FABRICATION OF A NANOIMPRINT

LITHOGRAPHY MASK FOR IMPROVED

INFRARED DETECTORS

Isha Datye

Faculty Mentor: Dr. Sanjay Krishna

Graduate Student Mentor: John Montoya

The Center for High Technology Materials

The University of New Mexico, Albuquerque, NM 87131

Undergraduate Student of

The Department of Electrical and Computer Engineering

The University of Illinois at Urbana-Champaign

Urbana, IL 61801

ABSTRACT

Infrared photodetectors will require new technologies on the pixel level to provide

spectral information for the development of polarimetric and color images.

Current infrared photodetectors have nearly identical pixels over a broad spectral

range, resulting in black-and-white images. Scientists have been researching the

idea of an infrared retina, which is similar in function to cones in the human eye,

to produce multi-color images. A multi-color infrared camera system can be

accomplished by tuning individual pixels to a specific infrared “color” with the

aid of resonant structures patterned onto a photodetector’s surface. In addition, a

resonant structure can also improve a detector’s detectivity (D*, a measure of the

signal to noise ratio) or increase the operating temperature. One of the key

limitations of present day technology is the difficulty in making deep

subwavelength structures on a large scale. This research paper will focus on the

fabrication of a nanoimprint lithography mask to pattern resonant structures on a

scale that would make this multi-color technology ready for mass fabrication.

Research has been conducted on nanoimprint lithography and it has proven to be

a very efficient method to pattern structures on substrates with nanoscale

precision. The goal for this project was to pattern complicated resonant structures

on a substrate for the development of a multi-color infrared camera.

1. INTRODUCTION

The infrared region of the electromagnetic

spectrum has wavelengths from 0.75

microns to 1000 microns, longer than that of

visible light, which has wavelengths from

about 400 nm to 750 nm. Infrared detectors,

photodetectors that respond to infrared

radiation, have made significant

improvements since they were first

developed. Although there are several

different divisions within the infrared

region, the mid-wavelength (MWIR) and

long-wavelength (LWIR) infrared regions

are most important for infrared detector

technologies, since they include the

wavelengths at which most objects emit

radiation. For example, humans emit

radiation at a wavelength of 10 microns,

which is in the LWIR range [1]. There is an

increased emphasis on obtaining

hyperspectral and hyperpolarimetric

sensitive detectors for night vision, missile

tracking, medical diagnostics, and

environmental monitoring applications [1-

3]. The development of frequency-selective

surface technology on the pixel level has the

potential to provide enhanced infrared

detection for a desired wavelength of

radiation. Since it was first developed in the

1960s, infrared imaging technology,

especially in the area of focal plane arrays,

has made significant improvements in

producing an image. A focal plane array, a

device that converts an optical image into an

electrical signal that can then be processed

or stored, is the core of a long wavelength

imaging sensor [14]. The first generation

consisted of a single pixel or a one-

dimensional array of pixels that required a

mechanical sweep to produce a two-

dimensional image [1]. The second

generation now consists of a two

dimensional array of pixels to produce an

image, eliminating any need for moving

parts [1]. Third generation infrared cameras

will consist of a two dimensional array of

pixels that can pass spectral information at

room temperature, which is similar in

function to cones in the human eye [1].

Although significant improvements have

been made by the infrared detector

community, infrared images that are truly

multi-color are not readily available. A few

examples are given in figure 1 to

demonstrate images taken with conventional

photodetectors. As one can see, these images

are based on the intensity of light to provide

a false color image. Next generation

photodetectors will be able to provide

spectral information based on the

wavelength of light.

1.1 Background

In the past decade, new infrared detector

technologies, such as quantum dot infrared

photodetectors (QDIPs), quantum well

infrared photodetectors (QWIPs), quantum

dots-in-a-well infrared photodectors

(DWELL), and superlattice structures (SLS),

have been developed with ever increasing

operating temperatures. Infrared

photodetectors that can operate at room

temperature can significantly reduce their

cost of operation and therefore expand their

use for everyday applications [2]. QWIPs,

generally made with GaAs materials [17],

are already well-known and are available

commercially [3]. However, they have many

shortcomings and are generally thought to

be inferior to QDIPs [2]. For example,

QDIPs do not require diffraction gratings to

couple normally incident light [2]. Because

of this, there is one step less in the

Figure 1. (a) [18] Infrared images of the human

body showing areas of muscle pain in the back

and post-operative inflammation in the knee.

(b) [19] Infrared detector images showing a jet

helicopter and jet engine.

a) b)

fabrication of QDIPs than in the fabrication

of QWIPs. QDIPs are similar to QWIPs in

structure; the quantum well is substituted

with a quantum dot [17]. QDIPs are

generally constructed with InAs dots on

GaAs substrates [17]. An image of a QDIP

structure is shown in figure 2. QDIPs can

operate at higher temperatures as a result of

having a lower dark current [2].

The DWELL structure, a cross between

QDIPs and QWIPs with InAs quantum dots

in an InGaAs quantum well [3], has also

been proven to have low dark currents, and

higher operating temperatures [3]. Similar to

QDIPs, these detectors allow normal

incidence, which ultimately provides better

control over the operating wavelength [16].

A DWELL structure is shown in figure 3.

InAs/GaSb type-II strain layer superlattices

also operate at higher temperatures, have a

higher detectivity, high efficiency, and some

multi-color capability [2], although not on a

large scale. This is the most promising

technology, but it also the most expensive.

2. MOTIVATION FOR PROJECT

Although the current infrared detector

technologies have made many

improvements in their images, researchers

are still interested in exploring new

technologies that can take even better

images and can incorporate more elements

such as color, polarization, and dynamic

range.

Currently, all of the pixels in an infrared

camera are nearly identical, creating black-

and-white images instead of images of

different colors [15]. Scientists would like to

change this by integrating multispectral

capability on the pixel level. An example of

multispectral imaging is shown in figure 4.

They have been researching the concept of

an infrared retina, which would act similarly

to the cones in a human eye in the

information conveyed in the images [1]. In

order to create this infrared retina, either

plasmonics or matamaterials can be used.

Current infrared detector technology, can in

general, be improved with the aid of

resonant structures of a given design to

provide a higher operating temperature,

spectral information on the pixel level, and a

higher detectivity [3]. This project will focus

on the development of a nanoimprint

lithography mask for the fabrication of these

resonant structures. Initially, a simple

nanoimprint lithography mask will be

created to imprint an array of posts. These

simple structures can aid in the fabrication

Figure 3. Image of a quantum dots-in-a-well

structure, showing the InAs quantum dot in an

InGaAs quantum well.

Figure 2. [20] The image on the left shows a 10-

layer InGaAs/GaAs QDIP structure, and the

image on the right shows a diagram of a QDIP in

an electric field.

of surface plasmon (SP) diffraction gratings,

which have been shown to enhance the

signal received by an infrared photodetector

[13]. A simple nanoimprint lithography

mask can be used to establish a fabrication

process for the creation of nanoimprint

masks with complicated features, such as

metamaterials. Metamaterials are artificial

materials that have properties usually not

found in nature. They offer a huge

advantage over SP structures because they

can be created for pixels with a small size

[5]. They can be engineered to have a

negative index of refraction, meaning that

the light is bent around the object rather than

transmitted through or reflected away from

it [5]. Narrow-band perfect absorbers can be

created because of the electric permittivity

and magnetic permeability of metamaterials

[5]. They can control and interact with

infrared radiation only if their structures

have wavelengths similar to those of the

infrared light waves with which they interact

[5].

3. RESEARCH OBJECTIVE

This project focused on the fabrication of a

mask through different forms of lithography,

such as interferometric and electron beam

lithography, to pattern metamaterial

structures. Lithography is a technique used

to transfer patterns onto a substrate.

Interferometric lithography is a process in

which a laser beam is split into two beams,

one going directly to the sample and one

going to mirror and then reflected to the

sample, thereby creating an interference

pattern [8]. The pitch of grating d is

determined by the formula

d =

where is the wavelength of the laser beam,

n is the index of refraction of the medium

(in air, n is 1), and is the angle between

the beam and the surface normal to the

sample. In electron beam lithography (EBL),

a beam of electrons is scanned in a pattern

across a surface with photoresist to create

small structures in the resist that can then be

transferred to the substrate.

EBL is generally used to make integrated

circuits and masks [10, 12]. Although useful

for creating interesting, repetitive patterns,

electron beam lithography cannot be

performed on a large scale because it is

extremely slow and expensive [10].

However, it can be used on a single sample

and then nanoimprint lithography can be

used utilized on a mass scale. Nanoimprint

lithography, as opposed to electron beam

lithography, is relatively simple, fast, and

inexpensive [9]. In addition, it has a high

throughput and resolution [4]. Nanoimprint

lithography (NIL) can be performed in three

different ways, the first called thermal NIL,

the second called ultraviolet NIL, and the

third called substrate conformal imprint

(a)

(b)

Figure 4. (a) [21] In the top right picture, an object is

detected that wouldn’t be seen with the human eye.

(b) [22] In the bottom three pictures, different parts of

a flame can be seen with an infrared detector.

lithography (SCIL) [4]. The general process

for NIL is shown in figure 5. In thermal

NIL, a mold is pressed into a resist on a

substrate at a high temperature, the substrate

and mold are cooled while pressed together,

and the mold is released from the substrate,

leaving a pattern on the substrate [9]. In

ultraviolet NIL, a mold is pressed into a UV

resist on a substrate, the resist is cured by

exposure to UV light, and the mold is

released from the substrate, leaving the

pattern on the substrate [4]. SCIL, developed

by Philips Research and Suss MicroTec, is

an improved version of UV NIL by

providing ways to get even better resolution

and patterning over larger areas [4]. In

addition, SCIL provides a low force and a

low temperature processing condition for

nanoimprint lithography, which is an

advantage for the fabrication of focal plane

arrays [4]. Photolithography, a simpler

method used to transfer a pattern to a

photoresist, would not be beneficial to use in

this case; it is diffraction limited, so it

doesn’t allow feature sizes as small as those

needed for the fabrication of metamaterial

structures that can be obtained using IL and

EBL [14, 15]. We wanted our feature sizes

to be around 250 nm. Our plan was to begin

with interferometric lithography to establish

the fabrication process for a mask and to

pattern simple structures, and then continue

with electron beam lithography to pattern

more complicated metamaterial structures.

Finally, the mask from EBL would be used

as a mold for mask fabrication through

nanoimprint lithography. This will

ultimately help to replace the optical filter

and put it on the chip level, which is desired

since the optical filter can be very expensive

and bulky. Also, the optical filter transmits

light of certain wavelengths and blocks the

rest of it, producing only one color at a time

[13]. By patterning metamaterial structures

on the pixels, the different geometry of the

structures will produce different colors in

the images taken by an infrared detector. As

mentioned before, metamaterial structures

have a higher absorption efficiency, which

will enhance the optical signal to an infrared

detector and produce more electrons, which

will allow the infrared detector to operate at

higher temperatures [5].

Metamaterial structures also have the

potential to create a more narrow spectral

response, which will improve multispectral

imaging [11]. This technology will be

different from other infrared detector

technologies because the mask patterned

with these structures can be used on any

infrared detector, rather than only on

specific infrared detectors.

4. METHOD

We began with a 530 nm thick quartz wafer,

since quartz is stronger than many other

materials. In addition, it is clear, making it

easier for us to see through it for mask

alignment in later steps. Using a dicing saw,

we inscribed 230 nm thick cuts into the

wafer to make it easier to later cleave into

square samples. We deposited about 35 nm

of chrome on the side of the wafer without

the dicing lines using metal evaporation. We

used chrome because it is a strong metal for

Figure 5. [23] Schematic of NIL fabrication

process.

the dry etching process, and the IL process

allows metallic nanostructures to be

transferred easily [6]. Next, we used a

spinner to spin-coat ICON-16 anti-reflection

coating, which prevents reflections from the

quartz wafer, at 2700 RPM, and did a soft

bake at 200°C for 60 seconds. We then spin-

coated SPR-505A photoresist, a material

coated on a surface to create patterns, at

3000 RPM and did a soft bake at 95°C for

60 seconds. At this point, we cleaved the

wafer into separate square samples along the

lines we inscribed with the dicing saw. This

way, we had multiple samples to test for

different exposure times using

interferometric lithography. The laser that

we used had a beam with a wavelength of

355 nm, frequency of 60 Hz, and energy of

75 mJ. The pitch of grating for our samples

was 500 nm. Initially, we exposed a few

samples without rotating the sample in the

sample holder to create one dimensional

lines. We tested exposure times varying

from 5 to 13 seconds, and with each sample

we incremented the time by 2 seconds. After

each exposure, the sample was placed on a

hot plate heated to 110°C for 60 seconds,

and then developed using MF-702 for 60

seconds. The developer removes the

photoresist from the parts of the sample not

exposed by the laser beam and leaves

patterns across the surface of the sample. At

this point we looked at our samples under a

scanning electron microscope (SEM) to see

how the lines looked. Once we found a

sample with solid, unconnected lines, we

used that exposure time to pattern two

dimensional posts on new samples. We once

again took SEM images to examine the

posts. The next step in the fabrication of the

mask was dry etching the ARC, which was

done with a reactive ion etching (RIE)

machine. In this process, gases and plasma

are introduced in a chamber. We used

oxygen gas for the dry etching. We tested a

few different etch times and then looked at

our samples under an SEM to see if we were

successful. Our final recipe for the RIE was

10 mTorr chamber pressure, 10 sccm

oxygen gas, a radiofrequency (RF) power of

15% of 200 W, and an etch time of 1.75

minutes.

The next step in our procedure was to wet

etch the chrome. We used a wet etchant,

called CEP-200, and tested different etch

times—10, 15, and 20 seconds—on a few

samples. After looking at these samples

under an SEM, we determined that the

chrome needed to be etched for at least 20

seconds, because we could still see the

chrome on the wafer with the etch times less

a)

b) c)

d) e)

f) g)

h) i)

j) k)

Figure 6. Visual representation of the interferometric

lithography process. a) quartz substrate, b) chrome

deposition, c) ARC coating, d) photoresist coating, e)

IL, f) developing sample, g) dry etching ARC, h) wet

etching chrome, i) removing ARC and photoresist, j)

dry etching quartz, and k) removing chrome.

than 20 seconds. At this point in our

experiment, we didn’t have enough samples

to test the dry etching of the quartz, so we

repeated the entire process—dicing saw cut

lines, metal deposition, interferometric

lithography, dry etching the ARC, and wet

etching the chrome. Then, we removed the

photoresist and ARC by using another

plasma dry etching machine with oxygen

gas. Since our facility, the Center for High

Technology Materials (CHTM), does not

have the capability to dry etch quartz, we

had to go to the Center for Integrated

Nanotechnologies (CINT) at Sandia

National Laboratories to use their machine

for dry etching. We used a Trion Tech

fluorine dry etching machine with a chamber

pressure of 10 mTorr, ICP radiofrequency of

350 W, RIE radiofrequency of 35 W, 45

sccm CF4, 5 sccm Ar, 5 sccm O2, and an

etch time of 120 seconds. We went back to

CHTM to take some SEM images to see if

the etching worked. After removing the

chrome with the chrome wet etchant and

taking a few more SEMs, we had completed

the process for the fabrication of a mask. A

visual process for the fabrication of a mask

using IL is shown in figure 6. We then

applied this process to the fabrication of a

new mask using electron beam lithography

to pattern more complicated shapes than

posts. We began with a new quartz wafer on

which we spin-coated a photoresist, called

poly methylmethacrylate (PMMA),

specifically for electron beam lithography, at

3000 RPM. Next, we went to CINT to use

their electron beam lithography machine.

This process was quite complicated and took

a few hours to complete, unlike

interferometric lithography, which only

takes a few minutes. We did a direct write to

create the pattern on our substrate. We

developed the sample using MIBK diluted

1:3 for 1 minute. After this, we deposited

around 35 nm chrome using a metal

evaporator and dry etched the quartz at

CINT, using the procedure outlined above.

A few SEM images confirmed that the

patterns were transferred to the quartz.

Although the next step of our project was to

fabricate many masks using the sample from

electron beam lithography as a mold through

nanoimprint lithography, we unfortunately

were not able to access the machines and

materials needed to do so.

5. RESULTS

We were able to successfully make one

dimensional lines and two dimensional posts

using interferometric lithography, as seen in

the SEM images in figure 7. The lines and

posts have a diameter of 250 nm.

After creating the patterns in the photoresist,

we dry etched the ARC to transfer the

pattern to the ARC. We tested a few

different etch times, and even though

different etch times may have successfully

dry etched the ARC, some of the times

reduced the diameter of the posts

significantly. We wanted the posts to still be

similar in diameter to how they were before

the dry etching. We determined that 1.75

minutes was the etch time that correctly

transferred the pattern to the ARC and kept

the diameter of the posts, as shown in figure

8. We knew we had dry etched the ARC

because we could see a slight horizontal line

Figure 7. The sample on the left was exposed for

7 seconds, and the sample on the right was

exposed for 3.5 seconds, and then rotated and

exposed for another 3.5 seconds.

showing the separation between the

photoresist and ARC.

We then wet etched the chrome using

different etch times and took SEM images of

the different samples. After comparing the

images, we believe that 20-30 seconds was

enough time to wet etch the chrome, as seen

below in figure 9.

After wet etching the chrome, we removed

the photoresist and ARC using a plasma RIE

machine with oxygen gas. Then, we went to

CINT to dry etch the quartz. We dry etched

approximately 100 nm per minute. To

determine the etch rate, we took a quartz

substrate that had half of the quartz exposed

and the other half with 35 nm of chrome.

We dry etched the sample for 5 minutes and

measured an etch depth of 0.5 microns using

an alpha-step machine. For our samples, we

dry etched the quartz for 2 minutes and

achieved an etch depth of 200 nm. An

example of this is shown in figure 10. We

wanted close to a 1:1 ratio between the

diameter of the post (250 nm) and the depth

of the post (200 nm).

After using the SEM at CHTM, we went to a

different facility, The Center for Micro-

Engineered Materials, to use their SEM to

obtain different types of images. We took

some secondary electron (SE) images and

some back-scattered electron (BSE) images

to get new information from the images, as

shown in figure 11. SE images show the

topographical contrast in the images, and

BSE images show the contrast in the

material. We performed electron beam

lithography on a sample to pattern a

metamaterial structure. Although we were

able to transfer the pattern to the quartz

Figure 10. Titled view of the substrate after we

dry etched the quartz. There is still chrome on the

posts, but we believe we were able to successfully

etch the quartz.

Figure 9. SEM image of a sample that was wet

etched for 20 seconds to remove the chrome. The

quartz can be seen in between the posts. The

photoresist can be seen on top of the posts (the

darker spots).

Figure 8. Both of these images are SEMs of a

sample after the anti-reflection coating was dry

etched for 1.75 minutes using oxygen gas. The

image on the left is the top view and the image on

the right is the side view of the same sample.

successfully, the patterns did not look

exactly as we intended them to look.

The metamaterial structure that we

attempted to pattern on the substrate is

shown in figure 12. The patterns were

distorted and non-uniform, as seen in figures

13 and 14, but we were not completely sure

why this happened. This may have happened

due to surface charging from the electron

beam lithography machine. We believe

reducing the current and coating the sample

in gold prior to performing EBL would

reduce the charging effects. If we had more

time, we would vary the parameters to try to

perfect the EBL process. This process would

take another two weeks to perfect, since

there are still many unknowns, such as level

of current and amount of gold. Another

reason why the patterns appear to be

disconnected and non-uniform is poor

adhesion of the photoresist. This can be seen

in figure 14, where parts of the cross do not

appear. One way to prevent this is to clean

the substrate thoroughly before applying

photoresist, by baking the substrate for a

long period of time and then cleaning it with

acetone and isopropanol. Although our

results were not perfect, we were still able to

show that it is possible to pattern these

metamaterial structures on a quartz

substrate.

6. CONCLUSION

In this paper, we have outlined the current

infrared detector technologies and the

reasons for continued research in this field.

We discussed the need to increase the

functionality of pixels on an infrared camera

by patterning metamaterial structures

through lithography to enhance the function

of the focal plane arrays and improve the

optical signal to an infrared camera. We

have shown that it is possible to pattern

these structures and that, if produced on a

mass scale using nanoimprint lithography,

they show much promise in ultimately

improving infrared detectors. This

technology is different than the other

technologies mentioned earlier in this paper,

Figure 12. Image of the metamaterial structure

that we patterned on the quartz substrate.

Figure 11. Both pictures are top views of the

substrate after the quartz was dry etched. The top

image is an SE image, and the bottom image is a

BSE image.

since it can be used on any infrared detector;

it is not specific to a certain type of infrared

detector.

As mentioned before, these metamaterial

structures have the potential to improve the

images taken by an infrared camera by

incorporating polarization, dynamic range,

color, and multispectral capabilities in the

images.

7. FUTURE WORK

If we had more time to continue this

research project, we would attempt to

improve the process for electron beam

lithography. We would like to be able to

successfully pattern metamaterial structures

on the quartz substrate. After obtaining the

required materials for nanoimprint

lithography, we will be able to use this

quartz substrate as a mold for NIL to

fabricate masks on a large scale. Then, we

will be able to use the masks from NIL to

print patterns on the focal plane arrays in

infrared detectors. NIL can be used to create

subwavelength grating patterns on the mask

[7]. Currently, EBL can only be

implemented for substrates of small sizes. If

we could perform EBL on a large sample,

then we could fabricate a mask that could be

used for any infrared detector. The final step

in this project would be to actually construct

devices with multi-color and multispectral

capabilities using the metamaterial

structures we fabricated.

8. ACKNOWLEDGEMENTS

I would like to thank the National Science

Foundation and their Research Experience

for Undergraduates program for giving me

the opportunity to perform undergraduate

research. I would like to thank my faculty

mentor, Dr. Sanjay Krishna, and my

graduate student mentor, John Montoya, for

all their help and guidance in my project

throughout these ten weeks. In addition, I

would like to thank some of the other

researchers at CHTM, including Dr. Alex

Raub, for helping me become familiar with

some of the steps in the IL process, Xiang

He, for helping me learn how to use the laser

and for helping with the dry etching process,

Figure 14. 30 degree tilted view of the

metamaterial structure patterned on the quartz

substrate.

Figure 13. Top view of the pattern after dry

etching the quartz. The pattern is non-uniform

and some parts of the crosses are disconnected.

and Ajit Barve, for helping me take many of

the SEM images in this paper. I would also

like to thank CHTM at UNM and Sandia

National Laboratories for allowing me to use

their facilities throughout the summer.

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