This invention relates to chemical vapor deposition. In particular the invention relates to improved throughput and process cleanliness.
Chemical vapor deposition (CVD) or chemical vapor infiltration (CVI) reactions are processes in which gas phase precursors are reacted together to form a solid phase coating material with resulting gas phase byproducts from the reaction being carried away and trapped or vented. CVD processes may be used in applications that require certain process attributes, including high purity coatings, non-line of sight processing, ability to infiltrate porous substrates and fill open voids with solid phase materials and extremely thin or very tightly controlled coating thickness layers. In most industrial CVD applications, the energy required to cause the reaction between the reactant gasses is supplied in the form of heat in a resistance or induction heated furnace or reactor. Typical CVD reaction temperatures range from 400° C. up to 1600° C. depending on the desired deposition phase, reaction chemistry and substrate material. In applications that require precise coating thickness control or infiltration of a porous substrate, such as the formation of a ceramic matrix composite via CVI, the process is conducted at reduced pressure to reduce the reaction rate and to extend the mean free path of the reactants.
The partial pressure of byproducts may increase by several orders of magnitude as they cool and pass through a vacuum system. Depending on the process and byproduct chemistry, the byproducts may stay in the gas phase through the entire vacuum system, or they may condense into a solid or liquid material. In processes where the byproducts tend to condense, great effort is usually used to control where the material condenses so that it can be easily cleaned without dismantling and cleaning the entire CVD system. The condensation of reactant byproducts in vacuum pumps can be particularly problematic causing pump failure in some instances. To circumvent this issue, various cold traps and particle traps are used to condense the byproduct ahead of the pump. Various additional systems may be employed such as heated vacuum lines to insure that byproducts stay in the gas phase until they reach a trap. The presence of reaction byproducts in a vacuum system or process chamber interrupt process yield and adds significant maintenance costs. The problem is particularly acute when the byproduct is hydroscopic and attracts moisture from the atmosphere each time the process chamber is opened for a batch change.