Tin Oxide transparent conductive oxide(TCO) films of less than 20nm in thickness was developed as a substrate for pen touch screens. The films were deposited using APCVD method by hydrolytic decomposition of stannic chloride. Electrical properties and stability to heat treatment were studied in relation to deposition conditions. We found the both properties depends mainly on fluorine concentration in the films. The stability was much improved at reduced fluorine concentration. The film which showed the stability to heat treatment at 600°C heat treatment was attained at the thickness of 20nm.