An electron cyclotron resonance (ECR) plasma source 16 cm in diameter using a planar ring-cusp magnetic field and a reentrant coaxial cavity was developed. The planar ring-cusp magnetic field produces a large-area ECR surface. The reentrant coaxial cavity forces microwaves to be introduced into the ECR surface from an annular window of a discharge chamber sidewall. This plasma source generated large-area, uniform, and stable plasmas. At 0.079 Pa of discharge chamber pressure for Ar, the plasma uniformity was 11.1% within a 12-cm-diam plane. Above 184 W of forward power, overdense plasmas were produced on the center axis at 0.079 Pa. The maximum Ar plasma density of 1.14x10(11) cm(-3) was achieved on the center axis at a discharge chamber pressure of 0.079 Pa with a forward power of 437 W.