  We report the development of a continuous-wave and pulsed X-band electron
spin resonance (ESR) spectrometer for the study of spins on ordered surfaces
down to cryogenic temperatures. The spectrometer operates in ultra-high vacuum
and utilizes a half-wavelength microstrip line resonator realized using
epitaxially grown copper films on single crystal Al$_2$O$_3$ substrates. The
one-dimensional microstrip line resonator exhibits a quality factor of more
than 200 at room temperature, close to the upper limit determined by radiation
losses. The surface characterizations of the copper strip of the resonator by
atomic force microscope, low-energy electron diffraction, and scanning
tunneling microscope show that the surface is atomically clean, flat, and
single crystalline. Measuring the ESR spectrum at 15 K from a few nm thick
molecular film of YPc$_2$, we find a continuous-wave ESR sensitivity of $2.6
\cdot 10^{11}~\text{spins}/\text{G} \cdot \text{Hz}^{1/2}$ indicating that a
signal-to-noise ratio of $3.9~\text{G} \cdot \text{Hz}^{1/2}$ is expected from
a monolayer of YPc$_2$ molecules. Advanced pulsed ESR experimental capabilities
including dynamical decoupling and electron-nuclear double resonance are
demonstrated using free radicals diluted in a glassy matrix.
