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Month: July 2020

Knowledge Base Fact Sheet

What Is A Mask Aligner?

1st July 2020

This document overviews the mask aligner and its use in semiconductor photo-lithography (IKB-068).

What is a mask aligner?

A mask aligner is a precision machine tool used in the semiconductor manufacturing process to transfer a pattern onto a wafer or substrate; these patterns are micro and nano in scale.

The patterns (structure) are created using a shadow transfer method, where the pattern (photomask) to be printed is placed between a light source and the substrate to be patterned (wafer). The substrate being patterned is first coated with a photosensitive material (photoresist), this material then reacts to the light projected from the shadow image. The resultant pattern is then developed using specific chemicals.

This process is commonly known as photolithography, more details of this micro-fabrication technique are covered in a separate document Photo Lithography Basics.

Mask aligners enable photolithography to be used to produce semiconductor devices, such transistors, sensors and medical components, etc.

Along with a method to uniformly coat the substrate with photoresist, a mask aligner is crucial to the photolithography process. A mask aligner is used to both precisely align the coated substrate to the photomask containing the structure to be patterned, and for then exposing the substrate with light to transfer the desired pattern onto the substrate.

In order to enable this complex process to take place and to produce structures with features down to 250nm, the mask aligner must be a precision engineered instrument. As these structures are so small, the wavelength of light used is also a factor in determining the final feature size.

The key components of a mask aligner include the following:

  • Ultraviolet light source
  • Optical elements
  • Mask holder
  • Substrate holder
  • Microscope

Ultraviolet light source:

A mask aligner would not be able to expose a substrate to light in a controllable manner, if it does not have a high-quality source of light. Traditionally, this has been a broadband mercury bulb, which provides a spectrum of light (see “Emission Spectrum of a typical mercury lamp” graphic). Despite two wavelengths being outside the specific UV spectrum (100-400nm), mask aligner light sources are commonly referred to as being UV, as the bulb also transmits in this range.

Dependent upon the photoresist used, the light should be transmitted in either the broadband range, or in a specific spectrum G: 436nm, H: 405nm or I: 365nm.

An alternate option to a mercury lamp is to use a UV LED array. These contain a number of LEDs that emit light at the G, H or I line wave-lengths, or a combination of all three to produce a similar broadband spectrum to that of the mercury bulb.

UV LEDs have seen advances in quality and performance in recent years, enabling them to replace the traditional mercury bulb. The introduction of the UV LED has brought a number of advantages. The first of these is a reduction in the power consumption of mask aligners, which brings with it a reduced running cost. In addition, the removal of mercury from the production facility brings health and safety benefits to the operator, plus there is a reduction of the volumes of mercury that must be safely disposed of when lamps reach the end of their lifetime. Other benefits include reduced maintenance, as the LED’s are only illuminated during the exposure period; thus no shutter mechanism is required, plus simplified facility requirements, since no additional cooling or extraction are needed with LED light-sources.

Optical elements:

After the UV source the light passes through a number of optical elements. These optical elements are used to shape the beam of light so that there is uniform illumination across the substrate to be processed.

Typical optical elements include:

  • Ellipsoidal Mirror
  • Cold Light Mirror
  • Heat Sink
  • Shutter
  • Fly’s Eye
  • Condenser Lens
  • Filter Plates
  • Front Mirror
  • Front Lens

The quality of these optical components is critical to the resolution achievable by the mask aligner. Without high quality lenses and mirrors that are kept in good condition and free of scratches or contamination, the resulting lithography will not be homogeneous over the whole substrate. There will be changes in the critical dimensions (CD), under or over exposed photoresist and overall a loss in the yield of functional devices.

Mask holder:

The mask holder is the component that holds in place the photomask containing the pattern to be transferred to the substrate. The mask holder must not allow the photomask to move whilst alignment and exposure are taking place.

Substrate holder:

The substrate holder, often referred to as the wafer chuck, holds the substrate in position within the mask aligner. The substrate is usually held in place by applying a slight vacuum to the chuck and with alignment pins used to mark the rough placement. The substrate holder can then be moved around relative to the mask holder, this allows the precise alignment of features on the mask to existing features on the substrate.

The substrate holder also compensates for any wedge or slope on the surface of the substrate through a process called Wedge Error Compensation (WEC). The WEC is crucial to ensure uniform results across the whole substrate. WEC is the process of ensuring the top surface of the wafer is parallel to the photomask and so the optical path travelled by the UV light is the same regardless of position on the wafer.

Microscope:

The microscope system within the mask aligner allows the user to view the photomask and the substrate and to align the relevant features to one another. Typically, there are two microscope arms which are used to locate the alignment targets on the substrate and the mask, and to then move the substrate on the wafer chuck into position.

Summary:

The quality of these five components outlined above will determine the resolution and alignment accuracy of the lithographic process.

Critical to the final resolution of the features being printed are a well-defined UV source and optics that reduce optical diffraction limitations and ensure uniformity of the light across the whole wafer.

The mask holder and substrate holder together are critical to the alignment accuracy, this is key when fabricating complex devices with many layers which must be aligned to one another.

If employed in a production environment, all elements of the mask aligner can be automated using pattern recognition to detect alignment targets and to correctly orientate the substrate and the mask, in addition to wafer and mask handling systems, to automate the loading and unloaded processes.

Mercury Lamp Emission Spectrum
Emission Spectrum of a typical mercury lamp





SUSS Mask Aligner with LED Light Source
SUSS Mask Aligner with LED Light Source









Example Semiconductor Wafer Lithography
Example Semiconductor Wafer Lithography

For further information on our range of Mask Aligners, please click HERE

Author

Date

Version

Author

Chris Valentine

Date

01 July 2020

Version

IKB068 Rev. 1

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Many Thanks… And A Reminder

1st July 2020

Andover, United Kingdom – In April, Inseto donated £7,000 to seven local charities, all of which are close to the hearts of the company’s employees. Within days of the donations being made all seven charities sent letters that not only expressed huge thanks to Inseto’s employees but also conveyed how important the donations have been and what a positive impact they have had.

Thank You Letter
Matt Brown, Managing Director, pictured with one of the “Letters of Thanks”

Matt Brown, Managing Director of Inseto, comments: “It was great to receive these letters of thanks, which we’ve shared throughout the company, but it was also nice to be reminded of the work these charities are doing during challenging times. It’s all too easy, particularly when social distancing measures are in force, to look inwards, and not beyond the immediate boundaries of our personal and work lives.”

Below, we quote snippets from the letters of thanks Inseto received.

Andover Foodbank
“The disruption to daily life caused by the current virus has affected everyone but its greatest impact is on some of the most vulnerable people. Thanks to your donation, those in need in our local community have received food boxes, toiletries and cleaning supplies. We continue to work with our partner agencies who are helping people stay out of long-term debt or those who are at risk of domestic violence. Schools have been some of our biggest referral agencies during this time, supporting parents to feed and look after their children. We are all living with uncertainty and concern but worrying about how to feed your family increases that enormously. I wish I could show you the relief on people’s faces when they receive one of the food boxes, it is tangible.”

Andover Mind
“A big thank you for your donation of £1,000, this was such a lovely surprise in these unprecedented times. Your donation will help us to continue to support our community, when our services are needed more than ever.”

• The Countess of Brecknock Hospice
“Thank you so much for your kind donation of £1,000.00. We are most grateful for this gift towards our work providing palliative care for our local community. Many of the services provided by the Hospice rely on support and donations to deliver exceptional care to patients and their families at a very difficult time.”

• Abel Foundation
“You’re amazing, thank you, your kind donation will help so many children and families, we couldn’t do what we do without you.”

• Two Saints
“We really appreciate this very generous donation from Inseto in these extraordinary times. Please pass on our thanks to everyone at Inseto for their kindness and please be assured that we will put your donation to excellent use to support the homeless and vulnerable people we work with.”

• Andover Crisis & Support
“The kindness of you all is much appreciated and especially welcome; particularly during these very difficult times which I am sure you will appreciate has led to a substantial increase in demand for our support services. The centre continues to be staffed 24 hours a day by our dedicated team and although we have had to change our way of working in order to comply with current safety measures we are doing all we can to support those in need.”

• Naomi House & Jacksplace
“Thank you for your generous gift of £1,000.00. Everyone at Naomi House & Jacksplace is truly grateful for the support of the team at Inseto UK. These are certainly difficult times for everyone, especially the medically vulnerable children, young adults and their families that use our services. Due to the current circumstances, we had to temporarily suspend respite care at Naomi House & Jacksplace, turning our focus to end-of-life care only. We’ve also seen a significant drop in fundraising incomes, with events and community revenues plummeting, and income from shops seeing a decline. So support like yours in needed now more than ever, and we cannot thank you enough for donating to us at this very difficult time.”

Matt Brown concludes, “It was very humbling to receive these letters of gratitude and to be reminded of how important it is to give back to the community. Together, the communications paint a common picture. Many, many people turn to charities in their hour of need, and those charities are heavily reliant on donations and fund-raising events. Take those away and those dedicated to helping are quickly placed in need of help themselves. Team Inseto was delighted to do its part, and we urge all companies to support their local communities.”

For further information please visit: www.inseto.co.uk