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What is MEMS Technology? Understanding MEMS in Wafer Testing

A MEMS probe card for wafer IC testing

Imagine an electromechanical system the size of a grain of sand or a functional device that can fit on a silicon chip—because that’s what MEMS technology can produce. Join us as we explore the technology and its critical role in semiconductor testing. We also shared our thoughts on how we expect things to change. First, what does MEMS stand for?

What is MEMS Technology?

“MEMS” is an acronym for Micro Electromechanical Systems. In electronics, it means tiny systems with both electric and mechanical functions. The electrical elements process data, while mechanical parts respond to the data.

“MEMS” can also mean the technology of making devices with tiny components. The devices are generally sensors or actuators, with parts ranging in size from a few millimeters to a few microns or less. (For comparison, human hair is 50 -100 microns thick).

MEMS technology has dramatically changed the electronics industry, shrinking device sizes to unimaginable levels and increasing their performance and functionality. Below, we examine the different ways of manufacturing these microsystems.

MEMS fabrication using the etching process
MEMS fabrication using the etching process
Resource: https://www.youtube.com/watch?v=UKf0offCyw

MEMS Fabrication Technology

Most of the techniques used to manufacture microelectronics can fabricate micro-electromechanical systems. These processes are either additive, subtractive, or both. They fall into these major categories: etching, lithography, and deposition. Each category has several different execution methods, as discussed below.

Etching

Etching uses chemicals or micromachining methods like a laser to selectively remove unwanted sections from a substrate. The result is a structure that traces the required circuit or other structures.

Etching in MEMS fabrication can be a wet or dry type. Wet-etching means using chemicals to create microscopic parts. Dry etching utilizes plasma or ion–milling technology.

Dry etching is the most effective and versatile method. It is more selective and automatable than wet etching. It’s also easier to control, resulting in precise elements and cleaner with no toxic chemicals like acids involved.

Deposition

Deposition involves adding layers of materials to a base layer or substrate. Later, the layers are etched or micro-machined. This fabrication method leaves a microscopic 3-dimensional structure on the substrate.

Deposition methods include epitaxy, sputtering, anodic bonding, and evaporation. It can also involve vapor deposition, where gaseous reactions create thin layers. Vapor deposition is either chemical or physical.

In chemical vapor deposition, gaseous materials react to form a structure on the substrate. Physical vapor deposition involves physically casting a material on the substrate.

Lithography

Lithography (or patterning) is the most popular MEMS fabrication method. A pattern representing the required structure gets transferred onto a substrate. Removing unwanted parts leaves a microstructure of the pattern imprinted.

Lithography can take many forms and involve various pattern-creation methods. One example is photolithography, which uses light. Others include ion beam lithography and electron beam lithography. It may also use X-rays.

Photolithography is the most used technique. It involves coating a substrate with a photosensitive mask. Upon exposure to UV light, the mask hardens in the desired parts. Further processing involving chemical etching produces a functional microstructure.

The construction of a MEMS probe card for wafer testing
The construction of a MEMS probe card for wafer testing
Resource: https://www.semanticscholar.org

MEMS Technology Application in Wafer Testing

The potential for MEMS technology is virtually endless. In the semiconductor industry, its advent made IC testing at the micron level possible. Manufacturers could achieve better testing results for the first time with increased accuracy and efficiency.

During large-scale IC production, a wafer holds several integrated circuits. The wafer is a silicon sheet or other semiconductor material. The ICs are later separated by slicing the wafer and packaged into the required semiconductor device.

Testing individual IC packages can be costly. Testing them while still on the wafer is a more economical option. It helps eliminate bad dies from the lot, and only suitable circuits proceed to the packaging stage.

Using MEMS probers, it’s now possible to test several circuits simultaneously, reducing the time spent on the process and lowering costs. These devices are custom-made and match the die under test, DUT.

MEMS probe card structure
MEMS probe card structure
Resource: https://www.semanticscholar.org

MEMS Probe Card Technology

A MEMS card is a wafer or IC testing device manufactured using micro-fabrication technology. It consists of tiny mechanical and electrical parts. The typical probe (and the most common style) is a circular PCB with connectors on the edges and a hole at its center.

The space in the middle contains an array of pins. These are test needles with sharp points. They contact the pads on the wafer, send test signals to the IC circuits, and relay the results via the card and prober to the ATE (Automatic Testing Equipment) machine.

A MEMS probing card can also have microscopic bumps instead of pins. The beauty of it is that it packs many test contacts in a tiny area, allowing it to test high pin count circuits with ease and increased reliability. The following section details these benefits.

One of the most significant benefits of MEMS probe cards is their reduced pin sizes.
One of the most significant benefits of MEMS probe cards is their reduced pin sizes.
Resource: https://www.sciencedirect.com

Advantages of MEMS Technology in Wafer Testing

MEMS devices are already present in an extensive range of products. The technology delivers new levels of performance and efficiency in specialized applications, such as semiconductor testing. When used to check microchips electrically, it offers these benefits.

Precision and Accuracy

The tiny probes make precise contact with the microscopic pads of IC circuits, increasing testing accuracy. The low inductance of the probes and reduced parasitic capacitance prevent signal distortion, further improving the results’ reliability levels.

High Density Probing

With microstructures, these probe cards are the best solution for testing densely packed IC wafers. Compared to the traditional probers with larger needles, it matches the pace witnessed in electronics today, where a smaller size is better.

Increased Scalability

The technology adapts to various wafer manufacturing needs and processes, supporting small and large-scale production or simple and highly intricate circuits. It also allows manufacturers to adapt testing devices to new requirements to match industry growth.

Manufacturing micro-card probes for testing miniature semiconductor devices
Manufacturing micro-card probes for testing miniature semiconductor devices
Resource: https://www.e-jwj.org

The Future of MEMS Technology in Semiconductor Testing

MEMS technology has proved its effectiveness in the semiconductor industry today, making wafer testing more precise and accurate. With micron-level parts, it keeps pace with the rapid miniaturization of new-age electronics, securing its place as the testing technology of tomorrow.

Currently, research is moving toward producing better microstructures. MEMS manufacturers and other key players are discovering new materials and fabrication technologies—these promise to improve the performance of micro-electromechanical systems and reduce their manufacturing costs.

In the coming years, we will likely witness the wafer testers become even smaller, more powerful, and cheaper than they are now. And that’s just a tip of what’s to come. Integrating artificial intelligence (AI) will increase MEMS testing capabilities tremendously.

Conclusion

MEMS technology has become so popular and widespread since its beginnings several decades ago. Today, it forms the foundation of IC circuit testing, delivering high performance and reliability. Judging by the current trends, micro-electromechanical systems technology will continue to improve and become more powerful than it is now. More importantly, the advancements will see probe card fabrication costs come down.

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