Automated Handling of Ultra Flat Wafers—Challenges and Opportunities

As semiconductor wafers acquire more applications throughout all kinds of industries, their levels of specialization in their manufacturing processes also get more complex. Among all, ultra flat wafers are built with the highest specifications, but they also require the most complex handling conditions.

As all industries' demands for ultra thin wafers increased, the need for automated transportation methods that don’t damage the wafer’s integrity also became apparent. Here are some of the current possibilities and challenges in wafer logistics, in an industry where substrates are constantly getting thinner.

Ultra Thin Wafer Attributes

Ultra flat wafers are thin slices of semiconductor material with a thickness generally under 200 μm. A micrometre (μm) or micrometer is a unit of length equaling 1 × 10−6 metre.

When the term ‘ultra-thin wafers’ emerged, only the mass production of 8-inch wafers of 120 and 70 μm had been achieved. However, today, wafers with thicknesses of 50 μm and 30 μm have also been proven to be viable.

What this demonstrates is how innovative and productive the wafer industry is, with innovation being a constant factor. And as new, and more delicate substrates are developed, industries need to catch up with their machinery and handling methods.

After all, moving these paper-thin discs is no trivial task. But, what makes ultra thin wafers such delicate materials? Here are some facts:

When evaluating the mechanical properties of a wafer for use, elasticity, plasticity, stiffness, strength, and hardness are measured. The reduction of thickness in ultra thin wafers can affect their elasticity, stiffness, and strength, making them easy to bend.
Thin wafers have unusual degrees of bow and warp, can sag due to gravity, tend to stick together and have razor-sharp edges.
Due to their thinness, these wafers can flutter in a breeze.

Any irritation of the wafers after placement could cause breakage, chipped edges, or other losses in the quality of the piece.
The gripping of ~50 µm silicon foils needs to be balanced, placing a well-distributed strength throughout the piece. If not, it can cause chips in the surface.

As you can see, due to the characteristics of the material, the handling of thin wafers in-today’s production lines require high standards of quality. Moreover, this has to be done in a way that fulfills the industry’s demand for thin wafers production.

The secret to that lies in automation. With the right technique and the proper tools, handling these delicate materials in mass production can be achieved.

Ultra-thin Wafer Transportation With Bernoulli Vacuum Grippers

In 2012, the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) conducted an important experiment on automated handling methods for ultra thin wafers. One of the most successful strategies they found was using Bernoulli Vacuum Grippers to transport wafers.

The Bernoulli Type Gripper is a specialized robotic gripper used to manage delicate objects. It's based on the Bernoulli principle, which describes the inverse relationship between the pressure and velocity of a fluid.

By taking advantage of this phenomenon, Bernoulli Grippers can lift and hold items with minimal contact, making them ideal for handling delicate workpieces like thin cloths, films, and ultra flat wafers.

Here’s the process the investigators described:

Lifting

The Bernoulli grippers pick up the wafer and lift it vertically. One of the risks to consider at this stage is the possible vibrations caused by the air drag and the inertia force

To avoid that, large area support of the wafer during pick-up is recommended.

Movement

The wafer is transported to the next step. Further movements in the air can cause vibrations, however, utilizing a vacuum-type gripper helps avoid even stronger damage to the wafer.

During their capability tests, the operating pressure recommended for the vacuum gripper was 3 bar.

Placement

During the placement procedure, wafers often need to be blown off. This can cause shifts in the position, which makes repeatable position accuracy in placement difficult. Additionally, observations showed an unsteady way of how the wafer is released from the gripper’s surface.

To avoid damage to the wafer at this last stage, the placement requires a much longer waiting time than the placement with other grippers. Although it can affect productivity, it’s a requirement to ensure the quality of the material, as any irritation could harm the wafer or cause chipping at the edges.

Pro Tip: The researchers remarked on the need to make an effort to find suitable parameter settings for a reliable pick-&-place application.

The Results of Automated Ultra Thin Wafer Handling

Fraunhofer Institute’s study demonstrated that it’s feasible to handle ultra thin wafers in a fully automated way with the Bernoulli-grippers, as long as the correct parameters are set, and longer time requirements are considered.

As research continues, more handling methods are bound to emerge. However, this research set a standard for the kids of parameter to consider when deciphering potential handling methods.

From the risks of air tension when moving the wafer, to the importance of distributing the vacuum’s force along the entries' wafer surface, these are crucial moments of ultra thin wafer's line of production where extra care needs to be taken.

Further Challenges to Consider When Handling Ultra Thin Wafers

If movements are too fast and sharp, wafers can end up getting completely detached from the gripper and fall down.
Thin wafers can suffer deformations due to air drag in a horizontal direction.
There are still doubts about whether the gripper area can damage the wafer during the pick-up phase.
The vacuum gripping of flat, ultra-thin wafers using cups causes heavy vibrations and punctual loads on the delicate crystalline substrate.
The placement takes extraordinarily long compared with today`s standard wafers. which has a huge effect on the overall production cycle time.