Double Exposure Laser Interference Lithography for Pattern Diversity using Ultraviolet Continuous-Wave Laser

Double Exposure Laser Interference Lithography for Pattern Diversity using Ultraviolet Continuous-Wave Laser

Yong-Won Ma

, Jun Han Park

, Dan Hee Yun

, Cheongyeol Gwak

, and Bo Sung Shin

1

Interdisciplinary Department for Advanced Innovative Manufacturing Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Korea

2

Department of Cogno-Mechatronics Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Korea

3

Department of Optics & Mechatronics Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Korea (Received June 10, 2019: Corrected June 17, 2019: Accepted June 25, 2019)

Abstract: The newly discovered properties of periodic nanoscale patterns have increasingly sparked research interests in various fields. Along this direction, it is worth mentioning that there had been rare studies conducted on interference exposure, a method of creating periodic patterns. Additionally, these few studies seemed to validate the existence of only exact quadrangle shapes and dot patterns. This study asserted the formation of wavy patterns associated to using multiple exposures of the ratio of the first exposure intensity to the second exposure intensity. Such patterns were designed and constructed herein via overlapping of two Gaussian beams relative to certain rotation angles, and with a submicron struc- ture fabricated based on a 360-nm continuous-wave laser. Results confirmed that the proposed double exposure laser inter- ference lithography is able to create circular, elliptical and wavy patterns with no need for complex optical components.

Keywords: Laser Interference Lithography, Multiple Exposure, Wave Pattern, Regular Submicron Pattern, Gaussian Beam

1. Introduction

Wave interference occurs when two or more waves, usu- ally of light or sound, superpose to form a resultant regular wave consisting of a series of maxima and minima. Laser interference lithography (LIL), one of the methods for fab- ricating regular submicron- or nano-patterns, employs two or more coherent beams. In such a method, the period of the interference pattern can be changed by the angle and the wavelength of light, bearing a great advantage as it is oftentimes accomplished in a simple and inexpensive man- ner over a large area, as compared to other periodic submi- cron-, or nano-patterning methods.

Line patterns of 1D periodic intensity distribution are produced by 2-beam interference, dot patterns by 3-beam and 4-beam interfer- ence, and other various periodic patterns by higher multi- ple beam lithography.

Comparatively though, it becomes more difficult to control the angle and slope of each beam in 3-beam exposures than in 2-beam exposures. In other words, fabricating the dot patterns complicates the optical

system where the desired results become challengingly dif- ficult to achieve.

As with the interference pattern, fabricating such with a UV lamp is an impossible position, but it becomes possible with the application of laser owing to the spatial and tem- poral coherence. However, more importantly, unlike the UV lamp, the laser does not possess uniform intensity dis- tribution owing to a Gaussian beam distribution, which suggests that the beam intensity distribution varies in circu- lar laser beam as well. When solving the problem, the shaping method is not a perfect scheme, and a light inten- sity homogenizer cannot produce good interference pattern due to the broken wavefront. Thus, it becomes extremely difficult to achieve uniform beam distribution, as well as ensure uniformity within the entire area of the pattern pro- duced with a small laser beam, unlike in the case of UV lamps used in production of microelectromechanical sys- tems (MEMS) based on photomasks. There are existing papers that show pattern diversity relative to intensity dif- ferences in the laser beam Gaussian distribution. There

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were also papers discussing the variation of the pattern due to the intensity difference of the laser beam Gaussian dis- tribution, given in particular a 1:1 ratio between the first and second exposures.

Nonetheless, as the absorption coefficient of the photoresist decreases after the first expo- sure, the absorptivity of the first and the second exposure is certainly not 1:1 as shown in Fig. 1.

In this paper, we simply use a very Gaussian beam distri- bution to change the ratio of intensity to exposure ratio. We describe the fabrication of a dotted, elliptical and wavy pat- tern using a conventional double 2-beam exposure, and explain the diversity of the pattern in relation to the beam intensity distribution.

2. Experimental and Simulation

The positive photoresist (PR) used in the experiment was ma-P 1205 from MicroChem Corp, and the laser was an MSL-FN-360 from CNI Inc., a 360-nm continuous laser with 30-mW of average power, 1.4 mm of beam diameter (1/e

), a single longitudinal mode, and a TEM

beam mode.

For clarity, a schematic diagram of the laser system is dis- played in Fig. 2. Mirrors M3 and M4 are each rotatable, and the period of the interference pattern can be determined by relative angles of the beams reflected. The period of the 2- beam interference pattern is ruled by:

(1)

where Λ is the period of regular pattern; λ is the wave- length of the light source; n is the refractive index of the medium and; θ

and θ

are the angles of exposure to the surface reflected by the mirror M3 and M4, which are each rotatable. θ

and θ

are generally the same and expressed as

θ. In this experiment, θ was exposed at 13°. The rotation stage is used to allow the wafer to be fixed and rotated for multiple exposures. Patterns were designed by simulation at an exposure angle of 13° and the experiment was con- ducted under the same conditions as the simulation; a sche- matic of the double exposure with the sample’s rotation is shown in Fig. 3(a).

After the first exposure, the wafer was rotated by the stage, and then a second exposure was performed at wafer rotation angles of 90

, 12

, and 24

.

The PR was coated on the wafer at 3000 rpm for 60 s;

further, the wafer was baked at 105

C for 90 s. The total amount of exposure was not allowed to exceed 30 mJ/cm

to avoid an overdose. The sample was developed by ma-D 337. For scanning electron microscopy (SEM) measure- ments, the sample was coated with platinum (Pt).

3. Results and Discussion

As the patterns were created via a laser interference method based on TEM

mode, the laser beam power exhibited such a distribution that caused pattern diversity.

Fig. 3(b) describes the double exposures expected to be clean. The sum of exposure H indicated by the red square is an overlapping area with the highest exposure; the sum of exposure M depicted by the orange square is an area exposed only once during the first or the second exposure and; the sum of exposure L indicated by the yellow square is an area that was hardly exposed. To produce a dot pat- tern in the positive PR using double exposures, enough exposures should be made in areas H and M so they could be subsequently developed, and only the L area would be left in the dot form. However, constructing a pattern with no design and simulation is a challenging task, as the laser intensity distribution varies not only in each dose of the H, M, and L areas as described in Fig. 3(a) but also in the H region, which has a sinusoidal intensity distribution. This is because such distribution is present in the 2-beam interfer-

Λ λ

n sin θ ⋅ [ ( ) sin θ

+ ( )

] ---

=

Fig. 1. Absorption coefficient graph according to wavelength and exposure.

Fig. 2. Schematic of laser interference lithography system.

ence equation, which is expressed as follows:

(2)

Sidharthan et al. asserted when constructing a dot when there is overdose in laser exposure intensity, a circular pat- tern forms and the entire area of H and M develops owing to the overdose

; moreover, the nanodot size becomes smaller than the period. On the contrary, if there was no overdose, there is variation in the pattern shape. Fig. 4 shows the double exposure pattern of a 90

-rotation angle at the same intensity area. Accordingly in previous studies, with large laser exposure intensity, PR would remain in the L regions; otherwise it would appear as in Fig. 4. A pattern with the same height should result when the same intensity was exposed; nonetheless in Fig. 4 M1 and M2 were not of the same height, indicating different absorption amount for each region. Fig. 1 displays the graph of the absorption coefficient relative to wavelength and exposure. After the first exposure, the absorptivity at the entire surface of the

PR decreased and became less sensitive to the second exposure, so that patterns such as dots were not accurately formed. Thus, to form a dot array, it was necessary to design an exposure amount in the second exposure that is larger than that of the first exposure.

I x y ( , ) = I1 I2 + +

I1 2 ⁄ cos φ1 φ2 ( – )

Fig. 3. Schematic of double exposure with sample rotation.

Fig. 4. SEM micrographs of the double exposure pattern: at a rotation angle of 90

( ×10,000 magnification).

Fig. 5. (a) Simulation of double exposure at a rotation angle of 12

; (b) Case 1: SEM image of double exposure; (c) a magnified SEM

image of Fig. 3(b); (d) Case 2: SEM image of double exposure and; (e) magnification of the SEM image in Fig. 3(d).

value, which develops as soon as the threshold value is exceeded; otherwise, PR does not develop, so that a pattern can be connected as in the case of a wavy pattern in Figs.

5(b) and 5(c). The reason for the pattern not being constant in the entire double exposure region was associated with

ian beam distribution.

Fig. 6 illustrates changes in the pattern shape with the weakening of the second exposure relative to the first exposure. From Figs. 6(c) and 6(d), a wave pattern was generated, allowing an estimation for the second exposure

Fig. 6. Simulation of double exposures at sample rotation angle of 24

(Ratio of the first exposure intensity to the second exposure intensity) (a) 1:1, (b) 1:0.8, (c) 1:0.6, (d) 1:0.4, and (e) 1:0.2.

Fig. 7. SEM image of double exposure: at rotation angle of 24

.

power to be reduced down to approximately 50% relative to the first exposure.

For comparison of Fig. 5 and the sample in Fig. 7, the sample angle was rotated at 24

. Consequently, the size of the pattern was smaller considering Eq. (1). A large num- ber of studies have produced a rectangular pattern with the 2-beam interference exposure; however, the intensity dis- tribution of the interference phenomenon herein was sinu- soidal, as suggested by Eq. (2). During the first exposure, a large amount of exposure occurred due to the PR sensitiv- ity; nonetheless, at the second exposure the absorption rate became smaller and thus, there was less amount of absorp- tion. Such result can be approached identically with that in Fig. 4. Generally, the wavy pattern (Fig. 6(d)) was con- firmed through the simulation, while an overall pattern diversity was demonstrated with the Gaussian beam.

Fig. 8(a) displays a ×5,000 magnification image of dou- ble exposure with a sample rotation of 24

, whereas Figs.

8(b) through 8(d) are the x20000 magnification equiva- lents. Moreover, Figs. 7 and 8 are SEM images taken from the same specimen, discernible with the pattern change.

With insufficient exposure amount (such as at the edge of the Gaussian beam), no dot pattern was formed, but rather, a dot hole (Fig. 8(b)). When the laser intensity was slightly higher than in Fig. 8(b), a pattern of connected waveforms was generated (Fig. 8(c)). In the case where the laser inten- sity was slightly higher than in Fig. 8(c), a deeper elliptic

hole appeared near the wavy pattern (Fig. 8(d)). Further increase in the laser intensity would result in the formation of an elliptical dot, as observed in the previous papers.

4. Conclusion

Employing a double-laser interference exposure under overdose demonstrated the fabrication of diverse patterns.

Experimentally, this study was able to generate the wave

pattern. Specifically, the pattern shape was designed via

double exposure simulation for the ratio of the first expo-

sure intensity to the second exposure intensity; the wave

pattern was generated when the second exposure power

was reduced to approximately 50% less than the first

exposure. Conventionally, the double exposure method

has been used to prepare a dot array; nonetheless, this

study demonstrated that an ellipse and a wavy pattern are

also possibly created via rotation of the sample at angles

of 12

and 24

. Initially, the pattern varied according to

the position in the Gaussian beam distribution, and

changed by the second exposure intensity. When the

overlapped area barely exceeded the threshold value, only

a dot-shaped hole was fabricated; however, the wave pat-

tern appeared when laser intensity was slightly higher

than the dose of the dot-shaped hole. As such, the present

study demonstrated the possibility of creating various pat-

terns through the utilization of the Gaussian beam distri-

Fig. 8. SEM image of double exposure according to intensity distribution: rotation angle 24

.

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