There is provided a chip for use in a combination microbalance and optical monitor sensor, the chip comprising a piezoelectric element and electrodes formed on a first surface thereof. At least a portion of the piezoelectric element is translucent. There is also provided a chip comprising a piezoelectric element and electrodes in contact with the piezoelectric element, at least a portion of at least the first electrode being translucent, and at least a portion of the piezoelectric element being translucent. Current can be applied through the electrodes to cause a region of the piezoelectric element to undergo vibration, and light can be passed through a portion of the region of the piezoelectric element which is undergoing the vibration. There is also provided a combination microbalance and optical monitor sensor, comprising a chip as described herein, and a method of simultaneously piezoelectric crystal vibration monitoring and optically monitoring a layer.