A, CMP technology equipment and consumables
CMP, that is, Chemical Mechanical Polishing, chemical mechanical polishing, CMP technology used equipment and consumables include: polishing machine, polishing slurry, polishing pad, after CMP cleaning equipment, polishing end-point detection and process control equipment, waste disposal and detection equipment. Polishing machine, polishing slurry and polishing pad are the 3 key elements of the CMP process, whose performance and mutual matching determine the level of surface flatness that can be achieved by CMP (Figure 1). Among them, polishing slurry and polishing pad are consumables.
Yue Fei once said, "Formation and after the battle, the art of war is the usual, the use of the subtle, depending on a mind." It means that it is the routine of battle to set up the formation and then fight, but the cleverness and flexibility of using it all lies in good thinking. It is with this concept, Yue Fei broke the Song dynasty against the Liao, Jin combat is concerned about the layout rather than the general disease of flexibility, repeatedly built a successful war. If the chemical mechanical polishing (CMP, Chemical Mechanical Polishing) of the full set of processes compared to warfare with soldiers, then the CMP process of consumables, especially the choice of polishing slurry is undoubtedly the key to the "use of the magic". Therefore, polishing slurry is one of the key elements of CMP, its performance directly affects the quality of the polished surface. The factors that affect the removal speed are: the chemical composition of polishing slurry, concentration; the type, size, shape and concentration of abrasive grains; the viscosity of polishing slurry, pH, flow rate, flow path, etc. The precise mixing and batch-to-batch consistency of the polishing slurry is essential to obtain sapphire wafer-to-sapphire wafer and batch-to-batch repeatability, and its quality is an important factor in avoiding surface scratches during the polishing process.
The composition of polishing slurry mainly consists of three parts: corrosion media, film-forming agents and additives, nano-abrasive particles. Polishing slurry should meet the requirements of fast polishing rate, good uniformity of polishing and easy cleaning after polishing. The hardness of the abrasive particles should not be too high to ensure that the mechanical damage to the surface of the film layer is relatively light.
According to the pH classification, polishing slurry is mainly divided into two categories: acidic polishing slurry and alkaline polishing slurry. General acidic polishing slurry contains oxidizing agent, co-oxidizing agent, resist (also called film-forming agent), uniform etching agent, pH adjusting agent and abrasive. The oxidizing agent acts as an oxidizing and corrosive agent on the surface of the object to be polished, and then removes the raised part of the surface through mechanical action to make the surface of the object flat; in addition, the oxidizing agent can oxidize the surface of the substrate to form a layer of oxide film to improve selectivity. Oxidizing agents play a role in increasing the rate of oxidation. The uniform corrosion agent can make the corrosion evenly, so that the surface is smooth and fine; the role of resist is in the polished object surface and the corrosion of the substrate to form a layer of association film, thus preventing corrosion to improve selectivity. And alkaline polishing slurry generally contains complexing agents, dispersants, pH adjusters and abrasives. Solid particles to provide grinding; for different corrosion substrate to choose a different complexing agent; dispersant is generally a large molecular weight non-ionic organic dispersant, its role is to ensure that the abrasive in the slurry does not occur flocculation and sedimentation phenomenon, and to keep the viscosity of the abrasive as low as possible, with good mobility, inhibit the reaction to generate particles and then be polished by the adsorption of the surface to speed up the mass transfer; alkaline composition is generally used For silica sol CMP polishing solution, SiO2 particles are required in the range of 1~100 nm, and SiO2 concentration is 1. 5%~50%. Since the Mohs hardness of SiO2 is 7 and the Mohs hardness of sapphire crystal is 9, the mechanical abrasion effect is less and the mechanical damage is greatly reduced.
B,the basic principle of CMP
From a macroscopic point of view, the basic principle of CMP is: the rotation of the polished wafer is pressed in the same direction as the rotation of the elastic polishing pad, while the polishing slurry between the wafer and the base plate continuous flow. The upper and lower discs are reversed at high speed, and the reaction products on the surface of the wafer to be polished are continuously stripped, and the new polishing slurry is replenished, and the reaction products are taken away with the polishing slurry. The newly exposed wafer plane is chemically reacted again, and the products are stripped down again and the cycle repeats, forming a super fine surface under the combined action of substrate, abrasive and chemical reactant, as shown in Figure 1, 2.
To obtain good quality polishing film, must make the polishing process of chemical corrosion and mechanical grinding effect to achieve a balance. If the chemical corrosion effect is greater than the mechanical polishing effect, it will produce corrosion pits and orange peel ripples on the surface of polished wafers; conversely, the mechanical polishing effect is greater than the chemical corrosion effect to produce a high damage layer on the surface.
From a microscopic point of view, the CMP mechanism of action is not yet a complete theoretical explanation. One of the theories is that the chemical mechanical polishing process is a process in which chemical action and grinding action reinforce and promote each other. The chemical mechanical polishing of sapphire is used as an example to illustrate.
Due to the presence of KOH, the kinetic process of CMP for chemical mechanical polishing of sapphire substrates is mainly composed of the following steps: 1) diffusion of reactant molecules (OH-) from the main body of the liquid to the outer surface of the wafer to be processed (external diffusion); 2) diffusion of reactant molecules from the outer surface to the inner surface, the rate of which is related to the thickness of the mass attachment layer. (3) the reactants are adsorbed on the surface of the sheet to be processed; (4) the reactants are chemically reacted on the surface of the sheet to produce products; (5) the products are desorbed from the surface; (6) the products are diffused from the inner surface to the outer surface of the reaction layer; (7) the products are diffused from the outer surface of the reaction layer to the main liquid. As shown in Figure 3.
In order to achieve better polishing results to ensure high surface flatness, low damage and no contamination, it is necessary to speed up the mass transfer process in the polishing process. Mass transfer consists of two aspects: timely arrival of the reactants to the surface and timely release of the reactants from the surface. The combined result of the two processes directly affects the CMP rate and surface quality. The CMP process in sapphire differs from other CMP processes in that the monocrystalline Al2O3 constituents have reached the highest valence and have a cubic structure with one Al atom surrounded by three O atoms and one O atom surrounded by two Al atoms, thus forming a hexagonal dense stacking pattern. From the chemical reaction equation and the structure of sapphire, it can be concluded that three Al-O bonds have to be broken for each AlO-2 generated, and the Al-O bonding energy is very high, which is crucial in the chemical mechanical polishing of sapphire. However, the reaction between the sapphire (monocrystalline Al2O3) surface and the OH- in the polishing solution is different from the reaction mechanism between Al2O3 powder and OH-, it is not just a simple reaction of each Al2O3 molecule with OH- to form AlO-2. Due to the unique structure of single-crystal Al2O3 (one layer of Al atoms and one layer of O atoms in the c  crystallographic direction), the Al-atoms or O-atoms on the surface first interact with the slurry to form Al-OH and O-OH hydrolysis layers, respectively, under the action of alkaline slurry. During the polishing process, the negatively charged SiO2 abrasive particles also form chemical bonds with OH- on the surface, and then with the suspended bonds on the sapphire surface. As the polishing pad rotates, the SiO2 abrasive particles carry away the Al and O atoms.
C, the application of CMP
The concept of CMP technology was first introduced by Monsanto in 1965. The technology was originally used to obtain high quality glass surfaces, such as military telescopes, etc. In 1988, IBM started to apply CMP technology to the manufacturing of 4M DRAM, and since 1991, when IBM successfully applied CMP to the production of 64M DRAM, CMP technology has been rapidly developed around the world. Unlike traditional purely mechanical or chemical polishing methods, CMP avoids surface damage caused by purely mechanical polishing and the disadvantages of slow polishing speed, poor surface flatness and polishing consistency caused by purely chemical polishing through a combination of chemical and mechanical effects. It uses the principle of "soft grinding hard" in wear, that is, the use of softer materials to polish to achieve high quality surface polishing. The most widespread application of CMP technology is in integrated circuits (IC) and ultra-large scale integrated circuits (ULSI) for polishing of silicon wafers as the base material. The international consensus is that global planarization is necessary to ensure the accuracy and resolution of lithographic image transfer when the device feature size is below 0.35 μm, and CMP is almost the only technology that can provide global planarization, and its application scope is expanding.
At present, CMP technology has developed into chemical mechanical polishing machine as the main body, in-line inspection, end-point inspection, cleaning and other technologies in one CMP technology, is the integrated circuit to microfabrication, multilayer, thin, flat process development products. It is also a process technology necessary for the transition of wafers from 200mm to 300mm or even larger diameters, increasing productivity, reducing manufacturing costs, and global flattening of substrates.
In the CMP process for tungsten metal (W), the typical polishing slurry used is a mixture of silica gel or suspended Al2O3 particles with a solution pH between 5.0 and 6.5. Acidic conditions are mostly chosen for CMP of metals, mainly to maintain a high rate of material removal. Generally, silica powder is softer than Al2O3 and less likely to cause abrasion on the wafer surface, making it more commonly used. the chemical composition of the polishing slurry used in WCMP is a mixture of hydrogen peroxide (H2O2) and silica or Al2O3 abrasive particles. During the polishing process, H2O2 decomposes into water and water-soluble O2, which reacts with W to form tungsten oxide (WO3). wO3 is softer than W, thus removing the W.
D, the main problems in the CMP process
The ultimate goal of polishing slurry research is to find the best combination of chemical and mechanical action, so that the polishing slurry with high removal rate, good flatness, good uniformity of film thickness and high selectivity can be obtained. In addition, easy cleaning, corrosiveness to equipment, waste disposal costs and safety issues should be considered.
Defect reduction is a constant topic in the CMP process, and in chip manufacturing in general. The semiconductor industry also has a corresponding "unspoken rule" for the CMP process, that is, the CMP process after the device material loss to be less than 10% of the entire device thickness. That is to say, slurry not only to make the material is effectively removed, but also to be able to accurately control the removal rate and the final effect. As the device feature size continues to shrink, the impact of defects on process control and final yield becomes more and more obvious, and the size of fatal defects is required to be less than 50% of the device size.
E. Development prospect of CMP
Gallium nitride (GaN) as the representative of the third generation of broadband semiconductor materials in recent years is very rapid development, gallium nitride (GaN) based semiconductor materials with high luminous efficiency, good thermal conductivity, high temperature resistance, radiation resistance, high strength and high hardness and other characteristics, can be made into high-efficiency blue, green light-emitting diodes and laser diodes (also known as lasers). But gallium nitride (GaN) material itself can not grow a single crystal, must be grown in its structure similar to the substrate material. At present, the internationally accepted substrate material is sapphire crystal. With the growth of market demand for gallium nitride (GaN) materials, the demand for sapphire will also grow rapidly.