteries are able to provide the high current required for high performance aerobatics while still remaining light. Typical flight times are 4 -- 12 minutes depending on the flying battery capacity. In the past electric helicopters were used indoors due to the small size and lack of fumes. Larger electric helicopters suitable for outdoor flight and advanced aerobatics have become a reality over the last few years and have become popular, their quietness has made them popular for flying sites close to residential areas and in places such as Germany where there are strict noise restrictions. Nitro helicopters have been converted to electric power by commercial and homemade kits; the smallest remote-controlled production model helicopter made is the Picooz Extreme MX-1 sold at many toy stores, electronics stores and internet stores, costing about $30. The next smallest is the standard Picooz helicopter.
Several models are in contention for the title of the smallest non-production remote-controlled helicopter, including the Pixelito family of micro helicopters, the Proxflyer family, the Micro flying robot. A recent innovation is that of coaxial electric helicopters; the system's simple direction control and freedom from torque induced yaw have, in recent years, made it a good candidate on small models for beginner and/or indoor use. Models of this type, as in the case of a full-scale helicopter, eliminate rotational torque and can have quick control response, both of which are pronounced in a CCPM model. Most cheaper models do not have a swashplate, but instead use a third rotor on the tail to provide pitch control; these helicopters have limited mobility. While a coaxial model is stable and can be flown indoors in tight quarters, such a helicopter has limited forward speed outdoors. Most models are fi
A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry; the process begins by coating a substrate with a light-sensitive organic material. A patterned mask is applied to the surface to block light, so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is applied to the surface. In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened by light, the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed. A positive photoresist is a type of photoresist in which the portion of the photoresist, exposed to light becomes soluble to the photoresist developer.
The unexposed portion of the photoresist remains insoluble to the photoresist developer. A negative photoresist is a type of photoresist in which the portion of the photoresist, exposed to light becomes insoluble to the photoresist developer; the unexposed portion of the photoresist is dissolved by the photoresist developer. Note: This table is based on generalizations which are accepted in the Microelectromechanical systems fabrication industry. Based on the chemical structure of photoresists, they can be classified into three types: Photopolymeric, photocrosslinking photoresist. Photopolymeric photoresist is a type of photoresist allyl monomer, which could generate free radical when exposed to light initiates the photopolymerization of monomer to produce a polymer. Photopolymeric photoresists are used for negative photoresist, e.g. methyl methacrylate. Photodecomposing photoresist is a type of photoresist that generates hydrophilic products under light. Photodecomposing photoresists are used for positive photoresist.
A typical example is e.g. diazonaphthaquinone. Photocrosslinking photoresist is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are used for negative photoresist. Off-Stoichiometry Thiol-Enes polymersFor Self-assembled monolayer SAM photoresist, first a SAM is formed on the substrate by self-assembly; this surface covered by SAM is irradiated through a mask, similar to other photoresist, which generates a photo-patterned sample in the irradiated areas. And developer is used to remove the designed part. In lithography, decreasing the wavelength of light source is the most efficient way to achieve higher resolution. Photoresists are most used at wavelengths in the ultraviolet spectrum or shorter. For example, diazonaphthoquinone absorbs from 300 nm to 450 nm; the absorption bands can be assigned to π-π * transitions in the DNQ molecule. In the deep ultraviolet spectrum, the π-π* electronic transition in benzene or carbon double-bond chromophores appears at around 200 nm.
Due to the appearance of more possible absorption transitions involving larger energy differences, the absorption tends to increase with shorter wavelength, or larger photon energy. Photons with energies exceeding the ionization potential of the photoresist can release electrons which are capable of additional exposure of the photoresist. From about 5 eV to about 20 eV, photoionization of outer "valence band" electrons is the main absorption mechanism. Above 20 eV, inner electron ionization and Auger transitions become more important. Photon absorption begins to decrease as the X-ray region is approached, as fewer Auger transitions between deep atomic levels are allowed for the higher photon energy; the absorbed energy can drive further reactions and dissipates as heat. This is associated with the contamination from the photoresist. Photoresists can be exposed by electron beams, producing the same results as exposure by light; the main difference is that while photons are absorbed, depositing all their energy at once, electrons deposit their energy and scatter within the photoresist during this process.
As with high-energy wavelengths, many transitions are excited by electron beams, heating and outgassing are still a concern. The dissociation energy for a C-C bond is 3.6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed; the resulting scission breaks the original polymer into segments of lower molecular weight, which are more dissolved in a solvent, or else releases other chemical species which catalyze further scission reactions. It is not common to select photoresists for electron-beam exposure. Electron beam lithography relies on resists dedicated to electron-beam exposure. Physical and optical properties of photoresists influence their selection for different processes. Resolution is the ability to differ the neighboring