PHOTORESIST COATING ON SILICON WAFERS BY KARTIKEY GROVER LALIT AGGARWAL
WHAT IS MEMS• It’s a technology of very small device.• MEMS are made up of components between 1 to 100 micrometres in size and generally range in size from 20 micrometres to a millimetre.• The fabrication of MEMS evolved from the process technology in semiconductor device fabrication.
Patterning• Patterning in MEMS is the transfer of a pattern into a material.• In patterning we use lithography.• Lithography is the transfer of a pattern into a photosensitive material by selective exposure to a radiation source such as light.• In order to do it we need photoresist coating.
CATEOGORIES OF PHOTORESIST• A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The portion of the photoresist that is unexposed remains insoluble to the photoresist developer.• A negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.
GENERAL CHEMICAL COMPOSITION OF PHOTORESIST• Different chemicals may be used for permanently giving the material the desired property variations:• Poly(methyl methacrylate) (PMMA),• Poly(methyl glutarimide) (PMGI)• Phenol formaldehyde resin (DNQ/Novolac),• SU-8.
THREE MAJOR PHOTORESIST COATING TECHNIQUES• SPIN COATING• SPRAY COATING• ELECTRODEPOSITION OF PHOTORESIST
SPIN COATING• It is the standard coating method for flat wafers in IC technology• In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force.
SPIN COATING• Solution viscosity and the spinning speed are the only parameters which influence the layer forming process• The thickness approximately decreases with the (increasing) square-root of the spin speed (in rpm)
ADVANTAGES OF SPIN COATING• Spin coating is a mature technique and uses commercially available equipment and resists.• used at all stages of processing on all types of substrate layers.• The high resist film thickness homogeneity as well as the short coating times makes spin- coating the most-applied coating technique at least in microelectronics.• The process optimization focuses only on these two parameters.
DISADVANTAGES OF SPIN COATING• The deeply etched features cause a physical obstruction to the solution flow, preventing complete coverage.• Depends on the position of cavity.• Sizes and shapes of the cavities also have influence on the resist uniformity• The reproducibility of spin coating is very less.
1. The lowest uniformityvariation can be observed atH/V = 2/3, 1, 3/2, whichindicates a better resistuniformity in the square orlarge rectangle cavities.
SPRAY COATING• It uses an Ultrasonic atomization technology that has unique advantage that is based on two features: its ability to generate very small droplets of an extremely narrow range diameter size, and its ability to apply these droplets gently on the substrate with minimum “bounce back” from the target surface.• To get the proper droplet size distribution of photoresist, a resist solution with lower viscosity is necessary
• By optimizing the coatingprocess a good uniformresist layer has beendeposited on wafer with375μm-deep cavities.• The uniformity of resistlayer obtained by this spraycoating technique is betterthan with the spin coating
ADVANTAGES OF SPRAY COATING• This technique uses much less resist than spin coating. The spray process requires no spin off photoresist so that it can effect up to 70% less resist consumption to spin process.• The resist thickness is repeatable over all cavities with the same size, regardless the position of cavities on the wafer• Spray coating has no thickness variation caused by directional effect of spinning
(a) Spin Coating (b) Spray Coating (c) spray coating with angle control
DISADVANTAGES OF SPRAY COATING• variations in resist thickness are observed if cavities with a large difference in size are present on the same wafer. The resist thickness at the bottom of a small cavity is thicker that the one in a large cavity. If the difference in dimension of cavities is large, it will lead to a large variation in resist thickness between cavities.• Photoresist spray coating has more variables than spin coating.
ELECTRODEPOSITION OF PHOTORESIST• The process used in electroplating is called Electrodeposition.• The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material• Wafer to be deposited is the cathode of the circuit.• the anode is made of the metal to be plated on the part.
ADVANTAGES OF ELCECTRODEPOSITION• The main advantage of this ED coating is the conformal resist layer independently of the geometry of the nonplanar features
• For that reason, ED coating is the most suited technique to pattern structures that run in and across cavities or when a smaller line width is required
DISADVANTAGE OF ED• This technique always requires a conductive (metal) surface.• The set up and process handling are more complicated than the other two coating techniques.• The coating bath should be checked and maintained frequently in order to get a reproducible process.
CONCLUSION• Spin coating is a simple method but it does not give a good and reproducible result on deep cavities of different sizes and randomly distributed across the wafer• Spray coating and ED deposition certainly bring better results but they need special equipment• ED deposition is perfectly suited for patterning metal layer