CMP Slurry Monitor: An Overview of the Sensors

A CMP Monitor sensor consists of five separate sensors:

  • pH sensor
  • Pressure sensor
  • Combined temperature and conductivity sensor
  • Electroacoustic (particle size and zeta potential) sensor
  • Concentration sensor

The first three sensors are conventional industrial instruments, while the electroacoustic and concentration sensors have been developed by Colloidal Dynamics specifically for the CMP application. This description therefore focuses on the latter sensors.

The Electroacoustic (or ESA) Sensor applies high-frequency alternating voltage pulses across the slurry. This causes the slurry particles to shake back and forth at a velocity that depends on their size and electric charge (or zeta potential). This particle motion generates sound waves, which in turn depend on particle size and zeta potential.

The input voltage pulse is applied across two flat, parallel platinum electrodes that are in contact with the slurry. The slurry flows vertically upwards through a polyphenylenesulphide (PPS) spacer in the sensor. One of the platinum electrodes is coated onto a rectangular glass delay line, at the opposite end of which is mounted a thin ultrasonic transducer.

The gated amplifier produces a pulse of high frequency alternating voltage of about 3 µs duration, which is applied across the parallel electrodes. The resulting high frequency sound wave produced by the motion of the slurry particles in the alternating electric field travels down the glass delay line and is detected by the ultrasonic transducer.

The measured electrical signal, called the ESA, contains information about the particle size and charge, and is the primary measurement made by the electroacoustic sensor.

The ESA measurement is repeated a number of times at a range of frequencies from 1 MHz to 20 MHz. The Fourier Transform of these signals is referred to as the ESA frequency spectrum. From this spectrum the particle size distribution and the zeta potential are ultimately calculated.

The Concentration Sensor consists of a short, thick-walled pipe made of PPS through which the CMP slurry flows vertically upwards. Ultrasound pulses produced by a thin transducer are reflected off the pipe-slurry interface. The particle concentration is calculated from the magnitude of the reflected signal using a routine that has been specifically tailored for CMP conditions.