White light interferometer offers nanometre-accurate thickness measurements of monocrystalline silicon wafers

Micro-Epsilon has further developed its interferometer range of white light interferometers to include a system specially designed for non-contact, high-precision thickness measurement of monocrystalline silicon wafers. This further expands the range of potential applications in the semiconductor industry. 

The new interferoMETER IMS5420-TH24 system incorporates a broadband superluminscent diode (SLED) with a wavelength range of 1,100nm, allowing thickness measurements of undoped, doped and highly doped silicon wafers using only one measuring system. Thicknesses from 50 to 1050 µm can be measured with a doping of up to 5 mΩcm from a distance of 24 mm and signal stabilities (z-axis resolution) of less than 1 nm can be achieved. Even for highly doped wafers with a specific resistance of 0.005Ωcm, thicknesses from 0.05mm to 0.85mm can be measured. 

With a measuring rate of up to 6kHz, the interferoMETER IMS5420-TH24 is suitable for in-line and at-line thickness measurements, for example, after and during grinding and lapping, as well as during quality control. Other applications for the measurement system include thickness measurement of glass, film or silicon carbide wafers. 

Multi-layer thickness measurements 

The interferoMETER IMS5420 is available either as a thickness measuring system or as a multi-peak (MP) version for thickness measurement of up to five layers (e.g. wafer thickness, air gap, film and coatings >50µm) with silicon thicknesses from 0.05 to 1.05mm. The measurable thickness of air gaps is up to 4mm. In addition, a version of the controller with IP67 protection, a stainless steel housing and suitable fibre optics/sensors is available for thickness measurements during lapping or for use in other harsh industrial environments. 

Pilot laser 

White light interferometers work with infrared, non-visible light (approx. 100 m), so the measurement position cannot be seen directly. To visualise the measuring position, all interferoMETER systems are equipped with a pilot laser that projects a light spot onto the measuring position. The pilot laser uses a patented method to provide feedback on the distance to the target and if the sensor is within the measuring range, a constant glow is emitted by the pilot laser. If the measuring object is outside the measuring range, the pilot laser flashes. 

Unmatched accuracy 

The interferoMETER range of high performance white light interferometers from Micro-Epsilon provide unmatched accuracy levels in absolute non-contact optical distance and thickness measurement. With measurement resolution from 30 picometres and linearity from ±10nm, these interferometers represent a step change in accuracy from its confocal measurement systems. The interferoMETER range comprises a controller, a sensor and a fibre optic cable. Unlike traditional laser interferometers that use a single wavelength and only perform relative measurements, the interferoMETER range operates with a broadband super-luminescent diode (SLED) and can make absolute measurements, ideal for step height and surface measurements. 

For easy setup and configuration, a modern, intuitive web interface is provided. In addition, various interfaces (EtherCAT, Ethernet, Profinet, EtherNet/IP, RS422, analogue) offer numerous integration options.