B. Anti Reflective Coating: The second resolution enhancing techniques or RET is the application of anti reflective coating. This also called ARC or arc. During the lithographic process, we saw that a thin layer of photo resist is applied on the top of the wafer and light is shone from the top, through the mask. Some of the light will be reflected from the top of the film and some of the light will pass through the film. After passing through the film, a part of the light will again be reflected on the wafer surface .The incoming light and the reflected light within the film can form what is known as a standing wave. Essentially, the light reflected from the wafer will interfere with the light coming from the top. This will distort the image, and hence we will not get the exact image on the mask.
In order to overcome this problem, the following procedure is used. First, on the top of the wafer, a film made of a material called ARC or anti reflective coating is applied. On top of ARC, the normal photo resist is applied. Now, when the light comes from the top, it will pass through the photo resist and then through the ARC. Then it will fall on the wafer. Most of the light will be absorbed by the photo resist, but some will not be absorbed. When it passes through this ARC, ARC will ensure that the light will not get reflected from the bottom of the wafer. This way there will be any interference and there will not be any standing waves.  The animation in figure 2.10 shows the lithography process with ARC.

Figure 2.14. Anti Reflective Coating in litho process

C Phase Shift Mask: The third RET is called phase shift mask or PSM. Earlier we saw that OPC can be used to account for diffraction to some extent. However, this trick can work only up to some extent. When the space between two features or two lines becomes very small, OPC will not work effectively. What is meant by very small? When these spaces and the widths are similar to the wavelength of the light used, then we can say that it is very small. For example, visible light is in the wavelength of 400 nm to 700 nm. Ultraviolet (UV) light is of wavelength less than 400 nm. In the microelectronic industry, one currently makes chips of less than hundred nanometer size. So, naturally, one cannot use visible light for photolithographic process. Even when UV is used, the usual wavelength is about 193 nm. The electromagnetic waves in the range of 30 or 40 nm are called extreme UV or EUV.  At present, (in the beginning of 2010), companies are creating chips with features of 65 nanometer size using light of 193 nanometer wavelength. This is possible only because they use phase shift mask (PSM).
What is a phase shift mask? How does it work?
First, we will look into the operation of a normal mask. Then we will see how the phase shift mask is constructed and how it works. Consider a normal mask and two openings, as shown in the animation in figure 2.15. Here we are assuming   that the mask and the wafer are of the same size. i.e. the mask is one-to-one mask or 1x mask. Even in the case of 4x or 5x masks, the arguments we make here will apply. However, it is easy to illustrate the issue in the 1x mask.

Figure 2.15. Normal mask. Diffraction effects

Remember that lights are electromagnetic waves. The electromagnetic field very close to the mask will be very close to ideal (blue rectangles). Ideally, if there were no diffraction, the light will come from the top and the mask feature and the wafer feature will be identical. But light will bend near the edges  and this is called diffraction. What happens is the “opening” on the wafer will be larger. This is shown by the curve which looks like ‘hills and valleys’.  The peaks correspond to the openings and the valleys correspond to the blocked regions.

When the space between the openings is very small, the valleys will be shallow (last part of the animation). Thus, the pattern printed on the wafer will be very different from the pattern on the mask.  When the gap between the two openings is very small in the mask, the corresponding openings on the wafer will begin to merge. We will not get two different lines but we will get one large line on the wafer. Here, even if OPC is used, we will not be able to get two different lines.
In the animation, we see that when the light comes through the mask, the phases of the light on the left side and on the right side opening are the same. Electromagnetic waves (including light) are a transverse waves and hence they have these two properties. One is the amplitude and the other is the phase. The intensity of the light is proportional to the square of amplitude. Whenever the amplitude is maximum, intensity   will be maximum, and when it is zero it will be zero. When the amplitude goes to the negative, the intensity will be positive because we are taking the square of the amplitude. The key point to note here is that in both openings, the electromagnetic wave (i.e. light) will have the same phase.
Now, we want to use the same light (i.e. same wavelength) and still get this small gap printed on the wafer correctly. In order to do that, we can use phase shift mask. Look at the animation in figure 2.16. We see there are still two openings separated by the same gap, but in one of the openings,  there is an additional material. This additional material is there to change the phase of the light. In this case, the phase of the light passing out of the first opening and the phase  of the light passing out of the second opening at the same point they will have 180 oor  p radians phase difference . Thus, the phase of the light in one of this is shifted or changed.

Figure 2.16. Phase Shift Mask principle

Here also the light spreads and a rectangular pattern is turned into a smeared (curved line) pattern, but the phases of the two waves are different. So, the amplitude of the combined electromagnetic wave will go through zero in between these two openings. Whenever the phase is positive in the left opening, the phase will be negative in the right opening and vice versa.  Since the amplitude passes through zero, the intensity (which is proportional to the square of amplitude) will also pass through zero. This way, we ensure that the intensity will not become too much in between the openings. This is how the phase shift mask works.
We will summarize the three different RET techniques. One is OPC (Optical Proximity correction). It makes changes to the layout and then correspondingly in the mask.  The second arc or ARC   makes change in the process. Before we coat the wafer with photo resist, we coat the wafer with anti reflective coating. The third technique is PSM which makes changes in the mask .Original layout is first generated and then modified to indicate wherever the change in phase is appropriate. The mask is made accordingly.

2. Depth of focus (Depth of Field):

In the beginning of resolution enhancement techniques section, we saw that the resolution is related to the wavelength and numerical aperture. We also saw that depth of focus is affected by the same parameters viz. wavelength and numerical aperture. We also made a note that we need a large depth of focus for a good process.
What is depth of focus?. Consider any of the spectator sports, such as cricket. In some of the sports magazines, sometimes we see the photo of a batsman. Usually the picture is taken from far away (from the boundary line). From the boundary line, the lens is adjusted so that the batsman is in focus. Then the image will be very clear and the resolution will be very good. But when the batsman is in focus in that picture, we can also notice that the bowler and the umpire will not be in focus and they will appear hazy.  The wicket keeper will appear slightly clearer if he stands close to the batsman. So, the distance between the lens and the point of focus (i.e. the batsman) is an important variable.
If we focus the lens exactly on the plane of the batsman, then the resolution will be very good, but the resolution of images of people behind and in front of the batsman in the same picture will not be good . On the other hand, it is possible to take a photo (perhaps with an autofocus camera) which will not be exactly focusing on one plane. In this type of photograph, (for example what we normally take when three or four rows of people are sitting or standing and then you take a photograph) everybody will be visible reasonably well, but none of them will be seen with high resolution.
What this means is that we can take photograph with many different objects (some in the front and some in the back) we can get either (1) all of them in the image with a reasonable image quality, but none of them with very high quality image or (2) we can focus on one object and get a very high quality image, but we will get very poor resolution on the other objects which happen to be in the front or the back. The former is achieved by reducing the lens aperture size and the later is achieved by increasing the aperture size.
How is this relevant for chip production? When we are placing a mask and projecting light through the mask using lenses onto the wafer, we want the image to come accurately on the wafer. What if the wafer is not exactly planar? If the wafer is not planar, (if there are ups and downs) and if the resolution required is very high, the image will not print properly.
To illustrate with another example consider a photographic film. We use light to make photographic reprints. We can make many copies of this and this has to be done in a dark room. The film has to be aligned with a lens and a light. We use a photographic film which is usually a plain paper with a light sensitive chemical coating on top. The film is held stationary, the light is shone through the film and the photographic paper is held straight at the bottom. Then the image  will print correctly. If the paper at the bottom is bent and wavy, then we will not get the correct image.
This is exactly what will happen if the wafer surface is not planar. The wafer surface will never be very planar in the atomic level, but if it is very non-planar (i.e. if there is lot of topography or  ups and downs), then the lithographic process will not print the images correctly. What can be done to correct this problem? We should remove the ups and downs on the wafer and this is done using a process called chemical mechanical planarization or chemical mechanical polishing (CMP).  CMP will help reduce the topography and get a more or less planar surface, but we will never get an ideal atomically flat surface for all the wafers. So the lithography process must be tolerant to some variation in the wafer planarity.
How can we measure the tolerance level to the variation in topography?
One way to do that is as follows: Take a very planer wafer (standard wafer). Take a mask with  a well known pattern  and then place it above the wafer and project the image on to  the wafer. Usually this is done with an auto set up, where the machine itself can find out the exact distance where the focus will be best.  Print the image and this should come correctly.  Next move the lens to the next shot and pull it away from the best focus deliberately by a small distance (0.1 micron or 0.2 micron) and then take the image. In this case, we are deliberately taking it out of focus and continue with the process. Similarly, in the next shot, move the lens towards the wafer (push it towards the wafer) by 0.1 micron and take the image.  In the next row of chips, increase (or decrease) the exposure, while following the same set of focus adjustments. Thus we are adjusting the focus and exposure in a matrix like fashion. This is called focus-exposure-matrix.

Thus the neighboring chips will be made with slightly different focuses and exposures. The remaining processes (such as deposition or etching) to make the lines will be identical. At the end of this sequence, the wafer will be tested to see whether all the features are created correctly. For  example,  if  the best focus itself is not giving the features properly then this process is really poor. But we may find that the best focus as well as 0.1 micron and 0.2 micron out of focus chips can yield good image, but 0.3 micron out of focus yield poor image. Then we can say that if the incoming wafer is not planar up to +/- 0.2 microns then the lithography process can handle that. Thus, the total ups and downs (maximum to minimum) can be 0.4 micron in this case. It also means that if the planarity is poor, (i.e. if the topographical variations are more than 0.4 micron) then we cannot transfer the image from the mask to the wafer successfully. The topography of the wafer has to be reduced by some method before it is sent to lithography.