70nm Periodic rib structure using new frequency-doubling technique
Author name
Glenn Hutchinson
E-mail: ghutchinson@digis.net
Abstract: A new frequency-doubling technique has been developed that halves the periodic dimension of repeated line
structures in a metal film. Using interference lithography, a repeating rib structure in aluminum has been developed with a
period dimension of less than 75nm. This structure, useful as a high flux polarizer, especially for the UV region, has been
made on 200mm wafers with over eighty percent useful areas. In the visible spectrum, the contrast ratio exceeds ~20,000:1
measurement limits.
Repeating parallel line patterns have been developed and made for many years using interference lithography.
Typically, these patterns are transferred to a metal film deposited on a transparent substrate, which then can be
used as a polarizing element. The effectiveness of this structure as a polarizer is a function of the line period
dimension, and in general the period should be at most half the wavelength1,2. When the period is generally
between half to twice the wavelength, , abrupt changes in the transmission and reflection properties occur, along
with pronounced effects of the incidence angle. Greater than this, the line/rib structure will act as a diffraction
grating. At dimensions less than /2, the incident radiation is separated into the two orthogonal polarization states,
p and s. The s-state polarization is parallel to the line orientation of the grid and is either absorbed or reflected,
while the orthogonal p-state polarization is transmitted. Better polarization performance, characterized mainly by
a greater extinction ratio, is a strong function of decreasing the period (pitch).
In interference lithography, a polarized coherent light source is split into two equal beams and then intersected to
produce a well understood interference pattern of repeating parallel lines3. Exposure of either a positive or
negative photoresist will create the multi-line grid pattern in the resist, which can then be transferred to the
underlying metal through anisotropic plasma etching. Different materials on a limited number of substrates have
been used in this process to create wire grid structures targeting different regions of the EM spectrum. Currently,
aluminum wire-grid structures with a pitch between 140nm and 150nm are commercially produced on glass
substrates for the visible spectrum. Two advantages of using the wire grid structure in polarization are the higher
acceptance angle and the ability to withstand much higher light fluxes and temperature. Optical properties of these
products are well documented. These commercially produced polarizers are fixed in pitch, and in general, the
optical properties are constrained by the trade-off between transmission and contrast ratio. Polarizer properties
with higher transmission values have lower contrast ratios, and vice versa, those with higher contrast ratios have
lower transmission values. Transmission values between 84 and 92 percent across the spectrum are expected4, and
corresponding contrast ratios are between 1200:1 and 300:1.
It is known that decreasing the period will dramatically increase the contrast ratio for this wire grid structure in
the visible region. Modeled data for the contrast ratio as a function of the period is shown in the graph below. In
this model, the ribs are pure aluminum, square in shape, and have a duty cycle of fifty percent. The rib height is
fixed at 120nm. The contrast ratio is an almost exponential function of the period.
Contrast Ratio vs Period
1000000
100000
CR
450nm
10000
550nm
650nm
1000
100
40
60
80
100
120
140
160
180
200
220
Period (nm)
Figure 1
Limitations of the currently available period structures also start to become apparent as the spectrum moves lower
towards the UV region. Both transmission and contrast ratio begin to decline rapidly at wavelengths lower than
400nm. Below ~360nm, the glass substrate starts to lose transparency, greatly affecting the transmission. In the
UV region, other substrates could be substituted with better transmission properties. However, the problem of
increasing the extinction ratio can only be solved through decreasing the period.
The period dimension for interference lithography is dependent on three parameters: the exposure wavelength
(), refractive index of the medium (n), and incidence angle (i). The equation for the period dimension is thus
expressed as follows5:
Period =  / 2n sini
Reducing the period therefore can be accomplished by decreasing the exposure wavelength, increasing the
refractive index of the exposure medium, or increasing the incidence angle. Practical constraints are placed on all
three variables for production purposes. The exposure wavelength needs to be single frequency and have enough
energy to be feasible for production. The exposure medium should be non-absorbing, or have a k value as low as
possible at the exposure frequency. It must not affect or interact with the photoresist, and it should also be nonhazardous and non-corrosive if possible. Decreasing the period by increasing the incidence angle will increase the
exposure time. Longer exposure times lead to problems associated with profile degradation due to ambient
vibrations. Also, the throughput cycles should be minimized as much as possible for manufacturing efficiency.
I-line lithography (=365nm) has the requisite single frequency power and has been used in the semiconductor
industry for several generations. In addition, the appropriate photoresists and BARCs are widely available. The
interference lithographic process has been developed and in use for many years and is well understood.
Immersion lithography using water as the exposure medium has also been further employed to reduce the period.
Then, only the incidence angle then needs to optimized to balance the exposure time with an acceptable period
dimension.
Reducing the period further along the above-mentioned lines would therefore necessitate a change in both/either
the exposure wavelength and/or exposure medium. A more practical medium than water has not been found, and a
change in exposure source would necessitate a complete change in the resist and exposure technique. This will
surely result in a dramatic cost increase when moved from R&D into full production mode.
A novel technique was developed that reduces the period by half, effectively doubling the frequency. From a base
period rib structure of about 140nm, the resulting period is half that, about 70 nm. The I-line exposure process,
with a commercially available I-line PR and BARC, is still used in the new technique. The base period then could
be adjusted by changing the incidence angle. The doubling frequency technique would then halve this base
dimension.
The cleanliness or sharpness of the final structure is dependent upon the cleanliness of the base rib profile.
Stability of the exposure environment and system is critical to minimize exposure washout. Much effort has gone
into homogenizing and stabilizing the immersion fluid during the exposure, though there seems to always be
small temperature and density strata in the fluid causing regions of differing refractive indices. As these small,
fluidic regions are better homogenized, the resulting resist structures will be improved. The small line width
variations of the resulting resist ribs are caused by these small fluctuations in the immersion fluid. After exposure,
a modified develop process was used. Past experience suggests that a high spin speed and evaporation rate causes
the thin PR ribs to collapse. In these cases, several ribs lean in towards each other, and the pattern is not
transferred to the underlying aluminum film. There may be some improvements on the develop procedure that
should be further developed. After develop, the final process step is the etch. The etch process is fairly complex,
with process parameters determined by the substrate, metal film and final structure dimensions. Optimization of
these parameters determine final product properties and characterization. Because of the smaller rib dimensions,
the anisotropic nature of the etch needs to be increased or undercutting of the ribs will occur. Erosion of the rib
top is a function of the resist to metal selectivity, and must be considered. Cleanout of the trench is important for
the transmission property. Effort is made to etch down into the substrate. Duty cycle, the ratio of rib width to
period is important as well, and can be controlled in the etch process by switching to a more isotropic etch. The
finer period necessitates a lower etch power than is standard to lessen rib erosion. A more efficient ratio of the
chlorine etchant gases would increase the sidewall etch rates, affecting the duty cycle. Further studies to improve
the etch process were not done due to research constraints.
This new technique roughly doubles the throughput time of the wafer, though the time through the etch process
might be reduced to that near the nominal throughput. This initial development has only been done on single layer
aluminum films, but is most likely possible on other metals. Frequency doubling on multi-layer films may be
feasible as well.
Several samples were made using this newer technique, and the resulting SEM pictures from three samples are
shown in the following pictures.
Sample 1
The above picture clearly shows the resulting structure made from the doubling frequency process. The exposure
balance may be slightly off as seen by the non-uniformity in the trench widths. Improved etching should reduce
the duty cycle, which should be in the 50 percent range. There may also be some non-intended increase of the rib
width shown by the conductive metal layer sputtered on the samples for SEM preparation. Though slight, the
actual thickness of this layer is not ‘much smaller’ than actual rib width dimensions.
Sample 2
The above picture shows more clearly the base period, which appears to be ~137nm. The true period is probably a
little larger, closer to 140nm, as calculated using the above period dimension equation. The doubling exposure
was not balanced enough for better separation of the doubled ribs from the base ribs. Further etching would
separate the second modal ribs from the first, and would also increase the trench width of the base structure.
Sample 3
In this picture, the exposure balance was worse than the other two. Again, the rib widths should be reduced to
better increase the transmission. In all the above cases, better balancing of the exposure process as well as
improving the etch process should increase the transmission properties. Rib aspect ratios are between four to six
according to the above pictures.
The bimodal nature of the doubling frequency is noticeable in all the above pictures. It most likely is the case that
there is flexibility in the A-rib to B-rib ratio limits before the final optical properties are grossly affected. Better
control of the A-rib width to the B-rib would require more research concerning the frequency balancing. No
modeling was done however to predict the effects of unbalanced bimodal rib widths on the optical properties.
Optical measurements were made for visible spectrum properties. Processing was done on 200mm Corning 1737
glass wafers. To the visible eye, six of the eight samples made looked good. The usable area of the good wafers
looked to be upwards of eighty percent. The areas of the wafer that were not as visually good, seem typical of the
non-uniformities routinely encountered in standard production. Though clean, the wafers appeared darker (less
transmissive) than standard production wafers. Before the SEM pictures were taken and the final structure better
understood, the optical properties were measured since the SEM destroys the samples.
The %Tp across the spectrum was fairly low, about 60 percent across most of the wafers. This was noticed before
measurement, but it seemed better to the naked eye. The transmission values for the samples were worst for
Sample3, at roughly 58 percent, and best for Sample 2 at roughly 64 percent. As expected, the contrast ratio was
extremely high. It would be difficult to accurately measure and report the CR with the measurement tool used,
since the upper limit of the tool is ~20,000 : 1. Across the spectrum, the extinction ratio can therefore only be
reported as greater than 20,000 : 1. Reflective properties were not measured. The areas measured and reported
should be adequately represented by the SEM pictures.
The low transmission was later explained from the SEM pictures by the large duty cycle. Ideally, the duty cycle
should be close to fifty percent. It is the duty cycle as well as the rib height, which mostly determines the
transmissive properties for a fixed period.
This new technique effectively doubles the exposure frequency, and has the benefits of using existing materials,
chemicals, and processes to reduce critical dimensions by half. As the race to further reduce size continues, this
technique can bypass the need for newer, more expensive chemicals and processes, at least demonstrated in the Iline process. Further development can sharpen these first results, giving much finer polarizers with better
performances. Whether or not the doubling frequency technique is applicable to other wavelengths and/or
chemicals has not been researched.
References
1.
2.
3.
4.
5.
E. Hecht, Optics, (Addison-Wesley Longman, New York, 1998)
J. Bennett, Polarization, Handbook of Optics, McGraw-Hill, New York, 1978
JJ Wang, F. Walters, X. Liu, “High-performance, large area, deep ultraviolet to infrared polarizers
based on 40nm line/78nm space nanowire grids”, Appl. Phys. Lett. 90, 061104 (2007)
C. Pentico, E. Gardner, D. Hansen, R Perkins, “New, high performance, durable polarizers for
projection displays”, SID 01 Digest 1287-1289 (2001)
M. Farhoud, J. Ferrera, A. Lochtefeld, et al, “Fabrication of 200nm period nanomagnet arrays using
interference lithography and a negative resist”, J. Vac. Sci. Technol. B 17(6)