Lithography comes from the Greek words lithos, which means ‘stone’, and graphein, which means ‘to write’. It is the process used to print by transferring a mirror image of the pattern from the flat surface. It was developed in 1976 by Alois Senefelder, a German writer and actor, as a cheaper method of printing theatrical publications. Since then, lithography has been one of the preferred printing techniques, especially in publication companies.
Lithography is also used in semiconductor industry as a method for printing the layout of integrated circuits onto the wafer. The layout is patterned in a mask and then transferred to a light sensitive layer of the wafer substrate. This lithography process used in microfabrication of integrated circuits is called photolithography (sometimes referred to as optical lithography, or UV lithography).
Photolithography involves several steps as demonstrated in Figure 1. Each of these steps is briefly discussed below but this article will focus more on the light sources used during mask exposure.
1. Surface Preparation (Wafer Cleaning, Adhesion Promotion)
The patterns in photolithography are formed in a light sensitive material called photoresist. However, photoresists usually do not adhere very well on the wafer surface. Thus, prior photoresist application, the wafer surface is treated with wet chemicals to remove any contaminations on it (i.e. atmospheric dust, coarse particles, and bacteria) and with liquid or gaseous adhesion promoter to assist photoresist coating.
2. Photoresist Application (Spin Coating)
Photoresist is applied on the wafer through spin coating wherein the wafer, with liquid photoresist solution dispensed on it, is spun rapidly. As the wafer spins the excess resist are tossed out of it, leaving a solid resist with a thickness of 0.1 to 2.0 micrometer.
3. Pre-Bake (Soft Bake)
After spin coating, the photoresist film is baked to evaporate the residual solvent on it and to densify the resist layer. This step usually thins out the resist layer by 25% of the original thickness.
An integrated circuit is comprised of layers of different patterns properly aligned to one another. To ensure accurate overlaying of each layer, every photolithography process undergoes alignment procedure.
5. Exposure and Development
Exposing the photoresist to light induces photochemical reactions on it, wherein a portion of the resist becomes soluble to a chemical solution known as developer. There are two types of photoresists: positive and negative photoresists. Positive photoresists are typically insoluble to developer but once exposed to light become soluble. Negative photoresists, on the other hand, are soluble to developer when unexposed and become insoluble when exposed to light.
In IC microfabrication, the pattern is reproduced in the photoresist film by selectively exposing the resist with light. The master IC pattern is projected through a glass sheet called photomask covered by an opaque material like chromium, in which the pattern is laid out. The projected image is transferred to the photoresist, forming an image of the pattern onto the wafer
6. Post-Bake (Hard Bake)
After the photoresist exposure, the wafer is baked to remove any traces of the developer in the photoresist film, and to solidify and stabilize the developed film. Post bake is an optional step and is not important for cases that require soft photoresist like metal lift off patterning.
7. Photoresist Removal (Stripping)
Once the developed layer is stabilized, the photoresist must be stripped off the substrate using a solution that removes the adhesion of the photoresist on the substrate. Another method of photoresist removal is by etching it with oxygen, and then oxidizes it.
Light Sources in Photolithography
Image projection from photomask to the wafer is executed on a tool called wafer stepper. To meet the trend on smaller, faster and more complicated chips, wafer steppers must print at higher resolution with cost-effective throughput by using narrow bandwidth and intense light sources, which has limited the types of light that can be used in photolithography.
Mercury Arc Lamps
Earlier steppers are operated using the light from mercury-arc lamps at spectral lines of g-line. Mercury g-line is a visible blue light with a wavelength of 436 nanometers. But as the size of the transistors shrunk, the need to print at finer features forced the manufacturers to shift to the h-line spectrum at 405 nanometers of wavelength and then later to i-line at 365 nanometers.
Mercury arc lamps maintain a steady state output through a constant current at DC voltage of 50 to 150 Volts. The process starts by plasma ignition at high frequency of 10 kilovolts or higher across its positive (cathode) and negative (anode) electrodes. This ionizes the inert gas (mercury and argon or xenon) inside the bulb, then evaporates and contributes it to the discharge. However, the resulting plasma does not have enough energy to support high voltage conduction; hence, voltage will drop between 50 to 150 Volts.
Operating at high electric charge can increase the temperature inside the bulb to as high as 2000 °C. At this level there is a possibility that condensed form of the inert gas will accumulate at the sides of the bulb with lower temperature. As a preventive measure against condensation, reflective coatings were placed at both ends of the bulbs. Moreover, for the electrodes to survive such temperature, electrodes made from refractory metals like tungsten were used in the bulb.
The two catastrophic failures that the mercury arc lamps usually experience are light output degradation and bulb fracture. As the bulb conducts, some of its electrodes get deposited inside the bulb, which reduces light output and lowers the stepper’s throughput. In addition to that, the pressure inside the bulb can exceed 30 atm as the lamp operates, increasing the possibility for the bulb to crack. To maintain the efficiency of the stepper and to prevent lamp explosion, the lamp is replaced after several hours of operation.
Aside from lamp replacement, mercury arc lamp users also employed air exhaust adjustment as a safety measure against catastrophic failures. In mercury arc lamps, only less than 1% of the power supplied were converted to actinic light (the light usable for photolithograph) while the rest are used by the exhaust air adjustment. Air exhaust adjustment removes any excess mercury in case of lamp explosion. Moreover, it improves the stability of light output by monitoring and controlling lamp temperature. Voltage and current are also monitored and controlled as part of failure prevention procedures. Lastly, since some of the materials in the mercury arc lamp are hazardous to the environment like the toxic mercury and radioactive thorium, chemical handling and disposal and procedures are established and properly executed by the users.
In mid 1990s, deep UV (DUV) mercury -xenon arc lamps emerged. This type of DUV system have a wavelength of 200 to 240 nanometers. However, as Moore’s Law predicted, the number of transistors in an integrated circuit continued to double over time (approximately every two years) and mercury arc lamps can no longer support such requirement. Thus, when excimer lasers were proposed in photolithography, most of the mercury arc lamp users shifted to this technology.
Excimer lasers offered a better resolution than mercury arc lamps. With a bandwidth requirement of one picometer or less, the industry was able to shrink the transistors to below 45 nanometers using excimer lasers. Excimer lasers are gas-based, pulsed light systems, producing light through pulses at a rate of 4 kHz. Pulsing at higher rates allows emission of an intense amount of light at a short period of time without inducing any damage on the optical components. The system uses mainly inert (Kr, Ar, Xe) and halide (F, Cl) gases charged with strong electric field. In photolithography, one of the most commonly used excimer laser is the KrF laser or krypton fluoride laser. The repetition rate of KrF lasers is 4 kHz at a duty factor of 10% can be equated to about 12.6 billion pulses/year.
Excimer lasers are operated at larger and more complex machines than mercury arc lamps. These systems are often installed far away from the stepper, outside the clean room, to save clean room floor space. Because of their intrinsic directionality, excimer lasers can be placed as far as 25 meters away from the stepper. The light is delivered by a beam delivery unit – an optical path from the laser to the stepper comprised of a series of mirrors and lenses aligned along the path, cancelling any significant loss of energy travels, which may have been brought by the distance that the light travels. The mirrors in the unit bends the beam to avoid any obstructions along the path. While the optical lenses takes care of the divergence of the laser. Proper alignment is maintained by the beam delivery unit through mirror controls and active beam sensors. Moreover, its lenses have antireflective coatings to maintain the light intensity and beam alignment. Beam delivery units were enclosed and flushed with nitrogen to keep the lasers from any contaminants. In Class-I laser systems, this enclosure also serves as a protection for the lenses and mirrors from high intensity light damage and as a safeguard for the manufacturing operators from DUV light that wanders away from the path.
One major barrier on production usage of excimer lasers are the fallouts on electrical components because of the stress induced by the high voltage requirement of the machine. Fortunately, the 20 kilovolts supply needed to produce an electrical discharge that will generate laser light was later reduced to 12 to 15 kilovolts. This voltage reduction has minimized the stress on electrical components, improving the laser’s reliability. Moreover, improvements were also seen during the evolution of laser technology from thyrathrone-based electronics to solid-state electronics.
Like mercury arc lamps, excimer lasers were also been challenged by possible cracking, which is in laser’s case, glass damage at higher power. To minimize glass damage, peak energy was reduced by lengthening the light pulses. But it should be carefully taken note that the system’s total energy should not be affected by the peak energy reduction. Moreover, longer pulses will also reduce the laser’s bandwidth.
Another concern on excimer lasers is increasing their repetition rate further than 4 kHz. Its repetition rate is limited by the facility of gas replacement on the electrodes. Fluorine gas must be continuously replenished because of the high atomic level of fluorine that will cause instabilities on the system. The fluorine gas used in one pulse must be replenished before the next cycle. If the repetition rate is further increased, the time between pulses will decrease, which means gas replenishment must be performed at a faster rate. But there are reliability concerns in faster replenishment rates one of it is the need for better fans and motors. Over time, gas refilling has improved by replacing the materials used as insulator and seals from organic materials like Teflon to ceramic and pure metal materials.
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