15 Photolithography

S.S. islam

Introduction

Photolithography, or optical or UV lithography, is a microfabrication technique employed to pattern parts of a thin film or bulk of a substrate. The desired geometrical pattern is created on a photomask. This pattern is transferred from the photomask onto a light sensitive material, often termed as photoresist of resist on a substrate. Further chemical treatments are done to engrave the exposed pattern on the substrate. Additionally, a new material can also be deposited in the desired pattern under the photoresist. This technique is widely used in semiconductor industries to produce modern CMOS devices.

Fundamentally, photolithography is somewhat similar to conventional photography in the fact that the pattern or feature in the resist is produced by exposing light onto it. Light can be exposed either directly (without mask), or with a projected image by using the photomask. Printed circuit boards are also produced by following this method with high precision. Following steps of the process are similar to the lithographic printing. This technique is extensively used owing to its ability to create extremely fine patterns (of the order of few nanometers in size). This technique can be used to produce objects with controlled size and shape. It is a very cost effective technique to create patterns over an entire surface. The important limitation of the technique is the requirement of substrates with flat surfaces, and is not an effective process to produce curved or twisted shapes. Additionally, the process is carried out in extreme clean environments.

Historical Development

The term has Greek origin, where photo means light, litho means stone, and graphy means writing. Literally, photolithography is a printing technique (originally used limestone printing plates) wherein light plays a very crucial role. The technique was first used in 1820s by Nicephore Niepce, who used Bitumen of Judea (naturally occurring asphalt) as photoresist. A layer of bitumen over a metal/glass/stone plate became less soluble upon exposing light on it. Thus, it became difficult to remove this layer, whereas the unexposed part could be rinsed with an appropriate solvent. The substrate could be removed by suitable treatments, such as etching out the metal, to produce a patterned printing plate. However, the sensitivity of bitumen towards light is poor and very long durations of light exposure were needed. Nonetheless, since it is economic, and has excellent resistance to acid attacks, it was extensively used even in early 20th century. Oskar Suss, in 1940, developed positive photoresist by using diazonaphthoquinone. Positive photoresist works in opposite manner to the negative one, where an originally insoluble coating becomes soluble after exposure to light. Louis Playback, in 1954, developed dycryl polymeric letter-press plate, thereby accelerating the plate preparation process.

Working Procedure

The photolithography process involves multiple sequential steps which can be summarised as follows:

A. Cleaning: It is the first step in the photolithography process and involves removal of both organic and inorganic contaminants that might be present on the wafer surface. This is usually achieved by wet chemical treatments, such as RCA cleaning method which involves cleaning by the use of solutions of H2O2. Other solutions used to clean the wafers may include trichloroethylene, acetone or methanol.

B. Preparation: The wafer is heated to about 150 °C for about 10 minutes to remove any moisture present on the wafer surface. Previously cleaned and stored wafers must be first cleaned before use. To promote adhesion of the photoresist with the wafer, liquid or gaseous adhesion promoter chemicals, e.g., bis(trimethylsilyl)amine, hexamethyldisilazane (HMDS), etc., are used. In case of SiO2 wafers, the surface reacts with HMDS to produce tri-methylated silicon-dioxide. This compound is extremely repellent towards water and obscures aqueous developer solution from penetrating between the photoresist layer and the wafer surface. This presents the lifting of fine photoresist structures, thereby damaging the pattern to be developed. To avoid this, the wafer is covered and placed on a hot plate to dry at about 120 °C.

Figure 1 Schematic representation of the steps involved in a photolithography process using positive photoresist.

C.   Appliying Photoresist: Photoresist can be applied on the wafer surface by spin coating. To do this, a viscous solution of photoresist is dropped on the wafer and the wafer is spun to produce a uniform layer of the resist on the wafer surface. Spin coating is performed at 1200-4800 rpm for about 30- 60 seconds. The thickness of the developed layer typically varies from 0.5 to 2.5 m. The uniformity achieved with spin coating is within 5-10 nm. Fluid mechanical modelling is applied to explain this uniformity in film thickness. It is found that the speed of movement of photoresist is differential through the film, it moves faster at the top and slower at bottom. The movement of resist is restricted at the bottom due to the viscous forces which bind is to the wafer surface. Therefore, the topmost layer of the photoresist is removed quickly from the wafer edges, while bottom layer creeps slowly in radial directions along the wafer. Thus, any bump or ridge of the photoresist is removed, resulting in a very flat layer. Evaporation of the liquid solvent from the photoresist solution coated over the wafer surface also affects the resulting thickness of the produced films. After application of photoresist, the wafer is heated at around 90-100 °C to remove excess resist solvent, this is usually termed as ‘prebaking’ the wafer.

D. Exposure and Development: After prebaking, the resist is subjected to light exposure. The exposure to light changes the solubility of the photoresist such that some parts of it can be easily dissolved in ‘developer’ solution, analogous to the developer used in photography. In case of the positive resist (most commonly used type of resist), exposed parts become soluble in the develop; whereas in case of negative resist, the unexposed parts can be dissolved and exposed parts of the resist become insoluble.

The wafer with photoresist is again baked to decrease the possibility of standing waves resulting from destructive and constructive interference of the incident light. Deep ultraviolet (or UV) lithography also uses a chemically amplified resist (CAR). UV lithography is more sensitive towards post exposure baking duration, time, as well as delay, since majority of the exposure reactions (such as creating acid, making the polymer soluble in the basic developer) occur during post exposure baking.

The developer solution is also applied via spin coater, just as the resist. Earlier developer solutions mostly included sodium hydroxide (NaOH). Unfortunately, sodium contamination is highly undesirable in MOSFET technology, as it degrades the insulating behaviour of gate oxide. Sodium ions tunnel across the gate, thereby causing changes in threshold voltage of the transistor, such that it may turn ON before or after the threshold. To avoid this, metallic ion free developer solutions have been developed, e.g., tetramethylammonium hydroxide (TMAH).

The wafer after putting developer is hard baked (120-180 °C for 20-30 minutes), to enhance its durability during future treatments (e.g., wet chemical etching, etc.).

E.   Etching: Both wet or dry etching can be performed. Wet etching uses aqueous chemistry while the dry etching uses plasma to remove the uppermost layer from the substrate from the area which is not protected by the resist. Semiconductor manufacturing uses dry etching, because it can be made anisotropic, thereby avoiding considerable undercutting of the photoresist patterns. It assumes great significance when the width of the patterns/features is comparable to the thickness of the material being etched (or aspect ratio is close to one). Wet etching is usually isotropic and is highly desirable in microelectromechanical systems, wherein suspended features are to be released from underlying layer.

The development of anisotropic low defect dry etching has allowed developing very fine features in resist via photolithography, which can be easily transferred to the desired surfaces.

F.   Photoresist Removal: An aqueous ‘resist stripper’ can alter the resist such that it does not adhere to the surface anymore. Another technique to remove the resist is by oxygen plasma treatment, which oxidizes the resist. This process is called ashing, and is similar to dry etching. Alternatively, 1-methyl-2-pyrrolidone (NMP) can also be sued to dissolve the photoresist. After dissolving the photoresist, the solvent can be removed by heating at 80 °C.

Exposure or Printing Systems

Exposure system produces an image onto the wafer surface by using photomasks. A photomask selectively allows the passage of light from some areas and blocks it in the rest of the areas. Alternatively, massless lithography can also be used, wherein a precise beam is directly focused on a wafter without masking it. However, this technique is not a common practice in commercial applications. Exposure systems can be classified with respect to the optics used to transfer the image from the mask to the wafer. These are:

Figure 2 Wafer track section of an aligner using UV light source. The filtered fluorescent lighting in photolithography cleanrooms do not have UV or blue light so as to avoid exposing photoresists. The spectrum of light emitted by such fixtures gives virtually all such spaces a bright yellow color.

a.    Contact and Proximity Systems: These are the simplest type of exposure systems. In case of contact system, the photomask actually touches the wafer and a uniform light it exposed on it. In proximity system, there is a small distance between the mask and the wafer. In both systems, entire wafer is covered by the photomask and the patterns is created simultaneously on each die. Both techniques require illumination of the wafer with uniform light intensity. The mask should be precisely aligned to the features present on the wafer. With increasing dimensions of the wafers in today’s technology, the use of these techniques has become difficult.

Contact type systems may cause damage to the photomask as well as wafer. Due to this reason, this technique is not used at commercial scale. However, owing to the economic equipments and high optical resolutions, these technique are still used in research and prototype processes. Resolution achieved in proximity system is close to the square root of the product of wavelength and gap distance. Since the gap distance is approximately zero, the resolutions achieved in proximity lithography are comparable to the more advanced lithographic technique called ‘projection’. Because of the low costs involved in installation of the systems, these systems can be used in nanoimprint lithography.

b.    Projection Systems: Projection systems are extensively used in VLSI technology as lithography technique. In this technique, only one die or an array of dies (termed as ‘field’) are exposed to light by using projection masks. The mask is projected on the wafer several times to produce the entire pattern.

Photomasks

A computerised data file is used to create the image for the mask. The computerised file can be produced by L-edit, Layout Editor, CleWin etc. This data file is converted into a series of polygons, which are then written on a square shaped fused quartz substrates covered with a layer of chromium using a photolithographic process. A beam of laser (laser writer) or electrons (e-beam writer) can be employed to expose the pattern produced by the data file. The beam is then moved over the substrate surface in either a vector or raster scan manner. Where the photoresist on the mask is exposed, the chrome can be etched away, leaving a clear path for the illumination light in the stepper/scanner system to travel through.

Resolution in Projection Systems

The resolution of these systems is limited by the wavelength of the light used as well as the ability of the reduction lens system to capture enough diffraction orders from the illuminated photomask. Present photolithography equipment employ deep ultraviolet (DUV) light from excimer lasers having wavelength of 248 and 193 nm. Thus the minimum feature sizes can be reached upto 50 nm. Excimer laser based lithography technology is termed as ‘excimer laser lithography’. Owing to the possibility of achieving high resolutions, excimer laser lithography has played a significant role in the continued advancements of the so-called Moore’s Law for past two decades.

Minimum printable feature size by using a projection system can be expressed as:

where, CD is the minimum feature size (or the critical dimension). k1 (or k1 factor) is a coefficient accounting for processing factors, and its typical value is equal 0.4 for manufacturing. The feature size in projection lithography can be decreased by reducing k1, which in turn can be tuned by computational lithography. λ is wavelength of the light, and NA is the numerical aperture of the lens.

From above relation, reducing the wavelength of the light used is an effective alternative to decrease the feature size. Additionally, the numerical aperture can be increased to obtain a tightly focused beam and a smaller spot size, thereby further reducing the feature size. Nonetheless, these design methods are further retained by another parameter, known as the depth of focus (DF), which can be expressed as:

k2 is another coefficient depending upon the process. The thickness of resist and thus the topography of the wafer, both are restricted by depth of focus (DF). The topography of the wafer can be flattened via polishing prior to high resolution lithographic steps.

Light Sources

The earliest photolithography experiments employed UV light from mercury based gas discharge lamps, which also used a combination of mercury with noble gases like Xenon. Such lamps emit light with a broad spectrum with significant peaks in UV range. This spectrum could be filtered to obtain individual spectral line. Mercury lamps used in lithography exhibit spectral lines at different positions, these are: 436 nm (g-line), 405 nm (h-line), and 365 nm (i-line). However, with the advancements in semiconductor industry, higher resolution is required to fabricate denser and faster chips. Additionally, the throughput must be higher, i.e., cost effective designing techniques. The mercury lamp based photolithography could not meet these demands.

The problem was resolved by Kanti Jain, while working at IBM in 1982, when he used excimer laser lithography to obtain higher resolutions. This technology has now been extensively used in the production of microelectronics. Kryton fluoride (248 nm) and argon fluoride (193 nm) lasers are the most commonly used lasers in deep ultraviolet excimer lasers. Since, the excimer lasers work with a certain gas mixture, thus varying the wavelength is not trivial, as the method of generating the new wavelength is completely different, and the absorption characteristics of materials change.

Figure 3 Dependence of resolution on the wavelength of the light used in photolithography.

Feature sizes of as low as 50 nm can be obtained in optical lithography by using argon fluoride excimer laser (193 nm) and liquid immersion methods. This is also called as immersion lithography and allows using optics with NAs above unity. Ultrapure deionized water is generally used, which results in a refractive index higher than air between the lens and wafer. The water is continuously circulated to avoid thermal induced distortions. With using water, NAs of ~1.4 can be obtained. To further increase the NA, fluids having higher refractive index must be used.

Figure 4 The wavelength cannot be made infinitely small, as these wavelength (lower than 185 nm) are absorbed by air.

Using liquid immersion lithography, the feature size can be reached upto 65 nm. These days, high index immersion lithography is being used to achieve further high resolutions. For instance, IBM has recently demonstrated feature sizes of as low as 30 nm.

Certain lasers can be employed for indirect generation of non-coherent extreme ultraviolet (EUV) light having 13.5 nm wavelength for use in extreme UV lithography. Additionally, visible and infrared lasers have also been demonstrated for use in lithography. In these cases, photochemical reactions occur due to multi-photon absorptions. The use of these light sources has applications in creating true 3D objects and process non-photosensitized (pure) glass-like materials with excellent optical resiliency.

Fabrication of Quantum Dots

Quantum dots (QDs) can be produced by photolithography method. However, these QDs have been reported to have dimensions more than 10 nm, and the production of every smaller QDs is difficult to achieve even with the high-frequency UV light. Therefore, electron-beam lithography is used for the synthesis of quantum dots. In this approach a highly focused electron beam is scanned to produce desired patterns onto the surface of the substrate. The surface is usually coated with an electron sensitive film termed as resist. The resist material normally consists of a polymeric compound. If the resist consists of a long chain polymer with high molecular weight (~105 units) it is called as negative tone whereas short chain polymeric resist are known as positive tone. In case of negative tone resist exposure to electron beam induces further polymerization of the chain length but for positive resist a reduction in the chain length occur. The electron beam alters the solubility of the resist, thereby allowing selective removal of either the exposed or unexposed regions of the resist by immersing it in the developer solvent. The purpose is to create very small structures in the resist that can subsequently be transferred to the substrate material, often by etching. The resolution of this electron beam process not only depends on the minimum width of source electron beam but it also depends on the development process. The source of electron beams are usually hot lanthanum hexaboride for lower-resolution systems, whereas, higher-resolution instrument uses field electron emission sources, such as heated W/ZrO2 for lower energy spread and enhanced brightness. The advantages of electron-beam lithography include high degree of flexibility for designing nanostructured systems (such as quantum dots, wires or rings) with resolutions below 10 nm. However, the spatial resolution needed to observe significant quantization effects is much higher.

Figure 5 Synthesis of quantum dots by E-beam lithography

Additionally, a new form of lithography called imprint lithography is best suited for producing QDs. Instead of etching a semiconductor wafer with light, a negative image is formed on a hard SiO2 wafer using electron-beam lithography. This mould is used as a stamp and is physically pressed onto a silicon layer to form the QDs. This technique allows precise positioning and size control of the QDs, and is being investigated for use in classical and quantum computing applications. Lithographic methods and subsequent processing often produce contamination, defect formation, size non-uniformity, poor interface quality, and even damage to the bulk of the crystal itself.

Figure 6 Schematic of patterned synthesis of QDs by imprint lithography.

Questions

1.   Who developed Excimer laser technology?

2.   Discuss the advantages and disadvantages on lithography technique.

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