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Bumping up production

13 June 2019

Further to Micro Matters' coverage on Inseto supplying STFC with their latest, semi-automated mask aligner, STFC have contributed a full case study for your consideration, looking at how this technology benefits the alignment of wafers, among other applications:

The UK is renowned for its world-leading engineering capabilities, and for nurturing designs from early concept all the way through to production. Also, when advanced manufacturing is required, the UK is particularly adept at collaborating and sharing areas of expertise. 

This healthy attitude, augmented by access to facilities and equipment, is enabling SMEs to enter markets that might otherwise be inaccessible due to technical challenges and/or financial constraints. In addition, teams from universities can prototype their concepts; and many academic spin-offs have de-risked their launches in this way.

A shining example of the above can be found in the activities of the Science and Technology Facilities Council (STFC). Part of UK Research and Innovation, STFC has a broad science portfolio and works with academic and industrial communities to share its expertise in fields that include micro- and nano-engineering, and wafer-scale manufacturing.

Innovations Technology Access Centre (I-TAC) within STFC provides access to flexible, fully equipped laboratories and manufacturing facilities for start-ups. They can either lease their own self-contained lab, or have access to a ‘hot lab’ on an hourly, daily, weekly or monthly basis. This model reduces the up-front capital costs the start-ups would otherwise face. 

In 2018, I-TAC purchased a SUSS MicroTec MA8Gen4Pro mask aligner through Inseto, SUSS MicroTec’s exclusive representative in the UK. The MA8Gen4Pro is used for the lithography of wafers up to 200mm in diameter.

Mask Aligner

The MA8Gen4Pro is suitable for industrial research applications, as well as full production runs. It is used for the operator-assisted alignment of wafers and can be used for standard and advanced packaging applications, such as 3D wafer-level chip-scale packaging and MEMS.

Some 100 companies and groups from universities are leasing I-TAC’s facilities and working on projects that include lab-on-a-chip, sensors for gas turbines and devices for head-up displays. Some users are starting from bare silicon, whereas others have had their wafers spun elsewhere and use I-TAC for the next processing steps, such as die connect. Though other foundries in the UK offer semiconductor manufacturing services, I-TAC’s model is about the provision of equipment and training.

For many years, the centre has had the ability to process 100 and 150mm wafers. But whenever requirements came in for 200mm, it was necessary to outsource; noting here that switching to larger wafers is not always about economy-of-scale. Some process nodes are only available for larger wafers.

Until recently, the number of requests for 200mm projects was insufficient to justify investment in a 200mm mask aligner.

The Right Connections

STFC Interconnect, a group within the Council’s Technology Department, designs and manufactures advanced packaging solutions. These are typically for scientific instruments that require bespoke sensing capabilities. For instance, most of the chips the group handles are sensors intended for operation in particle physics experiments or the x-ray and gamma ray region of the EM spectrum.

Everything STCF Interconnect does is focused around bare die, to allow photons or particles into the chip, condition them and provide a meaningful signal out for data acquisition instruments. Data is output in parallel, typically at a rate of about 1,000 frames per second (1kfps), though one the group produced recently operated at a staggering 5Mfps. 

Accommodating the parallelism requires several hundred physical connections to a closely coupled readout IC. Traditional wire-bonding around the periphery of a die ceases to be practical. Instead, a ‘flip-chip’ technique developed in-house is employed to connect sensor die to its accompanying readout IC.

One project the group is currently working on has 65,000 pads on the connection layer. These are 20µmin diameter and on a 55µmpitch. STFC Interconnect will soon be placing a small indium bump on each pad. The fabrication of the bumps will be through a standard photolithography method. Photo resist will be placed on the wafer, which will then be spun to assure even distribution. A mask patterned with an array of circles will be placed in the mask aligner, and UV light shone through to selectively crosslink the photo resist.

On track

Once the bumps have been placed, the wafer will be cut to produce more than 100 die. Each die will measure at just 196mm2and carry 65,000 bumps. As for I-TAC’s services, several of its customers have now received training on and used the MA8Gen4Pro.

The investment in the MA8Gen4Pro supports I-TAC’s objective of training SMEs and start-ups, and STFC Interconnect’s duty to provide advanced packaging solutions for some of the council’s scientific instruments.


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