the Best Wafer Inspection System in the world. . . probably

This machine was designed primarily for the inspection of Silicon Wafers.

These wafers are used in the Semiconductor Industry to produce the chips which are the building blocks of most modern electronic equipment.

The value of each wafer can be hundreds or in some cases even thousands of pounds, but they can be very easily ruined by faulty handling equipment.

The machine described here has facilities for visually inspecting both sides of the wafers, and also for the microscopic inspection of the intricate circuitry on each die (chip) on the wafer.

Following there is a collection of pictures and a lot of technical stuff about this machine, which at some point seemed to involve me in everything from the fundamental Laws of Physics though to the latest software techniques - and a lot of pretty tricky stuff in between.

* but it was worth it - I think!

A brief summary of the Inspection Process

• A cassette of wafers is placed onto the machine's loading tower
  - this moves vertically to allow the wafers to be individually transferred from the cassette to the machine
  
  
• The machine scans the cassette to detect which slots are occupied, and to check that the wafers are properly located
  - the results of the scan are then presented to the operator for approval
  
  
• The operator chooses which wafers to inspect, and which type of inspections to perform
  - this is achieved through a customised windows-style GUI
  - input to the GUI can be either with the touchscreen or via the joystick
  
  
• Each selected wafer is then automatically placed onto the Aligner Module where its profile is sensed to enable accurate positioning
  
  
• The wafer's identification code is placed under an OCR Camera for visual recognition
  - each wafer has a unique code, which is inscribed on the wafer as either a bar-code or an alphanumeric string
  
  
• The topside of the wafer is then presented to the operator for inspection, mounted on a tilting and rotating chuck
  - the rotation and orientation of the chuck is controlled with the joystick
  - a bright light is used as an aid to locate any contamination or physical defects
  
  
• The wafer is then picked up from the chuck and the reverse side is placed in front of a small video camera
  - the position of the wafer is controlled with the joystick
  - the live image is displayed on the monitor
  
  
• The final stage of the operations is to transfer the wafer to an XYtable on the microscope
  - the motion of the table under the microscope is also controlled by the joystick, in increments of 0.08 microns
  
  
• During each inspection a dynamic and fully interactive representation of the wafer with all previously recorded data points is displayed on the monitor
  - new data points can be input via the touchscreen
  - for each wafer the results of each inspection are stored in a database for analysis
  - there can be up to two wafers being handled at any time, one actually being inspected and one being prepared

From left to right the main features are:

• Main control system containing CPU
• Indexer Tower with a cassette of wafers
• LCD monitor with touchscreen
• Topside and Rearside inspection housing
• Motor drive towers at rear
• Microscope with XYtable attached
• Multi-purpose Joystick

The main control system consists of the CPU and control boards for motor drives, sensor input, vacuum control and a frame grabber that is shared between the OCR system and the video camera.

The joystick is used to tilt and rotate the wafer during the topside inspection and to rotate the gripper mechanism that is used for the rearside inspection.

It also controls the XYtable on the microscope - the motion can be finely controlled such that even under the highest magnification the wafer can appear to be moving very slowly, or by pressing the joystick "boost button" the whole of the wafer surface can be crossed in less than one second.

And finally, the joystick is also used to control the screen cursor for interaction with the GUI as an alternative to the touchscreen.

In this picture the inspection cover has been removed and you can see the three-armed gripper mechanism holding a wafer.
The cover itself contains a bright light to illuminate the wafer during the topside inspection, and a small video camera to view the rearside of the wafer.

The gripper can pivot upwards to present the rearside of the wafer to the operator for initial inspection.

The whole gripper assembly can be raised to position the wafer vertically in front of the video camera which is located inside the inspection cover.

A live image of the wafer can be displayed on the monitor at 4X magnification.

Due to the stepped construction of the gripper jaws, they can hold wafers from 4" to 8" diameter at approximately the same jaw positions.

The aligner is the mechanism which maps the position of the wafer to the machine
- this needs to be done before any of the other inspection processes can occur.

When the wafer is first picked up from the cassette it may be facing in any direction
- also it may not have been placed centrally in the slot and so the position of its exact centre is unknown.

The aligning process is as follows:

• The wafer is rotated above a slit which contains an analogue light sensor. The output signal is proportional to the length of the slit which is obscured, and this can be used to deduce the profile of the wafer and hence locate the flat or notch on its edge.
• The flat or notch is rotated towards to lie between the two prongs of the vee-block mechnanism. Each prong contains a simple sensor which can detect the presence of the wafer edge.
• By a rapidly converging series of orthogonal moves the edge of the wafer is positioned so that it is just triggering both sensors simultaneously. As two points known to lie on the circumference have now been found then the position of the centre is easily calculated.
• Now that the rotation and centre of the wafer have been found, it can be accurately handled from this point onwards.

Here we see a wafer held by vacuum on the end of the linear arm mechanism.
It is being offerred to the microscope chuck at which point the vacuums will be switched and the transfer will occur.

If you are wondering how it can jump the gap from the arm to the chuck, then let me tell you that this is actually a temporary mounting for the microscope chuck and not the real thing. But at least it shows the principle involved - doesn't it??

This is the main PCB that I spent many weeks designing.
It is a 4-layer board with the main tracking layers sandwiched between a layer for the ground plane and a layer for the power tracks.
The connectivity is extremely complex with approximately 1000 individual channels, many of which have multiple connections.

I also designed several other reasonably complex PCBs for various modules in the system, but the interesting aspect of this board is that it contains a link to essentially every part of the system.

It is the common junction through which every signal has to pass at some point, hence my reference to the "spinal column".

The PCB manufacturers told us that this was the most complex board they had ever made. When it arrived I made a point of populating it personally and wiring all of the connectors. I plugged everything in, turned it on, thinking anxiously of the hours of endless checking and painstaking attention to the most trivial details - and it worked perfectly first time!!

This is the testrig used to develop the system. It was used to mount the control components which were eventually to be fitted into the machine housing.
Although it may not look the most perfect development environment it was a great way of keeping everything together and also providing easy access.

This is an example of a typical wafer to be handled - it is 6 inches diameter (150mm), and contains almost 300 dies - an average type of wafer.
( The Metric System is widely used in the Semiconductor Industry, but wafer diameters are commonly quoted in inches! )

• Wafers sizes range from as small as 2 inches up to 12 inches.
• The number of dies on each wafer can range from a few large dies up to several thousand.
• The thickness of a wafer can vary from about 1mm for very sturdy wafers to less than 50 microns when they become extremely fragile bending as easily as paper.
• The most common material used is silicon - each wafer is sliced from a single crystal in the shape of a cylinder specially grown for this purpose.

Here is an example of a cassette used to store wafers - this one is for 4 inch (100mm) wafers, and has 25 slots.