Feature: Sensors
Sensors for semiconductor manufacturing and quality control
By Glenn Wedgbrow, Business Development Manager, Micro-Epsilon UK S
emiconductor wafer manufacturing is a demanding environment, with all production steps subject to the most stringent requirements.
Sensors monitor and control handling systems, machine parts, optics and other associated processes, so they must meet high standards of performance and precision and be robust.
Measuring in wafer production Ingot sawing is just one step that requires high precision. During sawing, the single crystal rod is cut into thin wafer slices using an internal hole saw. Wire saws, on the other hand, allow several slices to be cut from the bar at the same time. Here, non-contact sensors monitor the sawing processes; see Figures 1 and 2. Initially, laser profile sensors measure
the complete geometry of the silicon rod. To do this, the ingot passes a measuring station with several laser scanners; see
Figure 3. In a second step, the laser scanners check the dimensional accuracy of the orientation notches, which are later required for exact alignment of the ingot. Te blue laser scanners are used for this purpose, which generate a highly precise and interference-free measurement signal. In the inner hole sawing process, wafers
are sawn out of the silicon block. Here, eddy current sensors monitor the axial movement of the saw blades; see Figure 4. As eddy current sensors are insensitive to dust and dirt, they provide precise measurement results, even in adverse environmental conditions. Te eddy current sensors measure the
distance to the saw blade support without contact and so are wear-free. Te result is a homogeneous and uniform cut of the silicon wafers. In wire sawing, several wafers can be
cut from the silicon ingot simultaneously. Te thin wires have a diameter of just over 100µm. Due to abrasion, the wire wears out and loses about 5-10% of its
34 February 2023
www.electronicsworld.co.uk
diameter. Terefore, wire wear must be continuously monitored. Here, too, eddy current sensors are used, which record both the height of the wire and wire sag. Te wafer slices created during sawing
must be planar. Deflectometry systems record the flatness of the wafers with just one pass. 3D sensors project a striped pattern onto the wafer surface for this purpose. Integrated cameras record the reflected fringe pattern, and the included soſtware subsequently enables quick and easy evaluation. Distortions of the wafer will cause an equally distorted striped pattern. A highly-accurate 3D representation enables the detection of such deviations at the micrometer level.
Cleaning wafers For cleaning wafers, ozone is added to the cleaning liquid. Colour sensors, like those from Micro-Epsilon, are used to determine the concentration of the contained ozone, by monitoring the colour of the liquid; see Figures 5 and 6.
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