Source: Content by Semiconductor Industry Watch (ID: icbank) original, author: Anon.
Spring Festival at home, with many Fab factory engineers and front-line lithography engineers to communicate a lot, everyone invariably mentioned the problem of domestic photoresist, so the views of everyone were summarized, so that everyone more intuitive understanding of their eyes of the domestic photoresist situation.
The first part mainly talks about the lithography process and technical difficulties in the eyes of professionals
Lithography, as the name suggests, is engraved with light, and the process of transferring the pattern on the mask (mask) to the wafer with light as a tool sounds very simple, but in order to achieve the transfer of this shape, the process is very complicated and there are many technical difficulties. From a results-oriented point of view, there are three main things to do - CD (critical dimensions), OVL (overlay: engraving accuracy) and defect (graphic defects).
(1) To do a good JOB of CD, it is necessary to achieve it through two steps of reticle design and process process. Let's talk about reticle design and OPC. In order to finally get the desired size of the pattern on the wafer, you should first define the corresponding size of the pattern at the corresponding position on the reticle, and then transfer it to the wafer through a certain proportion of conversion. Theoretically, what kind of shape the reticle design looks like, by transferring to the wafer in a certain proportion, what kind of graphics should be obtained, but the actual situation is more complicated. As Moore's Law continues to evolve, the size of the target graphics required for chip manufacturing continues to become smaller, and interference and diffraction phenomena become more pronounced, which will directly affect the transfer of the patterns. Therefore, in order to make the graphics on the lithographed wafer consistent with the design graphics, the graphics on the reticle need to be corrected, and this is the sibling department of lithography - OPC (Optical Proximity Correction).
Solving the OPC problem, the next step is the process. Before that, let's take a look at the entire lithography process, the lithography process includes the homogenization developer (track) and the scanner we are familiar with, the track is responsible for gluing and developing, and the scanner is responsible for exposure. Taking the positive glue as an example, wafer first goes into the trackk and coats it with a layer of photoresist, PAB (Post Apply Bake), after the baking process. Used to heat up and evaporate most of the solvent in the photoresist, curing the photoresist, improving adhesion), after cooling to room temperature, moving to the lithography machine for exposure, and then back to the track for development, and finally PEB (Post Exposure Bake, baking after exposure) to make the development process complete. The wafer then leaves the lithography chamber and enters the etching or ion implantation process.
The whole lithography process, from the CD point of view alone, there is a lot of preliminary work to be done.
First, the selection and evaluation of photoresist. From a research and development point of view, it is necessary to first evaluate whether this photoresist can meet the process requirements. Take Krf photoresist as an example:
The first step is to adjust the amount of photoresist spray, generally speaking, the single spray amount of photoresist should be controlled at about 1.0cc ~ 2.0cc, and the amount of photoresist is too small to easily produce poor coating (that is, the PR coating is not complete in the exposure of the edge of the silicon wafer), which directly affects the entire lithography process.
The second step is the thickness of the photoresist film, the film thickness should be considered is the entire photoresist of the profile, different fab plant requirements are not the same, most of the requirements of the photoresist film thickness of the range controlled within 1% of the target film thickness, such as the film thickness target value is 2800A, the film thickness change range to be stuck below 28A, some will require whether the film thickness of the 3sigma value is below target 1.5% and so on.
The third step is to see whether the process parameters of this photoresist are up to standard (generally compared with the benchmark photoresist). Among them, including DOF (Depth of Focus), in the depth of focus range, the quality of exposure imaging can be guaranteed, the DOF at the time of exposure must be much greater than the unevenness of the wafer surface, so as to ensure the yield of the lithography process. That is to say, to ensure whether the focal length of the process window can cover the maximum value of the surface difference of the wafer surface); EL (Exposure Latitude exposure tolerance, can also be called energy window, exposure tolerance of the adhesive is less affected by the exposure energy fluctuation or unevenness. Generally under the premise of meeting the DOF to ensure that the EL meets the standard, such as Krf photoresist generally to meet the DOF greater than 150nm, while the EL to reach 10%), CD uniformity (to ensure the uniformity of the SAME cd in the same position in the different shot of the wafer, mainly to see whether the CDU 3simga value is in spec); CD LWR (Line Width Roughness, the line width roughness of the photoresist, Refers to the deviation of the photoresist line width relative to the target value due to rough edges) and so on.
(2) OVL is another important measurement index in addition to CD, OVL is simply whether the graph of the current layer and the graph of the front layer are accurate, the more advanced the technical process, the higher the requirements for OWL. Because as the node continues to become smaller, the window left for OVAL is getting smaller and smaller, and OVL is bound to become more and more difficult to do. OVL measurement mode industry generally has two, IBO (Image Based Overlay) and DBO (Diffraction Based Overlay), IBO, as the name suggests, is to measure the current layer and the front layer of the Mark to take pictures, to see the stack is not in place, and DBO is to collect the diffracted light of the measured Mark of the current layer and the front layer, convert the optical signal into a digital signal and reflect it.
(3) The Defic (graphic defect) of the entire lithography process is mainly caused by equipment and process reasons. The pattern defects caused by the equipment can be solved by regular machine maintenance and cleaning; the graphic defects caused by the process mainly include the caravan defect and dev Defect, the Coating Defect is mainly determined by the characteristics of the photoresist itself, which is generally found and solved in the photoresist evaluation stage; the other is the Discovering Defect, which is mainly caused by the incomplete development process, which needs to be improved by continuously optimizing the development formula. Domestic manufacturers sometimes because of the basic knowledge of the material is not perfect, the use of light resistance additives is unreasonable, but also lead to some light resistance of the development defects far worse than foreign manufacturers. This process, from finding the root cause to the final solution, often takes a long period.
In general, CDs and Defects are actually more dependent on the characteristics of photoresists than OVLs, which are more dependent on systems and lithographers for regulation. The process window of lithography depends on the performance of the photoresist, even if the performance of the machine is optimized, the performance of the photoresist is not up to standard, and the lithography process cannot be carried out at all, it can be said that the photoresist supports half of the lithography process.
The second part mainly shares their eyes on domestic photoresist
Since entering the industry has been in the lithography department, the first attention and contact is foreign photoresist manufacturers, such as Xinyue, DOW, JSR, TOK, these large factories provide all the 45nm-14nm advanced process of photoresist we need. Until the Sino-US trade war started, foreign manufacturers began to limit our supply or even cut off some photoresists, which can directly determine the life and death of our wafer production line. In this context, we slowly shifted our attention to domestic photoresist, from the beginning of Beijing Kehua, to Shanghai Xinyang, and then to Xuzhou Bokang.
In this process, we are deeply aware of the gap between domestic semiconductor photoresist and foreign products, including insufficient process windows, photoresist profile differences, various Defect problems, as well as stable mass production and raw material channels, which are urgently needed to be solved. In turn, these problems will limit the development process of major Fab factories in China for a long time.
However, the road to localization of lithography materials is not without a way out, and the problems in photoresist performance can be improved by optimizing the formulation. You know, even if a certain UV1610 series photoresist from a large factory like DOF is known as "demon glue" in the industry, the initial evaluation process is not smooth, it has undergone more than a dozen formula optimizations, and the evaluation cycle alone took 1-2 years. To say how good its own characteristics are, it is not necessarily, the key is that its universality is strong, to Logic 28HK lithography process as an example, UV1610 series products are used in Well Loop, LDD Loop, SIGE Loop, SD Loop, etc., this photolithography capsule covers nearly one-third of the application of the entire 28HK lithography process. Its process window is not particularly excellent, but it can basically meet the process requirements, its universality and timely follow-up and real-time feedback from manufacturers, so that this photoresist can be mass-produced on 28HK, and its fame is also instantly launched. Therefore, looking at domestic photoresist companies, we must focus on whether their categories and models are enough, simply in a certain 1-2 photoresist to make a breakthrough, the significance is not very large, must have universality, can be widely used in different levels of Fab process. For example, companies that can solve high-end Arf and Krf, line and hole glue at the same time, Fab and this one deep cooperation can solve most of the urgent problems, and the overall cost of cooperation and import will be lower.
It is precisely because of these foreign manufacturers that they are arrogant and arrogant, and the service attitude of many domestic Fab customers has become perfunctory, they think that you have to die without my products, even if my service attitude is poor, you dare not use our products. For example, a big brand in Japan, they were initially cooperated with TSMC, many photoresist products are through TSMC this industry foundry big guy to make a name, the big guys are using, you small factories love to use it, I did not ask you to use, but I think it is this attitude of having no fear, arrogance and pride to give us a huge opportunity for domestic photoresist manufacturers. The performance of domestic photoresist products is insufficient, then continuous testing, continuous feedback, continuous optimization, and in the context of the country's vigorous support for semiconductor domestic materials, the localization process will be more efficient. At present, there are already excellent companies in China that stand out, some of which can almost meet the needs of most Fab factories, and the efficiency of R&D feedback is also very high, and the lack of experience is nothing more than experience.
In the field of advanced processes, the use of more or Arf immersion of the photoresist, and due to the increase in the number of lithography layers, for domestic photoresist manufacturers is a huge cake, regardless of whether the last of these advanced processes can break through and mass production, can be Arf immersion photoresist breakthrough of the domestic photoresist company are enough to prove their own technical and process advantages, will certainly occupy a world in the domestic photoresist materials market.
Finally, there are also some sharing messages for domestic photoresist companies. First of all, the company that dares to forge ahead in this field itself has already deserved the respect of all semiconductor colleagues, and the overall process difficulty is unparalleled in the entire material field. Secondly, it is still necessary to deeply bind with domestic Fab factories and carry out technology research and development and process improvement from its demand point. Specifically:
(1) Open up the supply chain of each Fab factory, for the mature process and the process that is still being developed, we should distinguish the priority and secondary, take the replacement of the mature process, patiently complete the evaluation of advanced process development, follow up in real time, do our best, and grasp the initiative;
(2) To maintain the relationship with the major Fab factories, research and development and production is the key to product evaluation, general research and development is responsible for the development of new technology nodes, so the evaluation of photoresist is mostly used in new nodes, the cycle is generally longer; and the production pays more attention to the replacement of mass-produced products, and the rhythm is more compact.
(3) Fab attaches great importance to the stability of material supply, and domestic photoresist companies should pay close attention to solving the problem of resin and even monomer, because it has been heard that the shortage of materials has affected the stable supply of the final photoresist.
In general, in the past two years, everyone has more and more confidence in the rise of domestic semiconductor materials, and also expects domestic materials, especially domestic photoresists, to be released in large Fab factories.