The carboxylic acid product is very soluble in the basic developer. Thus, a spatial variation in light energy incident on the photoresist will cause a spatial variation in solubility of the resist in developer. Contact and proximity lithography are the simplest methods of exposing a photoresist through a master pattern called a photomask Figure Contact lithography offers high resolution down to about the wavelength of the radiation , but practical problems such as mask damage and resulting low yield make this process unusable in most production environments.
Proximity printing reduces mask damage by keeping the mask a set distance above the wafer e. By far the most common method of exposure is projection printing. Lithographic printing in semiconductor manufacturing has evolved from contact printing in the early s to projection printing from the mid s to today.
Projection lithography derives its name from the fact that an image of the mask is projected onto the wafer. In fact, these lenses have become so perfect that lens defects, called aberrations, play only a small role in determining the quality of the image. Such an optical system is said to be diffraction-limited, since it is diffraction effects and not lens aberrations which, for the most part, determine the shape of the image. There are two major classes of projection lithography tools — scanning and step-and-repeat systems.
Scanning projection printing, pioneered by the Perkin-Elmer company [1. Exposure dose is determined by the intensity of the light, the slit width, and the speed at which the wafer is scanned.
These early scanning systems, which use polychromatic light from a mercury arc lamp, are , i. Step-and-repeat cameras called steppers for short expose the wafer one rectangular section called the image field at a time and can be or reduction.
These systems employ refractive optics i. Both types of systems Figure are capable of high-resolution imaging, although reduction imaging is required for the highest resolutions. Steppers have continued to dominate lithographic patterning throughout the s as minimum feature sizes reached the nm levels. However, by the early s a hybrid step-and-scan approach was introduced by SVG Lithography, the successor to Perkin-Elmer. The step-and-scan approach uses a fraction of a normal stepper field for example, 25mm x 8mm , then scans this field in one direction to expose the entire 4 x reduction mask.
The wafer is then stepped to a new location and the scan is repeated. The smaller imaging field simplifies the design and manufacture of the lens, but at the expense of a more complicated reticle and wafer stage.
Step-and-scan technology is the technology of choice today for below nm manufacturing. Scanners and steppers use different techniques for exposing a large wafer with a small image field. Resolution, the smallest feature that can be printed with adequate control, has two basic limits: the smallest image that can be projected onto the wafer, and the resolving capability of the photoresist to make use of that image.
In the meantime, projection tool numerical apertures have risen from 0. Before the exposure of the photoresist with an image of the mask can begin, this image must be aligned with the previously defined patterns on the wafer. This alignment, and the resulting overlay of the two or more lithographic patterns, is critical since tighter overlay control means circuit features can be packed closer together.
Closer packing of devices through better alignment and overlay is nearly as critical as smaller devices through higher resolution in the drive towards more functionality per chip. Another important aspect of photoresist exposure is the standing wave effect. Monochromatic light, when projected onto a wafer, strikes the photoresist surface over a range of angles, approximating plane waves.
This light travels down through the photoresist and, if the substrate is reflective, is reflected back up through the resist. The incoming and reflected light interfere to form a standing wave pattern of high and low light intensity at different depths in the photoresist.
This pattern is replicated in the photoresist, causing ridges in the sidewalls of the resist feature as seen in Figure As pattern dimensions become smaller, these ridges can significantly affect the quality of the feature. The interference that causes standing waves also results in a phenomenon called swing curves , the sinusoidal variation in linewidth with changing resist thickness.
These detrimental effects are best cured by coating the substrate with a thin absorbing layer called a bottom antireflective coating BARC that can reduce the reflectivity seen by the photoresist to less than 1 percent.
One method of reducing the standing wave effect is called the post-exposure bake PEB [1. It is important to note that the detrimental effects of high temperatures on photoresist discussed concerning prebaking also apply to the PEB.
Thus, it becomes very important to optimize the bake conditions. It has also been observed that the rate of diffusion of the PAC is dependent on the prebake conditions [1. It is thought that the presence of solvent enhances diffusion during a PEB. Thus, a low temperature prebake results in greater diffusion for a given PEB temperature.
For a conventional resist, the main importance of the PEB is diffusion to remove standing waves. For another class of photoresists, called chemically amplified resists, the PEB is an essential part of the chemical reactions that create a solubility differential between exposed and unexposed parts of the resist. For these resists, exposure generates a small amount of a strong acid that does not itself change the solubility of the resist.
During the post-exposure bake, this photogenerated acid catalyzes a reaction that changes the solubility of the polymer resin in the resist. Control of the PEB is extremely critical for chemically amplified resists. Diffusion during a post-exposure bake is often used to reduce standing waves.
Photoresist profile simulations as a function of the PEB diffusion length: a 20nm, b 40nm, and c 60nm. Once exposed, the photoresist must be developed. Most commonly used photoresists use aqueous bases as developers. In particular, tetramethyl ammonium hydroxide TMAH is used in concentrations of 0. Development is undoubtedly one of the most critical steps in the photoresist process.
The characteristics of the resist-developer interactions determine to a large extent the shape of the photoresist profile and, more importantly, the linewidth control. The method of applying developer to the photoresist is important in controlling the development uniformity and process latitude.
In the past, batch development was the predominant development technique. A boat of some wafers or more are developed simultaneously in a large beaker, usually with some form of agitation. With the push towards in-line processing, however, other methods have become prevalent. During spin development wafers are spun, using equipment similar to that used for spin coating, and developer is poured onto the rotating wafer.
The wafer is also rinsed and dried while still spinning. Spray development has been shown to have good results using developers specifically formulated for this dispense method. Using a process identical to spin development, the developer is sprayed, rather than poured, on the wafer by using a nozzle that produces a fine mist of developer over the wafer Figure This technique reduces developer usage and gives more uniform developer coverage. Another in-line development strategy is called puddle development.
Again using developers specifically formulated for this process, the developer is poured onto a stationary wafer that is then allowed to sit motionless for the duration of the development time. The wafer is then spin rinsed and dried.
Note that all three in-line processes can be performed in the same piece of equipment with only minor modifications, and combinations of these techniques are frequently used.
Subsequent stages in the process have more in common with etching than with lithographic printing. This method can create extremely small patterns, down to a few tens of nanometers in size. It provides precise control of the shape and size of the objects it creates and can create patterns over an entire surface cost-effectively.
Its main disadvantages are that it requires a flat substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.
Photolithography is the standard method of printed circuit board PCB and microprocessor fabrication. Directed self-assembly is being evaluated as an alternative to photolithography.
Photolithography is commonly used to produce computer chips. When producing computer chips, the substrate material is a resist covered wafer of silicon.
This process allows hundreds of chips to be simultaneously built on a single silicon wafer. The active layer is what provides the semiconductor with the ability to conduct current. In the electronics industry, its often necessary to produce large quantities of photoresists with low costs. This is the rationale to utilize the photolithography process , a technology that is used to make large-scale photoresist masks for chemical masking, etching, and deposition.
The photolithography process is as follows: Ultraviolet light is focused with a lens onto a photoresist layer. The light passes through the photoresist to strike the photoresist, which is then exposed to chemicals.
The chemicals then react with the exposed photoresist, etching the photoresist. You are commenting using your WordPress. You are commenting using your Google account.
You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Blog Feed.
0コメント