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This article discusses the modern manufacture of semiconductors. For earlier uses of photolithography in printing, please see lithography.
Photolithography or optical lithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a substrate. Often crystalline silicon in the form of a wafer is used as a choice of substrate, although there are several other options including, but not limited to, glass, sapphire, and metal. Photolithography (also referred to as "microlithography" or "nanolithography") bears a similarity to the conventional lithography used in printing and shares some of the fundamental principles of photographic processes.
Photolithography involves a combination of:
- substrate preparation
- photoresist application
and various other chemical treatments (thinning agents, edge-bead removal etc.) in repeated steps on an initially flat substrate.
A cycle of a typical silicon lithography procedure would begin by depositing a layer of conductive metal several nanometers thick on the substrate. A layer of photoresist -- a chemical that 'hardens' when exposed to light (often ultraviolet) -- is applied on top of the metal layer. A transparent plate with opaque areas printed on it, called a photomask or shadowmask, is placed between a source of illumination and the wafer, selectively exposing parts of the substrate to light. Then the photoresist is developed, in which areas of unhardened photoresist undergo a chemical change. After a hard-bake, subsequent chemical treatments etch away the conductor under the developed photoresist, and then etch away the hardened photoresist, leaving conductor exposed in the pattern of the original photomask.
Most types of photoresist are available in "positive" and "negative" forms. With positive resists the area that is opaque (masked) on the photomask corresponds to the area where photoresist will remain upon developing (and hence where conductor will remain at the end of the cycle). Negative resists result in the inverse, so any area that is exposed will remain, whilst any areas that are not exposed will be developed. After developing, the resist is usually hard-baked before subjecting to a chemical etching stage which will remove the metal underneath.
Lithography is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. Its main disadvantages are that it requires a substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.
In a complex integrated circuit, (for example, CMOS) a wafer will go through the photolithographic cycle up to 50 times. For Thin-Film-Transistor (TFT) processing many fewer photolithographical processes are usually required.
A wafer is introduced onto an automated "wafertrack" system. This track consists of handling robots, bake/cool plates, and coat/develop units. The robots are used to transfer wafers from one module to another. The wafer is initially heated to a temperature sufficient to drive off any moisture that may be present on the wafer surface. Hexa-methyl-disilizane (HMDS) is applied in either liquid or vapor form in order to promote better adhesion of the photosensitive polymeric material, called photoresist. Photoresist is dispensed in a liquid form onto the wafer as it undergoes rotation. The speed and acceleration of this rotation are important parameters in determining the resulting thickness of the applied photoresist. The photoresist-coated wafer is then transferred to a hot plate, where a "soft bake" is applied to drive off excess solvent before the wafer is introduced into the exposure system.
The simplest exposure system is a contact printer or proximity printer. A contact printer involves putting a photomask in direct contact with the wafer. A proximity printer puts a small gap in between the photomask and wafer. The photomask pattern is directly imaged onto the photoresist on the wafer in both cases. The resolution is roughly given by the square root of the product of the wavelength and the gap distance. Hence, contact printing with zero gap distance ideally offers best resolution. Defect considerations have prevented its widespread use today. However, the resurgence of nanoimprint lithography may revive interest in this familiar technique, especially since the cost of ownership is expected to be very low. The cost will be low due to the lack of a need for complex optics, expensive light sources, or specially tailored resists.
The commonly used approach for photolithography today is projection lithography. The desired pattern is projected from the photomask onto the wafer in either a machine called a stepper or scanner. The stepper/scanner functions similarly to a slide projector. Light from a mercury arc lamp or excimer laser is focused through a complex system of lenses onto a "mask" (also called a reticle), containing the desired image. The light passes through the mask and is then focused to produce the desired image on the wafer through a reduction lens system. The reduction of the system can vary depending on design, but is typically on the order of 4X-5X in magnitude.
When the image is projected onto the wafer, the photoresist material undergoes some wavelength-specific radiation-sensitive chemical reactions, which cause the regions exposed to light to be either more or less acidic. If the exposed regions become more acidic, the material is called a positive photoresist, while if it becomes less susceptible it is a negative photoresist. The resist is then "developed" by exposing it to an alkaline solution that removes either the exposed (positive photoresist) or the unexposed (negative photoresist). This process takes place after the wafer is transferred from the exposure system back to the wafertrack.
Developers originally often contained sodium hydroxide (NaOH). However, sodium is considered an extremely undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of gate oxides. Metal-ion-free developers such as tetramethyl ammonium hydroxide (TMAH) are now used.
A post-exposure bake is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. The develop chemistry is delivered in a similar fashion to how the photoresist was applied. The resulting wafer is then "hardbaked" on a bake plate at high temperature in order to solidify the remaining photoresist, to better serve as a protecting layer in future ion implantation, wet chemical etching, or plasma etching.
The ability to project a clear image of a very small feature onto the wafer is limited by the wavelength of the light that is used and the ability of the reduction lens system to capture enough diffraction orders from the illuminated mask. Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light with wavelengths of 248 and 193 nm, which allow minimum resist feature sizes down to 50 nm.
Optical lithography can be extended to feature sizes below 50 nm using 193 nm and liquid immersion techniques. Also termed immersion lithography, this enables the use of optics with numerical apertures exceeding 1.0. The liquid used is typically ultra-pure, deionised water, which provides for a refractive index above that of the usual air gap between the lens and the wafer surface. This is continually circulated to eliminate thermally-induced distortions. Using water will only allow NA's of up to ~1.4 but higher refractive index materials will allow the effective NA to be increased.
Tools using 157 nm wavelength DUV in a manner similar to current exposure systems have been developed. These were once targeted to succeed 193 nm at the 65 nm feature size node but have now all but been eliminated by the introduction of immersion lithography. This was due to persistent technical problems with the 157 nm technology and economic considerations that provided strong incentives for the continued use of 193 nm technology. High-index immersion lithography is the newest extension of 193 nm lithography to be considered. In 2006, features less than 30 nm were demonstrated by IBM using this technique. Other alternatives are extreme ultraviolet lithography (EUV), nanoimprint lithography, and contact printing. EUV lithography systems are currently under development that will use 13.5 nm wavelengths, approaching the regime of X-rays. Nanoimprint lithography is being investigated by several groups as a low-cost, non-optical alternative. Contact printing may yet be revived with the recent interest in nanoimprint lithography.
The image for the mask is originated from a computerized data file. This data file is converted to a series of polygons and written onto a square fused quartz substrate covered with a layer of chrome using a photolithographic process. A beam of electrons is used to expose the pattern defined in the data file and travels over the surface of the substrate 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 light in the stepper/scanner systems to travel through.
X-ray lithography can be extended to an optical resolution of 15 nm by using the short wavelengths of 1 nm for the illumination. This is implemented by the proximity printing approach. The technique is developed to the extent of batch processing. The extension of the method relies on Near Field X-rays in Fresnel diffraction: a clear mask feature is "demagnified" by proximity to a wafer that is set near to a "Critical Condition". This Condition determines the mask-to-wafer Gap and depends on both the size of the clear mask feature and on the wavelength. The method is simple because it requires no lenses.
A method of pitch resolution enhancement which is gaining acceptance is double patterning. This technique increases feature density by printing new features in between pre-printed features on the same layer. It is flexible because it can be adapted for any exposure or patterning technique. The feature size is reduced by non-lithographic techniques such as etching or sidewall spacers.
Work is in progress on an optical maskless lithography tool. This uses a digital micro-mirror array to directly manipulate reflected light without the need for an intervening mask. Throughput is inherently low, but the elimination of mask-related production costs - which are rising exponentially with every technology generation - means that such a system might be more cost effective in the case of small production runs of state of the art circuits, such as in a research lab, where tool throughput is not a concern.
- Soft lithography
- Liquid imaging
- ^ Hand, Aaron. High-Index Lenses Push Immersion Beyond 32 nm.
- Semiconductor Lithography — Overview of lithography
- Optical Lithography Introduction — IBM site with lithography-related articles
- Immersion Lithography Article — Shows how depth-of-focus is increased with immersion lithography
- Photolithography — Article showing contact, proximity, and projection lithography