A nano-scale sketch on a millimeter-scale wafer Part 1

A. Photolithography: the pre-carving sketch Before carving or cutting, we often sketch out the work to be done. This way, we ensure that we make the cuts and carves exactly where we plan. Photolithography is very similar to this sketching step. The semiconductor process can be described as a repetition of stacking and cutting. Using photolithography, we sketch out where these cuts will be made. B. Creating a blueprint We typically use tools like pens to create sketches. However, in photolithography, as the name suggests, uses light to imprint sketches onto film. This can be seen in [Figure 1]. First, a thin plate containing the desired pattern is created. The pattern on the plate either blocks or lets light through onto the film, creating the desired pattern. This plate used to control the light is called a “mask” or a “reticle”.

However, simply shining light on paper and creating a shadow will not engrave the shadow into the paper. We need the equivalent of film in a camera to imprint a pattern of the incident light. In photolithography, the PR (Photo Resist), which is applied before the light is shone through, plays this role. PR changes its properties in response to light. As shown in Figure [2], the PR is applied on the material to be cut, then light is shone through the mask. This alters the properties of the PR in the areas exposed to light. This alteration of properties allows for either the PR exposed to light or the PR not exposed to light to be selectively removed in the development step. In other words, the PR is left behind in the shape of the mask. This series of steps is called “patterning” because the pattern of the mark is developed on the material to be cut.

After the patterning step is a step of cutting the material, called “etching”. Etching (cutting) is carried out on the whole area at the same time. The PR remaining after development keeps the underlying material from being cut away, and allows the desired sketch to be produced. That was a brief explanation of the basic role and principles of the photolithography process. It would appear that photolithography is a rather simple process that involves little more than shining light through a mask. So why is it that the advancement of photolithography technology is garnering such attention from the semiconductor industry?

A. Light diffraction and interference hindering patterning Diffraction is a property in which light spreads out when passing through a narrow slit, while interference is a property in which two light waves meet and reinforce or cancel each other out. These are currently the two biggest obstacles to the patterning process in photolithography. As shown in Figure [3], light has a diffracting property, and cannot move in a straight line when passing through a narrow slit. Instead, it propagates as a fan-shaped wave radiating from the slit. The diffraction pattern tends to be broader if the slit is narrow or or wavelength is long.

Further, when light passes through two or more slits and is diffracted at each, the diffraction patterns overlap and create an interference pattern, as shown in Figure [4]. This is not a problem when the slits and the distance between slits are sufficiently wide relative to the wavelength, as seen in (a) of Figure [4]. But if the slit and distance between slits are narrow, as seen in (b), it becomes impossible to develop the desired pattern accurately on the PR. In other words, the narrower the lines on the sketch and the less space there is between the lines, the more difficult it is to develop the sketch precisely.

As process technology advances, transistors are becoming ever smaller. Accordingly, the lines involved in photolithography are becoming narrower and more densely packed. This translates to ever-increasing difficulty of the photolithography process. Just how have we been able to overcome these photolithography obstacles?

A. Multi Patterning Technology(MPT) - Multi passes if one isn’t enough! If the problem is interference between rays of diffracted light, then maybe we can spread the light rays farther apart? Placing the slits farther apart is one way to overcome the light diffraction problem. In Figure [5] (a), patterning is carried out using 4 slits. Here, as shown in (b) and (c), the 4 slits are divided into pairs, with the slits spaced farther apart. By patterning in this fashion, interference can be reduced.

B. OPC - Too small? Make it larger. Too big? Make it smaller! In archery, if an arrow aimed at the center of the target veers off course in a certain direction, we correct for this error by adjusting our aim in the opposite direction. In other words, we recalculate our aim to compensate for the error. A similar approach can be employed when creating masks to correct for errors in patterning. This process is called Optical Proximity Correction (OPC). In this method, as shown in the process of Figure [6], the mask itself is intentionally distorted using feedback from the final outcome.