The humble chuck plate plays an integral part in supporting the wafer throughout the front-end processing, not just to accurately position the wafer during testing. With applications for the semiconductor industry ever diversifying, so are the substrate and materials used to provide new products so the chuck plates are constantly adapting.
Wafer chucks are available in a variety of shapes, sizes and materials to meet an ever-increasing range of applications. While round chucks accommodate most semiconductor wafers, square or rectangular chucks can be used for certain substrates for everything from optical windows to flat panel displays, hard disk drives or photonic applications.
Then there is the hold-down. Most chucks are available with circular vacuum grooves patterns or micro-holes to hold the wafers in place, but even these can sometimes cause measurement issues.
Lift pins can be fitted to the chuck assembly, to automatically raise and lower the substrate through the wafer processing positions. Add to that thermal capability, there is a lot to a simple chuck plate.
Chucks can be manufactured with different base materials. Thermal conductivity of materials varies widely and should therefore be a factor in choosing the plate material. Aluminum is the most commonly used because of its good thermal conductivity, ease of manufacturing and cost. Other possible materials are bronze, brass, copper, stainless steel, ceramics or glass although considerations need to be made depending on application, environment and temperature.
Chucks are available with a variety of conductive or insulating surfaces, with options including anodized aluminum, nickel, zinc or gold plating. Whatever the material and coating, flatness specification typically ranges from 4 µm to 10 µm.
Flat panel display screen
Holding flat panel displays onto a probe station, requires modifications to the chuck plate to provide ideal testing conditions, given the range of possibilities in this market. Wentworth has provided a number of systems for different applications in this area.
The chuck was designed to test rigid and flexible display screens, to be used in a flexible glass-free ‘e-paper’ screen. This allowed four large display screen substrates to be mounted for simultaneous testing, as well as probing of the edge tracks.
To enable the testing of irregular probing geometries on large area devices or panels. Two height sensing, flying arm probes were added to test the edges of the large screen. These probes were able to move independently from the bespoke X/Y chuck stage, to which they were mounted.
360° chuck rotation is an unusual requirement. Whilst not mechanically challenging, it does provide some interesting issues around control, particularly regarding angular positioning and cable management. This was required for a couple of different applications:
The customer needed to simultaneously probe and sense optical devices in a particular orientation, without the probe needles obscuring any part of the device. Opposite sides of each device had to be probed during the process. A chuck solution with 360° movement was developed which allowed for the two opposite sides of each device to be tested, 180° at the time. This ensured that the detector above the device, which formed an essential part of the test process, was not obscured during any part of the testing.
A circular 360° movement chuck was designed to enable the customer to probe ‘bumped’ devices, using the same probe card set-up, from all four sides.
Singulated die chuck
We have designed a large number of this type of chuck plate for a wide range of customers. These are typically required for known-good-die (KGD), or where die separation is required to perform functional testing. Designs have either been mechanical hold-down or vacuum; where vacuum is selected, this can be achieved via chamber or micro-holes.
Chucks with high level of flatness have been designed to hold any number of dies, including different sized die on the same chuck. Subject to pad size, pattern recognition capability can then be used to locate each die for probing. This also allows the flexibility to not fully populate the die carrier, with the system still functioning correctly. If no die is present, this will not be added to the wafer map.
MEMS (Micro-Electro-Mechanical Systems)
When probing MEMS devices, it can be necessary to allow for the device under test (DUT) to physically move. When the device is an accelerometer, or more complex mechanical structure, movement may be required. Holding a wafer firm when it needs to move, is a delicate balance of engineering which Wentworth has achieved.
Using the latest manufacturing technique, we have produced a number of chuck arrangements that have both sufficient vacuum to securely locate the wafer, and cavities to provide the freedom for physical movement of the DUT.
Switchable configuration chucks
Different probing environments require a chuck plate to be configured in many different ways. Making a system that is easy and quick to configure, helps engineers obtain fast and accurate results. High voltage chucks can be standard or thermal and configured with kelvin, coaxial or triaxial connection.
This high voltage chuck, insulated up to 10 kV, was developed so it could also be used for extreme high current and provides capability for all MOS device characterization including Quasi-static/CV measurements. Using a Wentworth current/voltage selector switch, enabled cabling configuration to be switched in seconds without the need for extensive wiring changes.
There are a number of reasons our customers need to probe devices on a saw-frame. However, using this on a standard chuck plate can cause some issues.
The film can frequently ‘stick’ to the chuck surface and tear as the operator or frame handler is trying to remove it. Vacuum can leak from the normal chuck layout and cause the film to be adhered to the chuck plate surface. When the vacuum is switched off, pockets of localized vacuum remain, causing the film to stretch or tear when the handler tries to unload and load the next frame. The vacuum layout is important to this, as the traditional groove needs to be avoided.
In addition, changes are also required to the chuck material and design, to avoid this stiction. Wentworth’s proprietary wafer-frame chuck design ensures both wafers and wafer frames can be probed on the same system. The frame warping can be overcome, but metal frames perform much better and have longer life.
The measurement of photonic devices, requires light of many different wavelengths to be presented and/or measured.
If this is from the top of the device, measurement is much easier. When this is required from the underside of the device, our chuck plate design enables probing from one side whilst allowing wavelength and intensity data to be collected through the high purity glass.
Where wavelength compatibility is not suitable for specialized glass chucks, our wafer carrier – used for double-sided probing applications – could be a suitable option.
We have also designed device holders for the smallest of side emitting laser bars. This permitted the devices to be probed from the top, emitting from both sides.
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This blog covers just a small selection of our customers applications. In some cases, customers require two or more different configurations so we have developed an interchangeable system for quick and easy chuck plate change offering maximum return on your probe station investment. Follow us on LinkedIn to keep informed about new and interesting chuck plate applications.