The Photoresist Process and it’s Application to the Semiconductor Industry
Introduction
Photoresist materials are polymer resins that contain photoactive (light sensitive) compound (PAC) and an alkaline-soluble resin.1 Present-day photoresist and photoetching processes evolved largely from technology developed in the print ing industry. Circuit boards and microelectronics were undreamed of in 1852 hen W.H.F Talbot patented a photoecthing process that could be used on copper. The photoresist used was gelatin sensitized with bicromate salt; ferric chloride solution served as the etchant. Even earlier in 1826 J.N Niepce had found that certain types of asphalt where photosensitive, and he successfully etched patterns in pewter.2
Polymeric materials have found use in the electronics industry in both manufacturing process used to generate today’s integrated circuits and as component structures in the completed devices. The broad applicability of polymers arises from the ab ility to design and synthesize these materials with the precise functionalities and properties required for a given application. Polymeric materials have been used as lithographic imaging materials called resists.3
Radiation sensitivity (ultraviolet light) is the key property required of materials used for imaging the individual elements of an integrated circuit. Known as the microlithograhic process, it is the technology used to fabricate electronic device s and is critically dependent on the polymer-organic materials chemistry used to generate the radiation-sensitive imaging material know as photoresist. As the lithographic technologies evolve to allow fabrication of the smaller and more compact circuit el ements, new resist chemistries and processes will be needed.3
The intense drive towards designing and fabricating integrated circuits having individual elements, which are less than 0.3m m is a global effort. The microelectronics industry is driven by the need to build devices, which contain an increasing number of individual circuit elements on a semiconductor material (i.e. silicon wafer). The increased density enables the device to per form more functions in a shorter period of time than previously possible while maintaining a constant surface area. The ability to decrease the functional size is critically dependent upon the technologies involved in the delineation of the circuit patter n. These technologies are part of the overall microlithograhic fabrication process of photoresist.3
Photoresist Process
There are two types of photoresist: positive and negative. Positive resist are different from negative resist in response to actinic light and the resulting image, although the essential composition is similar; each contain sensitizes, resin, solve nts, and additives. Positive resist is colored and soluble in strongly alkaline solutions. They develop in mildly alkaline solutions. General chemical resistance is less than the negative resist and positives are more costly to produce. However, images fr om this resist are extremely accurate, require minimal processing technique, and involve few processing steps.2
Negative acting resist characteristically have high chemical resistance and good image reproduction qualities and are of low cost. They are widely used in the manufacture of circuit boards and microelectronic devices for these reasons. Due to the ir high chemical resistance, the negative acting resist are generally more difficult to remove than other resist.2
Positive -acting photoresist is applied uniformly in a thin layer on the circuit board or other base material and dried thoroughly. A photomask with opaque image areas delineating the desired pattern is brought in close contact with the photoresist sur face. With the photomask in place, the resist is exposed to a light source rich in ultraviolet radiation. The resist film beneath the clear areas of the photomask undergoes a chemical change that renders it soluble in the developing solution. After exposu re, the resist – coated base is immersed in the developing solution, which dissolves the exposed areas. The resist images may than be baked for greater chemical resistance and prepared.2
Negative acting photoresist behave in just the opposite manner. Negative acting photoresist is applied uniformly in a thin layer on the circuit board or other base material and dried thoroughly. A photomask with transparent image areas delineatin g the desired pattern is brought in close contact with the photoresist surface. With the photomask in place, the resist is exposed to a light source rich in ultraviolet radiation. The resist film beneath the clear areas of the photomask undergoes a physic al and chemical change that renders it insoluble in the developing solution. After exposure, the resist - coated base is immersed in the developing solution, which allows the unexposed areas to be removed without excessive effect on the hardened or expose d area. The resulting image can be further hardened by baking or rendered more visible by dyeing or both. The base can then prepared.2 See figure 1 for overview of Photoresist process.
Figure 1.4
Photolithography Process
A common technique for growing crystal silicon involves the selective cooling of molten polycrystalline silicon so that solidification occurs in a particular crystal direction. The single crystal silicon is pulled from the "melt" as it gr ows. This technique is known as the Czochralski method of the crystal growth. The silicon wafer is used as the circuit board.4
Photolithography is the process of transferring geometric shapes on the mask to the surface of a silicon wafer. It is a part of the Microlithography process. The steps involved in the photolithography process are wafer cleaning; barrier layer fo rmation; photoresist application; soft baking; mask alignment; exposure and development; and hard –baking.4
The wafers are chemically cleaned to remove particulate matter on the surface as well as any traces of organic, ionic, and metallic impurities. After cleaning, silicondioxide, which serves as a barrier layer, is deposited on the surface of the wa fer. After the formation of the silicon dioxide layer, photoresist is applied to the surface of the wafer. High speed centrifugal whirling of silicon wafers is the standard method for applying photoresist coatings in manufacturing. This technique, know as "spin coating", produces a thin uniform layer of photoresist on the wafer surface.4
Soft baking is the step during which almost all of the solvents are removed from the photoresist coating. Soft baking plays a very critical role in photo imaging. The photoresist coatings become photosensitive, or imageable, only after softbakin g. Over softbaking will degrade the photosensitivity of resist by either reducing the developer solubility or actually destroying a portion of the sensitizer. Undersoft baking will prevent light from reaching the sensitizer. Positive resist are incomplete ly exposed if considerable solvent remains in the coating. This undersoft-baked positive resist is then readily attacked by the developer in both exposed and unexposed areas, causing less etching resistance.4
One of the most important steps in the photolithography process is mask alignment. A mask or photomask is a square glass plate with patterned emulsions of metal film on one side. The mask is aligned with wafer, so that the pattern can be transfer red onto the wafer surface. Each mask after the first one must be aligned to previous pattern. Once the mask has been accurately aligned with the pattern on the wafer surface, the photoresist is exposed through the pattern on the mask with a high intensit y ultraviolet light. There are three primary exposure methods: contact, proximity, and projection.4
In contact printing, the resist coated silicon wafer is brought into physical contact with the glass photomask. The wafer is held on a vacuum chuck, and the whole assembly rises until the wafer and mask contact each other. The photoresist is expo sed with UV light while the wafer is in contact position with the mask. Very high resolution is possible in contact printing (1-micron features in 0.5 microns of positive resist). The problem with contact printing is that debris, trapped between the resis t and mask can damage the mask and cause defects in the pattern. 4
The proximity exposure method is similar to contact printing except that a small gap, 10-25microns wide, is maintained between the wafer and the mask during exposure. This gap minimizes mask damage. Approximately 2 to 4 micron resolutions are po ssible with proximity printing. 4
Projection printing avoids mask damage entirely. An image of the pattern on the mask is projected onto the resist –coated wafer, which are many centimeters away. In order to achieve high resolution, only a small portion of the mask is imaged. This smal l image field is scanned or stepped over the surface of the wafer. Projection printers that step the mask image over the wafer surface are called step and repeat systems. Step and repeat projection printers are capable of approximately 1-micron resolution . 4
Hard baking is the final step in the lithographic process. This step is necessary in order to harden the photoresist and improve adhesion of the photoresist to the wafer surface. 4
Microlithography (Microprocessing)
Microprocessors are built in layers on silicon wafer through various processes using chemicals, glasses and light. On the wafer, exposing it to extreme heat and gas grows the first layer of silicon dioxide. This growth is similar to the way rus t grows on metal when exposed to water. The silicon dioxide on the wafer, however grows much faster and is too thin to be seen by the naked eye. The wafer is then coated with a photoresist. The photoresist becomes soluble when exposed to ultraviolet light . Ultraviolet light is passed through a patterned photomask. The mask protects parts of the wafer from the light. The light turns the exposed areas to a gooey layer of photoresist. Each layer on the microprocessor uses a mask with a different pattern. The gooey photoresist is completely dissolved by a solvent. This reveals a pattern of photoresist made by the photomask on the silicon dioxide. The revealed silicon dioxide is etched away with chemicals. The rest of the photoresist is removed. This process l eaves ridges of silicon dioxide on the silicon wafer base to begin another layer, a second thinner layer of silicon dioxide is grown over the ridge and etched areas of the wafer base. Then, a layer of polysilicon and another layer of photoresist are appli ed. Ultraviolet light is then passed through a second mask exposing a new pattern on the photoresist. The photoresist is dissolved with solvent to expose the polysilicon and silicon dioxide, which are then etched away with chemicals. The remaining photore sist is removed, leaving ridges of polysilicon and silicon dioxide. Through a process called doping the exposed areas of the silicon wafer are bombarded with various chemical impurities called ions. Ions are implanted in the silicon wafer to alter the way silicon in these areas conducts electricity. The layering and masking processes are repeated, creating windows that allow for connections to be made between the layers. Atoms of the metal are deposited on the wafer, filling the windows. Another masking a nd etching stage leaves strips of metal that make the electrical connections. Roughly 20 layers are connected to form the microprocessors circuitry in 3-dimensional structure. The exact number of layers on the wafer depends on the design of the microproce ssor.5 The Process is outlined below in figure 2(steps 1-7).6
Step 1: Silicon wafer with dioxide layer 6
Step 2: Photoresist layer placed on wafer 6
Step 3: Ultraviolet light through mask 6
Step 4:Etching (exposed photoresist removed) 6
Step 5: Photoresist Hardened 6
Step 6: Doping 6
Step 7: Final Microprocessor 6
Resist Chemistry
Positive and Negative photoresist can be classified as one or two- component systems. A one-component system is usually based upon a polymer that undergoes a photochemical reaction. In a two-component system a sensitizer molecule (monomeric) is dissolved in an inert polymeric matrix. The sensitizer undergoes the photochemical change. Common positive photoresist consists of a phenolic resin matrix and a diazonaphthoquinone sensitizer. Polymethyl methacrylate (PMMA) is a classical one-compon ent positive resist.9
Examples of Resist Chemistry
(a) Two component negative resist 9
Matrix resin: poly cis-isoprene
Sensitizer: bisazide
The bisazide sensitizer under radiation gives nitrine + nitrogen
The nitrines react to produce polymer linkages and three-dimensional cross-linked structures that are less soluble in the developer solution 9
(b) Two component positive resist 9
Matrix resin: Phenol-formaldehyde copolymer (novolak)
Sensitizer: diazoquinone
The sensitizer is randomly distributed in the polymer matrix. Exposure to radiation renders matrix regions locally soluble in base 9
(c) One-component positive resist 9
Polymer: polybutene-1-sulfone
Radiation leads to chain scission, and hence a reduced molecular weight. This produces a more soluble material 9
(d) One component negative resist 9
Polymer: copolymer of glycidyl methacrylate and ethyl acrylate
A cross linking reaction is initiated by e-beam radiation the presence of an anion: 9
This propagates to lead to insoluble high-molecular-weight material 9
Technological Advances
Patterning polished wafers with an integrated circuit requires the use of Photoresists materials that form thin coatings on the wafer before each step of the photolithographic process. Modern photoresists are polymeric materials that are modifi ed when exposed to radiation (either in the form of visible, ultraviolet, or X – ray photons or in the form of energetic electron beams). A photoresists typically contains a photoactive compound (PAC) and an alkaline-soluble resin. The PAC, mixed into the resin renders it insoluble. This mixture is coated onto the semiconductor wafer and is then exposed to radiation through a photomask that carries the desired pattern. Exposed PAC is converted into an acid that renders the resin soluble, so that the resis t can be dissolved and the exposed substrate beneath it chemically etched or metallically coated to match the circuit design. 1
Practical properties of the resist are shelf life, cost, and availability. Key properties of the Photoresists include purity, etching resistance, resolution, contrast, and sensitivity. As the feature size of integrated circuits shrink, Photoresists mat erials are challenged to handle shorter wavelengths of light. The photolithography of current design is based on ultraviolet radiation in the wavelength of 365-436 nanometers, but, in order to define accurately the smaller features of future circuits less than .25micrometer, shorter wavelengths will be necessary. The problem is that electromagnetic radiation in such frequency regions is weaker. One solution is to use the chemically amplified photoresists, or CAMP. Its quantum efficiency or the number of c hemical events that occur when a photon is absorbed by the material measures the sensitivity of a Photoresists. In CAMP material, the number of events is dramatically increased by subsequent chemical reactions, which means that less light is needed to com plete the process. 1
Photolithography using 193-nm light may to be a viable route for the extension of optical lithography to the dimensions required for the manufacture of 1GB DRAM and advanced CMOS microprocessors with 180-140nm minimum feature size. Deep UV lithog raphy has been developed to scale minimum feature size of devices on semiconductor chips to sub half-micron dimensions. Recent advances in the chemistry of negative – resist systems have provided materials with wide processing latitude and high resolution that are used to manufacture advanced CMOS devices and achieve high aspect ratio patterns for micromachining applications.7
Old negative resist design where based upon free-radical –initiated photocross – linking or photopolymerization processes. The newly developed lithography tools for the semiconductor industry used the output of the mercury arc lamps in the near UV at w avelengths of 365nm to 436nm. They require a photosensitive system that would be able to form pinhole free thin films that where resistant to acids and bases used to pattern devices, with adhesion to unique semiconductor surfaces and metals. Kodak introdu ced a system called KTFR consisting of a bis-aryldiazide photosensitive cross-linking agent, which absorbed the near UV, with a polyisoprene cyclized polymer to provide the necessary film forming and adhesion properties. 7
A new positive resist material was developed based on diazo chemistry that had been patented by Azoplate. As opposed to KTFR, where the differentiation in the developing solvent between the exposed and unexposed regions lies upon a molecular weight inc rease in the system through cross-linking, diazo-type resist depend upon a dramatic change in polarity to achieve differential polarity. The basic resist is a two component system where low –molecular wieght phenolic-based resin is mixed with a diazoketon e derivative. The phenolic resin provides excellent film forming properties and is highly soluble in basic solution. The addition of diazonapthoquinone photosensitizers
acts as a dissolution inhibitor, and dramatically reduces the solubility of the unexposed film in basic solution.7
As limitations of the conventional optical lithography approach, potential extensions of current technology are examined more closely. On of these extensions is to limit the Photoresists thickness that is needed for recording the imaging info rmation. Because the low etch resistance of resist typically precludes the use solely of resist utilizing very thin film, a variety of alternatives have been explored. These range from elaborate trilayer schemes to relatively simple processes such as top –surfacing imaging and a number of combinations thereof. In all of these systems the aim is to limit the imaging resist thickness to a thin layer by confining the radiation near the surface of the resist. This improves process latitude and also reduces re flective notching and thin film interference effects. 7
Other advances in Photoresists technology include modifiers for improving the performance of Photoresists made of Novalac type phenol resins such as m-cresol,2,5-xylenol, 2,3,5-trimethylphenol and more complex phenolic compounds for high resolution I-l ine technology. Specialty acrylic monomers that provide transparency and anti dry etching properties such as tetracyclododecyl acrylate. And other monomers for Kr-F excimer technology such as meta and para acetoxystyrene.8
A wide variety of Bis-phenols, tris-phenols and tetrakis phenols that react with napthoquinone diazide form photo initiators. Additives, such as low molecular weight compounds are intentionally incorporated to achieve better performance. Addi tives for longer shelf life such as antioxidants that have similar structures to the Photoresists are used. 8
Conclusion
Over the past thirty years, chemist and engineers have been able to provide a wide variety of resists, both negative (insoluble upon exposure to radiation) and positive (soluble upon exposure to radiation) to answer the needs of a growing indus try. Negative resist, currently comprise the largest segment of the photoresist market. The choice of whether to use negative or a positive resist system depends on the needs of the specific application such as resolution, ease of processing, and cost. Ne gative resist continues to dominate the fabrication of printed wiring boards, where manufacturing throughout and cost are paramount issues. Positive resists are largely used for the patterning of high-resolution semiconductor device, although advances in the resolution capability of some negative systems, and the advantage of patterning them on certain device levels have generated increased interest and usage.
References
1) www.britannica.com/bcom/eb/article