SyLMAND X-Ray Lithography Process Overview

Uploaded by sylmandcls on 20.07.2011


The Synchrotron Laboratory for Micro and Nano Devices (SyLMAND) at the Canadian Light Source
is a unique microfabrication research facility in Canada.
Using the techniques of X-ray lithography (XRL) and deep X-ray lithography (DXRL), it will provide distinctive
capabilities not currently available in Canada and be highly complementary to other existing and emerging facilities.
The laboratory’s focus in the near term will be on research in, and the fabrication of, polymer microstructures
while new capabilities to build metallic devices are developed.

Let’s look at some of the unique advantages that X-ray lithography offers.
1) The capability to fabricate tall structures with up to 100:1 aspect ratios.
Aspect ratios are typically defined as the ratio of structure height to feature width.
2) Smooth and near 90 degree vertical side walls
3) Arbitrary lateral shaping
4) Structure accuracy in the sub micrometer range
Such combination of capabilities is hard to achieve with any other existing microfabrication technology.

These capabilities are made possible by the teaming of a highly collimated x-ray beam from a synchrotron source
and the smaller wavelengths of the x-rays themselves allowing for deep penetration into a light-sensitive photo resist.

There are several critical steps involved in the fabrication of a polymer microstructure using X-ray lithography.
Key drivers for the choice of the processing steps and sequence include:

This flexibility with the choice of substrates offers an advantage over other similar techniques.

a. Positive type photoresists like the commonly used poly methyl methacrylate (PMMA) of high molecular weight.
When these types of polymers get exposed to X-ray light, the molecular chains get broken in the exposed regions
to be washed away during the subsequent development process.

b. Negative type photoresists such as SU-8 or SUEX.
Here exposure to X-rays result in increased cross-linking between the adjacent chains to form a complex
three-dimensional structure that has higher average molecular weight than the original.
Because of this, the irradiated regions cannot be dissolved during the development process.

Major blocks of processing steps in the fabrication sequence are provided.
Based on the pre-requisites of the final microstructure or device, the individual processing steps for each sample
typically include optimized variations of the following:

1) Beryllium mask: High quality and cost (including sub-micron structures)

2) Titanium Membrane Mask: Intermediate quality and cost (2 µm feature sizes and above)

3) Graphite Mask: Intermediate quality and low cost (10 µm feature sizes and above)

It needs to be noted that each of the mask types have different formats.
The beryllium and the graphite masks are circular but the beryllium mask has limited layout area.

This is not a limitation of the mask material but more a limitation of the e-beam writing process that
creates the initial high resolution pattern to be transferred onto the beryllium mask.

In addition, the beryllium and the graphite masks are both substrates
while the titanium mask is a membrane of about 3 µm thick.

The importance of the proper choice of masks cannot be overstated
given that the final structure quality is primarily dependent on this.

Efforts are currently underway to develop a mask technology at SyLMAND and is expected
to be available in about a year or two’s time.

Once the mask choice is made and the mask fabricated,
we are ready to fabricate these polymer microstructures at SyLMAND.

Once the substrate for the corresponding application is chosen,
it then goes through a series of processes before it is ready for exposure with the mask on the beamline.

If a final metallic MEMS device is desired, a thin Titanium seed layer is applied
onto the substrate with a sputtering process.
The titanium layer is then oxidized.
This is done to provide a better surface for adhesion of the polymer resist later.
This step is skipped if the final device is a polymer microstructure.

Given the thickness of the PMMA resist for most applications (>100 µm), PMMA is typically bought with
known molecular weights in bulk sheets of 1.5mm in polymer thickness.

The PMMA is cut to the desired shape typically rectangular or circular, depending on the type of mask
and design layout that is to be used for the exposure.

The cut PMMA is then annealed in a vacuum oven at 110 degree C in a programmed sequence to remove
any residual stress in the polymer sheet before it is glued on to the substrate.
This step is critical as it will avoid future delamination issues from the substrate and prevents cracks
forming in the resists.

Once the PMMA sheet is annealed, it is now time to glue the annealed PMMA sheet
onto the titanium oxide side of the substrate.
Freshly prepared glue (a monomer of the methyl methacrylate) is applied onto the substrate and the
PMMA sheet, then pressed onto the silicon with a custom-made compressed air pressing system.
A thin layer of kapton sheet is placed between the resist and the glass plate
to avoid stiction issues while de-pressing.
The sample is left under the press overnight.
Under pressure, the monomer glue polymerizes and forms a nice glue layer
attaching the PMMA sheet to the substrate.

The glued PMMA is then thinned down to a custom thickness requirement with
a fly-cutting process that uses a diamond-tip.
A coarse cycle removes 100 µm PMMA from the top surface to bring it closer to the required thickness.
A finishing cycle then buffs it up to bring the PMMA to the required thickness
and to have a clear optical finish.

The sample is now ready for exposure with the mask in the beamline.

The mask is mounted on customized mask holders and is then loaded inside the experimental endstation
(the scanner) and bolted on with close proximity to the water-cooled copper ring.

A thin layer of kapton is used to cover the top of the polymer to act both as a pre-filter
as well as a physical barrier in the event of unexpected foaming in the top surface and to protect the mask.

Shims of precise thickness are then placed on the sample at pre-determined locations to introduce a
small proximity between the mask and the polymer surface.
Typical proximity gaps range from 100 µm for micron-sized features
to up to 1mm for large mm size features.

The sample is mounted in the sample holder inside the scanner.
Once fixed, the sample stage is moved up and attached to the scanning stage that already has the mask mounted.

Depending on the type of mask, resist thickness and the feature sizes of the microstructures,
a dose calculation is done using custom-developed software called DoseSim.
During the process of dose calculation, appropriate settings for the mirror system grazing angle and the
chopper settings are used to reduce both the intensity of the incident X-ray beam and to tune the spectrum.
The mirror system acts as a high-energy filter.
The higher the grazing angles of incidence, the softer the spectrum passing through to the sample.
The dose is calculated to have a minimum 3.5 kJ/cm^3 bottom dose while the ratio of
top to bottom dose is maintained to be less than 2.
A higher ratio will cause undesired heat effects including foaming of the resist surface.

Once the dose is calculated and the sample loaded, the beamline equipment are set to the calculated positions
and the pre-filters loaded inside the scanner.

Using the dedicated scanner software, the exposure settings are entered.
In addition to exposure dose, this includes the scan height, scan speed and the aperture settings
to define the beam size, including the contact force to be applied to hold the mask
and the sample together during the exposure scan.
The exposure macro then goes through the initialization process and pumps down the scanner chamber
to 2 mbar followed by flooding of the chamber with 100 mbar of Helium.
The Helium environment helps with better transfer of heat away from the sample and the mask
to the cooled external chamber walls.
The scanner software which handshakes with the beamline controls, then starts the exposure by opening
the vacuum valves and the photon shutter to let through the X-ray beam
to pass onto the sample from the synchrotron ring.
The scanner stages scan vertically to integrate the dose through the full height of the exposure scan.

When the required dose is applied on to the resist, the scanner finishes the exposure by shutting the
beam passage to the sample and closing the vacuum valves, and preparing
the sample stage for unloading the sample.
When the scanner chamber is vented, the sample stage is moved down to unload the sample with the
latent image of the mask features imprinted on them.

Immediately after exposure, the sample is immersed in a pre-mixed developer solution to wash away
the exposed areas of the resist to the x-ray beam.
This process is time-critical as the longer the sample sits outside after exposure, the harder
it becomes for the exposed resist to be removed.
The development process is finished by immersing the sample in a milder developing solution
and finally in deionized water.

At the end of the development process, a detailed inspection of the sample is undertaken to confirm the
integrity of the pattern transfer and to look for defects from thermal or translation arising from relative
motion between mask and substrate during exposure.
If defects are identified, the causes for defects are characterized and the entire process fine-tuned
to minimize and eventually eliminate the source of defects in subsequent runs.

If no defects are detected, the sample is passed along to the metrology stage for
imaging with a scanning electron microscope.
Once imaged, the samples are ready for use if they are meant to be polymer microstructures.

For metallic devices, the sample is passed along to subsequent electroplating processes using the metal seed
layer to fill the voids with metal, typically Nickel or Copper.
Once electroplated, the remaining polymer is removed through a flood exposure process,
followed by development.
The seed layer is then removed by bombardment with Argon ions inside a reactive ion etcher.

This video tutorial is intended to provide a general overview of the X-Ray Lithography technique
and the various processes involved in the fabrication of micro and nano structures at SyLMAND.
Only the commonly used processes are shown in the video and are not inclusive of all the capabilities at SyLMAND.

Interested researchers are highly recommended to contact SyLMAND staff to discuss their projects and needs.