Application spotlight: Wafer mapping

Moores Law. For decades it has been a driving force in the semiconductor industry, for production equipment manufacturers and their end-users alike. The law states, that the number of transistors in an integrated circuit doubles about every two years and with it, the complexity but also the performance.

This means that the structures on those tiny silicon plates are becoming more and more complex and delicate. This, in turn,  causes the manufacturing process to become more and more expensive. The cost for state-of-the-art manufacturing equipment is increasing exponentially. The catch in all of this is, that the retail price of those finished chips is actually decreasing. A fact which poses a challenge to all semiconductor manufacturers.

To stay profitable those manufacturers of ICs, memory chips and microprocessors have to find other ways to increase their margin. The first angle of revision is – of course – their manufacturing processes. The goal is to increase efficiency and yield, while at the same time reducing throughput time. Automation is the key word here.

Automated Material Handling

Semiconductor manufacturing facilities are fully automated, high-tech environments. And one aspect of the automation is material handling.

In mass manufacturing of semiconductor chips, silicon wafers are being handled and processed in batches. This results in a reduction of throughput time and an increase of efficiency and yield. Transport between different processing steps is usually done in special cassettes or FOUPS (front opening unified pods) as you can see in Figure 1.

Automated handling of semiconductor wafers
Figure 1: Wafer handling robot and Wafer transportation FOUP

In order to increase production volume and throughput, handling speed is also essential. However, the risk of breaking any of the wafers during the transfer from FOUP to the processing chamber must be kept as small as possible, since the value of one wafer can go into the thousands of Euros. No easy task, considering that a wafer is an extremely fragile, less than 1mm thick disk. No human being could perform the handling and transfer of wafers faster, more accurate, more reliable and as tireless as a robot.

Why is Wafer Mapping important?

But how does the robot know how many wafers there are actually in the FOUP. And if they are placed correctly. In some cases, it can happen that the wafer in the FOUP is cross slotted or double stacked. This is portrayed in Figure 2.

If the robot doesn’t know or detects this, it will crash into the wafer, causing broken wafers. Broken wafers cause machine downtime and losses, that can go into the tens of thousands of Euros.

Therefore, the so called “wafer mapping” step is crucial.

Possible loading faults in a wafer carrier
Figure 2: Possible loading faults in a FOUP

Process of Wafer Mapping

At the end of the robot arm, which is responsible for the wafer handling, there is the end-effector, which is mostly some kind of forked blade as you can see in Figure 3.

Wafer mapping sensor on robot end effector
Figure 3: Robot end effector, equipped with BALLUFF wafer mapping solution

This end effector will go inside the FOUP just far enough, so the wafer will interrupt the beam of the mapping sensor. By moving up and down all the way, the control system can “count” how many wafers there are and if one or more are not placed correctly.

Since the gap between the wafers, which are stacked in the FOUP is only very few millimeters thin, the whole end effector must be just as thin. This in turn requires the wafer mapping sensor at the end of the blade to be just as thin and as light as possible.

BALLUFF has developed a sensor which is especially designed and perfectly suitable for this purpose. With only 1,5mm thickness it is the thinnest you can get. However, the reliable switching distance is still at 800mm. This is more than enough, considering that the maximum wafer size is currently 300mm.
The extremely controlled and focused light spot with outstanding homogeneousness lets you precisely detect the edges of the few µm thick wafers.

BALLUFF solution

Tight and cramped installation environments and obstacles like bending radiuses, are also not a problem, since the patented BALLUFF solution (BOH product family) does not use fiber optic technology – unlike all competitors solutions. The light emitting and receiving technologies are entirely inside the 1,5mm x 2,4 mm x 7mm sensor head of our wafer mapping sensor BOH00EZ. This means the connecting cable is exactly just that. A cable. Without the limitations of fiber optic technology. This makes the BALLUFF solution unique and extraordinarily reliable.

Talking about reliability. Repeated trials and examinations have shown, that the BALLUFF microspot technology is much more reliable and has a significantly higher repeatability than the common fiber optic technology.

Edge resolution of Balluff micromote compared to common fiber optic
Figure 4: Comparison of the results of repeated trials of BALLUFF and others technologies

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