Photolithography

Photolithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a wafer or substrate. It bears a similarity to the conventional lithography used in printing.

Lithography involves a combination of etching, chemical deposition, and chemical treatments in repeated steps on an initially flat substrate. A part 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 -- is applied on top of the metal layer. The photoresist is selectively hardened by illuminating it in specific places. For this purpose a transparent plate with patterns printed on it, called a mask, is used together with an illumination source to shine light on specific parts of the photoresist. Then, the photoresist that was not exposed to light and the metal underneath is etched away with a chemical treatment. Finally, the hardened photoresist is etched using a different chemical treatment, and all that remains is a layer of metal in the same shape as the mask.

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, a wafer will go through the photolithographic area on the order of 20 to 30 times.

Technology

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 photo-sensitive 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 desired pattern is then projected 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 typcially 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 off of 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 feature sizes on the order of 130-90 nm.

There are indications that 193nm lithography can be extended to feature sizes below 45nm using 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. Dopants may be added to the water to give a higher refractive index.

Also in development are tools that will use the 157 nm wavelength DUV in a manner similar to current exposure systems. In addition, Extreme Ultraviolet (EUV) radiation lithography systems are currently under development which will use 13nm wavelengths, approaching the regime of x-rays.

Currently, there is uncertainty as to the technology succession in lithography. 157 nm equipment, once targeted to succeed 193 nm at the 65 nm feature size node, has now been pushed back to at least the 45 nm node. This has been 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. At the 45 nm node and beyond however, 157 nm technology may face competition from both 193 nm technology and EUV 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.

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 expontentially with every technology generation - means that such a system would be far more cost-effective for small-scale manufacturing applications.