Diazonaphthoquinone Based Resists Pdf 15
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In the middle of the 20th century thus a negative and a positive photoresist were available which also initiated the development of integrated circuits (transistors, microchips). Since then, dozens of resists have been developed which allow for a wide variety of applications.
While photoresists were primarily used in the manufacture of printed circuit boards [6], printing plates [7], transistors [8] and, beginning, also of integrated circuits in the 1970s and 1980s, resists are today applied in a wide variety of fields of the electronics industry. The focus is set by microelectronics and semiconductor production which accounts for almost 50 % of the resists used worldwide [9]. The circuit board industry consumes about 30 %; the remaining 20 % are spread across different industrial areas such as optoelectronics, sensor technology, electroplating, optical industry, micro-electro-mechanical systems (MEMS), nanotechnology, nanoimprinting, TFT displays (thin-film transistors) and photomask production [10]. Special processes and technologies for most applications were developed, opening up an almost inexhaustible field of industrial uses. And, not to forget, a significant amount of resists are also required for research purposes.
The multitude of conceivable applications explains the wide range of technology-specific lithography processes today, but all of them are still based on the photolithography standard process, which is in detail described in the following section.
In most coating procedures, a pre-treatment of the substrates (mostly silicone, but also gallium arsenide, metals, glass etc.) is required. Even brand-new silicon wafers carry OH groups on their surface, rendering the surface hydrophilic (Fig. 4). Since most resists are hydrophobic, the hydrophilic properties of the wafers must be eliminated. This is often done by vapour deposition of HMDS (hexamethyldisilazane) at 160 C.
HMDS reacts with the OH groups of the silicon wafer and creates a hydrophobic surface (Fig. 4) which facilitates resists adhesion. Alternatively, also a short dip in dilute hydrofluoric acid removes the hydrophilic groups on the silicone surface reliably. In this case however, the wafer must be processed quickly since otherwise new OH groups will form on the surface due to the air humidity.
So far, exclusively substrate coating with liquid resists has been described. For the coating of printed circuit boards with a market share of to date 30 %, basically exclusively dry films [18] are used, except from a few special microelectronics applications, e.g. as protective layers. Solid resists are offered as films (Fig. 9a) that are laminated onto circuit boards and thermally adhered to the substrate (so to speak, ironed-on, Fig. 9b). Only weakly alkaline solutions are required as developer, which is environmentally friendly. A further advantage is that basically the entire material is utilisable, due to only minor losses during the coating procedure (in comparison: during spin coating, 95 % of the resist is wasted). However, a certain film thickness is required for mechanical stability. With a usual layer thickness of 50 µm, resolutions of only up to 25 µm can be achieved; special films with a thickness of 5 µm allow for a resolution of up to 5 µm [19].
During irradiation, the exposed parts of a resist film are chemically changed and become soluble in the case of positive resists, while negative resists are rendered insoluble upon irradiation (see section 4).
Interference lithography is a rather rarely used method for patterning. The principle is the same as in interferometry [22, 23] or in holography [24]. The superimposition of two or more coherent light waves leads to the formation of a periodic interference pattern which consists of a series of intensity maxima and minima (superimposition or extinction) and can be recorded in light-sensitive layers (photoresists) (Fig. 13 and 14). This process can even be used for films with a thickness of up to 100 µm [25]. Suitable resists are e.g. SU-8, AZ 9260 or CAR 44.
During development, those areas of the resist which have become soluble upon exposure (positive resists) or areas soluble from the start (negative resists) are completely dissolved by the developer and only the desired (insoluble) structures remain. The optimal result of the development depends on the concentration of the respective developer and the development time, and both parameters must be precisely maintained. Three methods for development are commonly used: For immersion development, single wafers or cassettes containing several wafers are immersed in a developer bath, mechanically agitated, quickly removed from the bath and rinsed after the time has elapsed. For puddle development, the wafer is placed on a spin coater and a defined amount of the developer is deposited on top of the wafer. The wafer rotates gently back and forth in half-second intervals, the developer is then quickly spun off and the wafer rinsed with water. For spray development, the wafer is likewise placed on a spin coater and the developer solution is applied onto the wafer at moderate spin speeds using a spray nozzle. At the end of the development, the wafer is rinsed with water at higher spin speeds.
The majority of photoresists and many e-beam resists are developed with aqueous-alkaline developers which either contain metal ions or are metal ion-free (MIF). Metal ions interfere with many semiconductor processes; they modify the properties if e.g. a subsequent plasma etching step after development is intended. Metal ions (mainly sodium or potassium) are incorporated into the semiconductor material and change the microelectronic properties. For this reason, preferably MIF developers based on TMAH (tetramethylammonium hydroxide) are used. These developers are fast and aggressive, which is generally beneficial for the efficiency of the process. If however a higher contrast and/or resolution is aimed at or if very thick resist films are developed, developers containing sodium hydroxide, potassium hydroxide, phosphates or borates are often favoured.
Resist structures no longer required are in most cases removed at the end of the lithography process, unless (in rare cases) they do not interfere with subsequent steps. The resist structures themselves are only used for a few applications, e.g. in microfluidics [30] when micro-channels or chemical mini reactors (reaction volume < 1 ml) are fabricated from the resist. Several organic solvents or strong aqueous-alkaline solutions are available for removal. If resists were processed according to standard conditions, the removal generally poses no problem and can in many cases even be achieved with acetone. But if resist structures were previously exposed to high temperatures (> 180 C), intensive plasma etching processes or high-temperature implantations, stripping may be difficult or almost impossible. In this case, removers that can be heated up to 80 C (e.g. N-ethyl-2-pyrrolidone, AR 300-72) might work, possibly with additional support by ultrasound treatment. Otherwise, only intensive oxygen plasma remains as final solution to remove the unwanted resists.
Photolithography is the most important technique for the fabrication of microstructures and utilises photo- and radiochemical imaging methods. Apart from this, three other methods exist that are not primarily based on a direct structuring with light: Thermal scanning probe nanolithography, block copolymer self-assembly imaging method and imprint imaging. For the sake of completeness, these methods are briefly described at the end of this section.
To optimise the performance of microchips, one goal is to increase the integration density of the circuits by producing ever smaller structures. The size of the structures that can be realized also depends on the exposure wavelength; the shorter the wavelength of the light, the higher the resolution. Due to technical reasons, the first generation optical lithography used broadband UV with the typical wavelengths of 435 nm (g-line), 405 nm (h-line) and 365 nm (i-line) of the Hg spectrum (see Fig. 18). In the course of the further development, devices and resists emerged that could use the short-wave range down to 126 nm.
Broadband UV wavelengths are applied for the exposure of many different resists, most commonly positive resists based on naphthoquinonediazides or novolaks. But also some other resists (positive, negative, image reversal, chemically amplified or not chemically amplified) for special applications are exposed to broadband UV. Broadband UV lithography is thus the imaging technology with the greatest variety of applicable resist and processes. The highest resolution achieved with this lithography process is currently in a range of 0.25 µm.
KrF lithography is mainly characterised by the use of a powerful krypton fluoride (KrF) exciplex (excimer) laser [31, 32]. Due to their specific absorption properties, resists suitable for this technology are mostly chemically enhanced poly(hydroxystyrenes) with highest absorption (i.e. sensitivity) at a wavelength of 248 nm. Meanwhile, resolutions of 0.13 µm can be achieved with this technology.
If argon fluoride (ArF) excimer (exciplex) lasers with a wavelength of 193 nm are used for irradiation, the resolution can be improved down to 65 nm (0.065 µm). Resists suitable for such applications are chemically reinforced and based on polyacrylates, polycycloolefins and polycycloolefin/maleic anhydrides. Two different methods are currently applied for this purpose; one method uses air between the last lens and the resist layer (dry lithography), the other method uses water instead (water immersion lithography) (Fig. 19). Due to the improved optical properties (i.e. the refractive index), water allows for a greater depth of contrast [33].
Ion beam lithography is similar to e-beam lithography in that the method itself and the equipment are identical, but the emitting source (ions instead of electrons) and the deflecting lens system differ. Since the energy of focused ions is higher than that of electrons, less sensitive resists can also be successfully used in ion beam lithography with acceptable writing times. Due to the high energy input, an even better resolution is possible [44, 45]. 153554b96e
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