DRYMAC (PDR-C Series, ULVAC JAPAN Ltd.) is a dry roots vacuum pump with six stages. Casings and rotors are made of aluminum alloy. Its high thermal conductivity enables to control the pump temperature in wide range with uniformity. DRYMAC is thus available in wide variety of applications. Silicon nitride (Si-N) process in LP-CVD systems is one of the hardest applications for pumps. NH4Cl generated as a by-product is easily deposited inside the pumps. The deposition of NH4Cl makes maintenance interval shorter. We performed LP-CVD Si-N process using DRYMAC as a vacuum pump and observed NH4Cl amount deposited inside the pump. Keeping pump temperature high and introducing Nz gas ballast of 40 l/ min into the cylinder for dilution, no deposition of NH4Cl was observed. Temperatures and pressures in cylinders were also measured under the same pump running conditions. From these results, the partial pressures of NH4Cl in the cylinders were calculated. In case of above pump running conditions, the estimated partial pressures of NH4Cl in the cylinders were lower than the sublimation pressure of NH4Cl. The results of calculations were consistent with the observation of the cylinders of DRYMAC used in Si-N processes. We have established a method that enables us to find a pump running condition where no deposition occurs in the cylinders.