Lithography is a way to “print” designs onto another surface. While it is an ancient art in some ways, dating back to art in many ancient cultures, it is also an important part of modern technology used extensively in the semiconductor industry.
There are many lithography steps in the semiconductor industry. Often, lithography is performed before etching steps so that materials can be selectively removed by the etch, instead of etching the entire surface. Each pattern where silicon, oxide layers, barrier layers and others are removed require a pattern or mask.
Lithography is performed in several steps. First, a photoresist layer is applied with a spin coating process. Then, the photoresist is exposed to light of a certain wavelength, which causes it to chemically change. Then, some of the photoresist is selectively etched leaving behind the desired pattern.
More about photoresist
Photoresist is a chemical that changes during exposure to certain wavelengths of light. Often, it is a polymeric layer that crosslinks between chains during exposure, but other methods are possible. Regardless of the actual chemical change that occurs, the two common types of photoresist are “positive” and “negative.”
Suppose there is a silicon wafer with a thin layer of oxide on the surface, and the next processing step needs the oxide layer removed in certain places. A positive photoresist is applied, exposed and etched. With a positive photoresist, the etchant will attack and remove the photoresist that was not exposed. If instead, a negative photoresist had been applied, the etchant would attack the photoresist that was exposed.
Spin coating
Spin coating is one of the easiest techniques for depositing a coating onto a semiconductor wafer. With spin coating, the wafer is held in place by a vacuum chuck and then rotated at high speed. As it spins, a liquid chemical, such as photoresist, is poured onto the surface of the wafer. The excess photoresist is slung off the wafer, leaving behind only what is held through the surface tension of the liquid. Depending on the coating, the wafer may undergo a drying step with applied heat to solidify the coating.
Developing spin coating parameters initially can be challenging. The vacuum pressure must be strong enough to hold the wafer, but not to flex it or break it. The rotational speed must be fast enough to coat the entire wafer, but not so fast that the wafer can break free from the vacuum chuck. The flow rate and nozzle properties for the dispenser must be tailored to minimize waste, allow no drips, but coat the entire wafer, given the chemical properties of the photoresist. However, once the process is mastered, running a spin coater can be easily automated.
Exposure
After the photoresist has been properly cured, it will be exposed to light through a pattern or mask, called a reticle. The reticle may have several repeated patterns on it, depending on the specific design. A light source, often a single-wavelength laser, shines through a series of lenses and through the reticle onto the substrate. This exposes areas not blocked by the reticle to the light, developing the photoresist, and ultimately the pattern.
The exposure process can be characterized by two important pieces of technology: the scanner/track and the reticle.
Scanners and tracks
Because there may only be one pattern for a wafer that can hold tens to hundreds of chips, the light must shine through the pattern many times for each wafer. The scanner is the machine that performs all of the exposure operations, earning its name by “scanning” the light through the reticle at different angles (or moving the light and the reticle) to repeat the pattern.
Inside the scanner is also the “track,” which contains heaters and coolers that can be used to preheat the wafer, stop the heating of the wafer, or even perform the spin coating in some models. The track typically has a wafer-handling robot that is processing the next wafers while one wafer is being scanned.
Reticles
The reticle is most often constructed of a glass plate with chrome sections that block light exposure. Reticles must be clean, with sharp edges between chrome and glass, and handled carefully. Dust, static discharge and mishandling can damage a reticle. They are fragile, expensive and often have large lead times if they need to be replaced.
Controlling dust is one of the primary issues with reticles. They are stored in special pods that the scanner handles to prevent the surface of the reticle from being exposed. There is also a large piece of equipment that checks the reticle for dust and can blow away small dust particles with a nitrogen jet or characterize where dust particles are located in hopes that the dust particle will have low impact. Also, the reticle itself has a small frame with a piece of transparent polymer, called a pellicle, stretched over it. The idea with the pellicle is that dust may land on it, but will be out of focus for the light, minimizing its effect on the pattern.
It is important to realize that the reticle is the company’s intellectual property. When a team of electrical and computer engineers designs a layer, they are designing the reticle. It is the complete design and contains the layout of the transistors, interconnects and components for that particular layer. When a reticle is retired, it must be completely destroyed, versus falling into the hands of a competitor.
Etching
Once the exposure has been completed, the wafers can move onto etching. There are several common etching techniques, but they can be broken down into two categories: wet etching and dry etching.
Wet etching means a wet chemical is applied to the surface of the wafer, often by spin coating, spray coating or some immersion technique. The etchant removes the exposed or the unexposed photoresist, as well as the oxide layer underneath. The wet etching process uses corrosive chemicals, such as hydrofluoric (HF) acid, which comes with additional safety concerns.
Dry etching techniques can also be used to remove photoresist. Dry etching does not use corrosive chemicals in the same manner, but instead uses a variety of sputtering and plasma techniques to create reactive species to remove the photoresist.
Dry etching is not used as often for lithography as wet etching. It is more commonly used to remove silicon ahead of a metallization step. Wet etching is preferred for lithography as the etchant can be tailored to attack only one form of the photoresist, making it much more selective for this process. Dry etching can be selective, but only when applied through another mask, which adds the potential for alignment errors.
Conclusion
Lithography is a constantly evolving technology that has come a long way from the early days of printing for ceremonies and art. Today’s semiconductor scanners can print thousands of chips in a matter of minutes by focusing lasers through carefully designed reticles, sometimes using small droplets of water as lenses to print some of the tiniest man-made features on Earth.
