Invisible Security Printing on Photoresist Polymer

Download PDF
0 downloads 0 Views 1MB Size Report
Comment
Dec 6, 2017 - the absorption coefficient also changes significantly. Using the ... These techniques—such as security ink, simple marker, plasmonic security labels, and holographic techniques [5 ... After UV treatment, we cooled the sample down to remove ... The output THz signal O(ω) passing through a material is.
sensors Article

Invisible Security Printing on Photoresist Polymer Readable by Terahertz Spectroscopy Hee Jun Shin 1 , Min-Cheol Lim 1 , Kisang Park 1,2 , Sae-Hyung Kim 2 , Sung-Wook Choi 1 and Gyeongsik Ok 1, * 1

2

*

Research Group of Food Safety, Korea Food Research Institute, 245, Nongsaengmyeong-ro, Iseo-myeon, Wanju-gun, Jeollabuk-do 55365, Korea; [email protected] (H.J.S.); [email protected] (M.-C.L.); [email protected] (K.P.); [email protected] (S.-W.C.) Department of Molecular Science and Technology, Ajou University, Suwon 16499, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-63-219-9408

Received: 24 October 2017; Accepted: 4 December 2017; Published: 6 December 2017

Abstract: We experimentally modulate the refractive index and the absorption coefficient of an SU-8 dry film in the terahertz region by UV light (362 nm) exposure with time dependency. Consequently, the refractive index of SU-8 film is increased by approximately 6% after UV light exposure. Moreover, the absorption coefficient also changes significantly. Using the reflective terahertz imaging technique, in addition, we can read security information printed by UV treatment on an SU-8 film that is transparent in the visible spectrum. From these results, we successfully demonstrate security printing and reading by using photoresist materials and the terahertz technique. This investigation would provide a new insight into anti-counterfeiting applications in fields that need security. Keywords: terahertz spectroscopy; anti-counterfeiting; security; photoresist material

1. Introduction In the last few decades, security, forgery, and alteration problems have become critical issues for many areas such as national, industrial, and individual security. Security threats can result in serious crimes and huge damage to a person or country. A representative example of this is currency counterfeiting. The result of currency counterfeiting causes currency devaluation, which can depress the economic process severely. Accordingly, the increasing trend in counterfeiting causes huge damage to the industrial economy of many establishments and significant deterioration of relations between nations. In the last decade, millions of dollars have been spent to expose fake currencies by many governments and industries. For these reasons, several scientists have struggled to solve the problem of currency counterfeiting, and they have achieved useful applications using techniques such as the holographic method or visible luminescent ink [1–5]. In addition, commercial branded goods manufacturers and pharmaceuticals have adopted anti-forgery techniques to distinguish an imitation from an original [6,7]. To prevent fake passports, many technologies have been studied [8–10]. Security is also an important issue for food industries where techniques such as oxygen sensing using a colorimetric indicator are employed in packaging [11]. This is called intelligent packaging. Nowadays, security and anti-forgery techniques have become more advanced and simpler. These techniques—such as security ink, simple marker, plasmonic security labels, and holographic techniques [5,12]—are practically applicable, and these technologies provide the solution for security and protection against forgery problems. However, these techniques still have some problems. Using chemical materials for ink and marker can cause chemical toxicity. It is possible to mimic or duplicate other techniques. Moreover, complicated techniques would be economically infeasible.

Sensors 2017, 17, 2825; doi:10.3390/s17122825

www.mdpi.com/journal/sensors

Sensors 2017, 17, 2825 Sensors 2017, 17, 2825

2 of 8 2 of 8

Currently,terahertz terahertztechnology technologyhas hasemerged emergedas as one one of of the the techniques techniques for for security Currently, security and and anti-forgery. anti-forgery. Ever since since terahertz terahertz time-domain time-domain spectroscopy spectroscopy was was developed developed using using aa femtosecond femtosecond laser, laser, terahertz terahertz Ever technology has been attractive because it has several advantages, such as non-invasiveness, technology has been attractive because it has several advantages, such as non-invasiveness, non-destructiveness,and and non-ionization measurement. these advantages, terahertz non-destructiveness, non-ionization measurement. OwingOwing to theseto advantages, terahertz techniques techniques are aconsidered a good approach toward and anti-forgery. Consequently, during are considered good approach toward security andsecurity anti-forgery. Consequently, during the last few the last few decades, terahertz techniques have been successfully applied in security and anti-forgery decades, terahertz techniques have been successfully applied in security and anti-forgery applications applications for military [13,14], applications [13,14], security screening[15], applications [15], detection hidden object for military applications security screening applications hidden object [16], detection [16], artworks [17], pharmaceuticals [14], etc. Additionally, terahertz waves can be used for artworks [17], pharmaceuticals [14], etc. Additionally, terahertz waves can be used for data security data security communications [18] and imaging [19]. communications [18] and imaging [19]. In this this study, study, we we introduce introduce security security printing printing in in the the form form of of invisible invisible information information coding coding on on In photoresist materials by using terahertz time-domain spectroscopy and terahertz imaging technique. photoresist materials by using terahertz time-domain spectroscopy and terahertz imaging technique. Our investigation investigationrevealed revealedthat thatthe theterahertz terahertzcharacteristics characteristicsof ofSU-8 SU-8as asone one of of the the photoresist photoresist polymers polymers Our are significantly significantly changed changed after after ultraviolet ultraviolet (UV) (UV) light light illumination illumination due due to to chemical chemical change. change. SU-8 SU-8 is is are chemically stable against many acids and bases, and is highly transparent under near-UV and visible chemically stable against many acids and bases, and is highly transparent under near-UV and visible light [20]. [20]. However, However, when when SU-8 SU-8 is is exposed exposed to to UV UV light, light, this this material material would would be be sensitive sensitive to to terahertz terahertz light waves. Hence, the encoded information is readable with terahertz waves but transparent in the visible waves. Hence, the encoded information is readable with terahertz waves but transparent in the visible spectrum. Thus, the security printing code has been successfully realized in the terahertz region. This spectrum. Thus, the security printing code has been successfully realized in the terahertz region. studystudy can provide new applications for allfor areas that need or innovative anti-forgery with a This can provide new applications all areas that security need security or innovative anti-forgery simple approach by using THz time-domain spectroscopy. with a simple approach by using THz time-domain spectroscopy. 2. Materials and and Experiment Experiment 2. 2.1. 2.1. THz Security Security Printing Printing Material: Material: SU-8 We We used used commercially commercially available available SU-8 SU-8 dry dry film film as as the the photoresist photoresist material material with with aa thickness thickness of of 100 100 µm μm(SUEX (SUEX K100, K100, K1 K1Solution, Solution, Gwangmyeong, Gwangmyeong, Korea). Korea). The SU-8 SU-8 film film was was laid laid between between aa metal metal substrate substrate and and the the mask. mask. For UV light exposure, exposure, a UV UV source source with with aa wavelength wavelength of of 362 362 nm nm and and intensity intensity 2 was used and we exposed the aluminum mask to UV light patterned with characters 2 of 10 mW/cm of 10 mW/cm was used and we exposed the aluminum mask to characters of of “KFRI” “KFRI” for for 20 20min minin inaasealed sealedcase. case.The Themetal metalmask maskwas wasfabricated fabricatedwith withaasize sizeofof77×× 8 mm mm for for each each character character and and the the line line width width was was 1.5 1.5 mm. mm. After UV treatment, treatment, we we cooled cooled the the sample sample down down to to remove remove the the thermal thermal effect effect and and measured measured its its THz THz characteristics. characteristics. Figure 11 shows shows the the scheme scheme for for the the UV UV light light treatment treatment process process on on an an SU-8 SU-8dry dryfilm. film.

Figure 1. 1. (a) (a) Scheme of the and (b) processing after after Figure Scheme of the UV UV exposure exposure on on the the SU-8 SU-8 photoresist photoresist and (b) cross-linking cross-linking processing UV illumination. UV illumination.

2.2. THz THz Time-Domain Time-Domain Spectroscopy Spectroscopy and and Reflective ReflectiveImaging ImagingMeasurement Measurement 2.2. The terahertz terahertz time-domain time-domain spectroscopy spectroscopy (THz-TDS) (THz-TDS) system system consists consists of of two two parts—generation parts—generation The and detection. detection. AAmode-locked femto-second pulsed laser produced a pulse width of 90of fs and mode-lockedTi:Ti:sapphire sapphire femto-second pulsed laser produced a pulse width and center wavelength of 800 nm. The output laser beam was divided into two paths by a beam 90 fs and center wavelength of 800 nm. The output laser beam was divided into two paths by a beam splitter for pumping and probing THz waves. To emit THz waves, a photoconductive antenna (PCA) was adopted. The PCA was switched using the conventional method to produce THz waves. When

Sensors 2017, 17, 2825

3 of 8

splitter for pumping and probing THz waves. To emit THz waves, a photoconductive antenna (PCA) was adopted. The PCA was switched using the conventional method to produce THz waves. When the ultra-fast laser excites a biased PCA circuit, THz pulses can be generated by the photocurrent. The THz photoconductive emitter PCA was fabricated on a GaAs substrate and the antenna pattern was a gold dipole antenna. The coherent detection method for THz waves is similar to the THz generation processing. The emitted THz pulses were acquired with a time-domain sweep at the PCA detector. Further, the gating DC signal from the THz detector can be monitored by photoconductive sampling using a lock-in amplifier. In this study, we used a commercial THz-TDS system (TPS-3000, Teraview, Cambridge, UK). Reflective THz imaging was performed by using a two-dimensional scanning module on the THz-TDS. To avoid water vapor absorption, the whole reflection system (including THz-TDS) was filled with dry gas with less than 1% relative humidity. The angle between the incident and reflected signal of the focused THz beams was 30◦ . The focused beam diameter of the THz pulse was approximately 1 mm and the stage traveled in steps of 100 µm. We acquired the THz pulse for the time domain per pixel, and the acquisition time was 30 ms per pixel. 2.3. THz Signal Data Processing Optical parameters, such as complex refractive index and absorption coefficient of a material, e = n1 + in2 is related to can be extracted by THz-TDS. The complex refractive index represented by n the phase delay of the THz pulse and absorbance in the materials compared to the reference signal (without the material). In other words, refractive index n1 can be extracted from the phase delay, and n2 is strongly related to the absorbance. The output THz signal O(ω) passing through a material is related to the input THz signal I(ω), regarded as the reference signal, as [21]     dα(ω ) 2π O(ω ) = I (ω ) exp − exp i n1 (ω )d , 2 λ

(1)

where α(ω) is the frequency-dependent absorption coefficient, d is the thickness of the sample, and λ is the wavelength. The absorption coefficient α(ω) can be obtained from the difference between spectral amplitudes with and without the sample after Fourier transformation. It is related to the imaginary part of the complex refractive index n2 by   2 O(ω ) 4πn2 α(ω ) = − ln = . (2) d I (ω ) λ In addition, n1 is measured from the difference of the THz signal phase between the with- and without-sample cases: Φ − ΦO n1 ( ω ) = 1 + I λ, (3) 2πd where ΦI and ΦO are the phases of the input (without sample) and output (with sample) pulses, respectively, throughout the sample. 3. Results and Discussion At first glance, we measured the refractive index of SU-8 before and after UV exposure in the frequency range of 0.3–2 THz. We measured the refractive index of the SU-8 film without any treatment. To investigate the effect of UV light on SU-8, we exposed the same SU-8 film to UV light with a wavelength of 362 nm for 20 min. As shown in Figure 2a, there is a significant change in the refractive index of the SU-8 film after UV light exposure. Generally, when SU-8 is exposed to UV light, photon energy is absorbed into the SU-8 film and the photochemical reaction produces an acid [22]. Further, the acid acts as a catalyst during the post-baking exposure. This process promotes a cross-linking between epoxy groups and creates a three-dimensional network. Consequently, cross-linking by UV light causes the SU-8 film to contract and the chemical structure of SU-8 become denser compared

Sensors 2017, 17, 2825

4 of 8

to that before UV treatment. This phenomenon affects the change in the refractive index in the THz Sensorsfrequency 2017, 17, 2825 region significantly. Figure 2c shows the scheme of chemical structural change in SU-84 of 8

after UV treatment. Figure 2b shows the exposure-time-dependent refractive index of the SU-8 film

material. treatmentindex for 20 min, the refractive index increases up to a value ofas1.636. at 0.5After THz. UV The refractive increases with UV exposure time. This result explains that the In UV light exposure time is increased, the cross-linking is also increased between epoxy groups in the addition, the refractive index of SU-8 increases linearly. The refractive index change in SU-8 in various SU-8 material. After UV for 20(IR)—has min, the refractive index increases up to a[23–26]. value ofOng 1.636.et al. regions—such as visible, UV,treatment and infrared been previously investigated In addition, the refractive index of SU-8 increases linearly. The refractive index change in SU-8 in was investigated the refractive index of SU-8 in the visible region. In this investigation, an SU-8 film various regions—such as visible, UV, and infrared (IR)—has2 been previously investigated [23–26]. treated with UV light having an intensity of 15 mW/cm with a time interval up to 40 min. Ong et al. investigated the refractive index of SU-8 in the visible region. In this investigation, an SU-8 Accordingly, the refractive index of the SU-8 film changed less2 than 0.2% compared to that of the film was treated with UV light having an intensity of 15 mW/cm with a time interval up to 40 min. non-treated SU-8 film at visible region. Salazar-Miranda et al., have reported a refractive index Accordingly, the refractive index of the SU-8 film changed less than 0.2% compared to that of the change of less than SU-8 in the visible region after UV treatment. addition, thechange refractive non-treated SU-8 1% filmin at visible region. Salazar-Miranda et al., have reported aIn refractive index indexofchange SU-8 hasinbeen reported with a very small variation in the (351 nm) 1.3% in less thanof1% in SU-8 the visible region after UV treatment. In addition, the UV refractive indexand change the IR region. In our results, in contrast, the refractive index of SU-8 changed by approximately of SU-8 has been reported with a very small variation in the UV (351 nm) and 1.3% in the IR region. 6% moreInthan that before UV treatment in the THzoffrequency range. Consequently,6% the refractive index our results, in contrast, the refractive index SU-8 changed by approximately more than that before UV treatment in theat THz frequency range. more Consequently, index of SU-8 is strongly of SU-8 is strongly sensitive THz frequencies, than in the refractive visible, UV, and IR regions. sensitive at THz frequencies, more than in the visible, UV, and IR regions.

Figure 2. The refractive index on SU-8: (a) before and (b) after UV light (362 nm) treatment with

Figure The refractive onThe SU-8: (a) before and (b) after light (362 treatment with an an 2. exposure time of 20index min; (c) refractive index change ratio UV of SU-8 after UVnm) exposure at visible, exposure time of 20 min; (c) The refractive index change ratio of SU-8 after UV exposure at visible, UV (351 nm), IR (1550 nm), and THz (0.5 THz) regions; (d) scheme of chemical change in SU-8 after UV lightnm), treatment. UV (351 IR (1550 nm), and THz (0.5 THz) regions; (d) scheme of chemical change in SU-8 after UV light treatment.

The absorption coefficient of the SU-8 film is measured before and after UV exposure as can Theseen absorption of the SU-8 film isismeasured and after UVFrom exposure as can be be in Figurecoefficient 3. The absorption coefficient increased before after UV treatment. this result, of SU-8 is enhanced by 2–3% at measuredafter THz UV range. It can be inferred that result, due to the seen the in absorbance Figure 3. The absorption coefficient isthe increased treatment. From this cross-linkages from UV treatment, THz absorbance is increased. absorbance of SU-8 is enhanced by 2–3% at the measured THz range. It can be inferred that due to

cross-linkages from UV treatment, THz absorbance is increased.

Sensors 2017, 2017, 17, 17, 2825 2825 Sensors Sensors 2017, 17, 2825

of 88 55 of 5 of 8

Figure 3. The absorption coefficient of the SU-8 film before and after UV exposure. Figure 3. The The absorption absorption coefficient coefficient of the SU-8 film before and after UV exposure. Figure

To compare with non-photoresist polymers for the enhancement of the refractive index change, To compare with with non-photoresist non-photoresist polymers polymers for enhancement of refractive change, To compare for the the enhancement of the thepolymers refractiveatindex index change, we measured the refractive index of several representative non-photoresist 0.5 THz. The we measured the refractive index of several representative non-photoresist polymers at 0.5 THz. The we measured the refractive index of several representative non-photoresist polymers at 0.5 THz. polymers for investigating THz characteristics were polyethylene (PE), polyethylene terephthalate polymers for for investigating THz characteristics were polyethylene (PE), polyethylene terephthalate The polymers investigating THz characteristics were polyethylene (PE), polyethylene terephthalate (PET), polyimide (PI), polymethylpentene (TPX), and SU-8 films. The UV light exposure time was kept (PET), polyimide (PI), polymethylpentene (TPX), and SU-8 films. The UV light exposure time was kept (PET), polyimide (PI), polymethylpentene (TPX), and SU-8 films. The UV light exposure time kept constant at 20 min for the samples. Figure 4 shows the ratio of change in the refractive indexwas of each constant at 20 min for the samples. Figure 4 shows the ratio of change in the refractive index of each constant at 20 min for the samples. Figure 4 shows the ratio of change in the refractive index of each polymer at 0.5 THz. As can be seen in the figure, the refractive index of polyethylene, polyethylene polymer at 0.5 THz. THz. As As can can be be seen seen in in the the figure, figure, the therefractive refractive index index of ofpolyethylene, polyethylene, polyethylene polyethylene polymer at 0.5 terephthalate, polyimide, and polymethylpentene did not change significantly. These polymers can be terephthalate, polyimide, and polymethylpentene did not change significantly. These polymers can be terephthalate, polymethylpentene didofnot change significantly. These polymers effective underpolyimide, only 0.25%and enhancement. In the case SU-8, in contrast, its refractive index cancan be effective under only 0.25% enhancement. In the casecase of SU-8, in contrast, its its refractive index cancan be be effective under only 0.25% enhancement. In the of SU-8, in contrast, refractive index changed by more than 6%. The SU-8 material is affected by UV treatment approximately 24 times more changed by more than 6%. The SU-8 material is affected by UV treatment approximately 24 times more be changedwith by more 6%. The SU-8 materialinisthe affected by UV From treatment approximately times compared otherthan non-photoresist polymers THz region. this result, it can be 24 inferred compared with other non-photoresist polymers in the THz region. From this result, itresult, can beitinferred more compared with other non-photoresist polymers in the THz region. From this can be that a photoresist polymer is very sensitive in the THz region compared to non-photoresist polymers, that a photoresist polymer is very sensitive in the THz region compared to non-photoresist polymers, inferred that a for photoresist polymer is very sensitive in the THz region compared to non-photoresist and is suitable THz security printing. and is suitable forsuitable THz security printing. polymers, and is for THz security printing.

Figure 4. The change ratio of the refractive indices of polyethylene (PE), polyethylene terephthalate Figure 4. The change ratio of the refractive indices of polyethylene (PE), polyethylene terephthalate Figure polyimide 4. The change of the refractive indices polyethylene (PE), polyethylene terephthalate (PET), (PI), ratio polymethylpentene (TPX), andofSU-8 at 0.5 THz. (PET), polyimide (PI), polymethylpentene (TPX), and SU-8 at 0.5 THz. (PET), polyimide (PI), polymethylpentene (TPX), and SU-8 at 0.5 THz.

weinvestigated investigatedthe theTHz THz image a character-patterned film byexposure UV exposure Finally, we image of aofcharacter-patterned SU-8SU-8 film by UV using Finally, we investigated the THz image of a character-patterned SU-8 film by UV exposure using a reflective imaging withspectroscopy. THz spectroscopy. To obtain theimage THz image of the character ausing reflective imaging systemsystem with THz To obtain the THz of the character that is a reflective imaging system with THz spectroscopy. To obtain the THz image of the character that is that is patterned on the SU-8 film, we produced a metal maskaspatterned as laid “KFRI” laid it on patterned on the SU-8 film, we produced a metal mask patterned “KFRI” and it onand the SU-8 film. patterned on the SU-8 film, we produced a metal mask patterned as “KFRI” and laid it on the SU-8 film. the SU-8 film. Then exposed film20tomin. UV The lightsample for 20 was min.moved The sample was moved by Then we exposed the we SU-8 film to the UV SU-8 light for by a two-dimensional Then we exposed the SU-8 film to UV light for 20 min. The sample was moved by a two-dimensional X–Y stage and the THz images were obtained with a stage resolution of 100 μm and linear velocity X–Y stage and the THz images were obtained with a stage resolution of 100 μm and linear velocity

Sensors 2017,2017, 17, 2825 Sensors 17, 2825

6 of 8 6 of 8

of 30 ms per pixel, as shown in the real image of the THz imaging system shown in Figure 5f. The a two-dimensional X–Y stage and the THz images were obtained with a stage resolution of 100 µm UV-printed SU-8 film imaged by the THz system is shown in Figure 5b. As can be seen in this figure, and linear velocity of 30 ms per pixel, as shown in the real image of the THz imaging system shown the “KFRI” characters are not shown as a visible image. However, the characters can be shown using in Figure 5f. The UV-printed SU-8 film imaged by the THz system is shown in Figure 5b. As can be THzseen pulses. Figure represented the reflective THz pulsed images reflecting thetheUV-treated in this figure,5athe “KFRI” characters are not shown as a visible image. However, characters and non-UV-treated experimental scheme is the shown in Figure 5f afterimages UV light exposure; can be shownSU-8 usingfilm. THzThe pulses. Figure 5a represented reflective THz pulsed reflecting the phase of the THz pulse is delayed because the refractive index of SU-8 is increased as the UV-treated and non-UV-treated SU-8 film. The experimental scheme is shown in Figure 5f shown after in Figure This phase delay causes contrast change in the THz image. Hence, the characters UV5g. light exposure; the phase of athe THz pulse is delayed because the refractive index of SU-8“KFRI” is increased as shown in as Figure 5g. in This phase5c. delay causes a contrast in that the THz image. can appear significantly, shown Figure If the sample is thin change enough, is, the timeHence, of the THz the characters “KFRI” can appear significantly, as shown in Figure 5c. If the sample is thin enough, that pulse passing through the sample is shorter than the pulse width of the THz pulse, the Fabry–Pérot is, the time of the THz pulse passing through the sample is shorter than the pulse width of the THz effect affects the THz pulse due to multiple reflections. In this case, however, the peak of the THz pulse, the Fabry–Pérot affects the THz due to multiple reflections. In thisofcase, pulse at time-domain caneffect be separated withpulse a reference THz signal regardless thehowever, Fabry–Pérot the peak of the THz pulse at time-domain can be separated with a reference THz signal regardless of effect. Hence, the THz pulsed image can be obtained successfully. Moreover, if the THz pulse with a the Fabry–Pérot effect. Hence, the THz pulsed image can be obtained successfully. Moreover, if the short pulse width less than the thickness of the sample can be generated, the THz pulse would be THz pulse with a short pulse width less than the thickness of the sample can be generated, the THz obtained reflections. pulse without would bemultiple obtained Fabry–Pérot without multiple Fabry–Pérot reflections.

Figure (a) The reflective THz pulseatatUV UVnon-treated non-treated and SU-8 area. TheThe images of characters Figure 5. (a)5.The reflective THz pulse andtreated treated SU-8 area. images of characters patterned on SU-8 film by (b) optical; (c) reflective THz pulses; (d) 400 GHz and (e) 700 GHz frequency; patterned on SU-8 film by (b) optical; (c) reflective THz pulses; (d) 400 GHz and (e) 700 GHz frequency; (f) an experimental scheme and real image of THz reflective imaging; (g) schematic of phase delay and (f) an experimental scheme and real image of THz reflective imaging; (g) schematic of phase delay absorption before and after UV treatment. and absorption before and after UV treatment.

In addition, the cross-linking between the epoxy groups could enhance the structural density of

In addition, the cross-linking between the epoxy groups could enhance the structural density of SU-8. This processing causes increased absorbance in the THz region (Figure 5f). This result shows SU-8.that This processing causes increased absorbance in the THz region (Figure 5f). This result shows the changed absorbance can also cause the image contrast. From this, we can read the image that of thethe changed absorbance SU-8 can also cause frequency the imageofcontrast. From this,aswe can in read the 5d,e. image of character-patterned at a single 400 and 700 GHz, shown Figure the character-patterned SU-8can at abesingle frequency of 400 700 GHz, asInshown Figure Accordingly, the characters observed prominently at aand single frequency. the THzinpulse and 5d,e. Accordingly, the characters can be observed a single In the single-frequency images, gradation of valuesprominently in the images at can be seen.frequency. This is because theTHz SU-8pulse film and is slightly rumpled aftergradation UV treatment. Wheninthe film is exposed to UV the UV the intensity single-frequency images, of values theSU-8 images can be seen. Thislight, is because SU-8 film decreases with the depth the SU-8 surface. eachthe depth the is slightly rumpled after UVoftreatment. WhenConsequently, the SU-8 filmthe is density exposedchanges to UV at light, UVofintensity film. Hence, the path of the THz pulse can deviate at the boundary between the UV-exposed and decreases with the depth of the SU-8 surface. Consequently, the density changes at each depth of the contrast gradation in THz film.normal Hence,area. the This pathcauses of thetheTHz pulse can deviate atimages. the boundary between the UV-exposed and normal area. This causes the contrast gradation in THz images. 4. Conclusions We studied the THz characteristics of SU-8 as a photoresist polymer and its applications to security printing and anti-forgery. We measured the refractive index of an SU-8 film by UV light

Sensors 2017, 17, 2825

7 of 8

4. Conclusions We studied the THz characteristics of SU-8 as a photoresist polymer and its applications to security printing and anti-forgery. We measured the refractive index of an SU-8 film by UV light exposure time dependency in the THz range. The refractive index of SU-8 changed significantly as the UV exposure time increased. It is due to the change in the chemical structure after UV exposure by cross-linking between epoxy groups. Consequently, the value of the refractive index increased from 1.539 to 1.636 at 0.5 THz. However, there was no significant change in other non-photoresist polymers such as polyethylene, polyethylene terephthalate, polyimide, and polymethylpentene. To demonstrate the security application, we also obtained the THz image of characters patterned on the SU-8 film by using reflective THz pulses. The THz pulse and THz single frequency can be used to distinguish images between UV treatment and non-treatment successfully, but the characters cannot be seen in the visible image. It can be concluded that SU-8 is a good candidate for security applications in the THz range. Consequently, these results can be applied to currency, passport, food packaging, and other areas by using a simple method consisting of a photoresist polymer, UV light, and THz system. For future work, to apply this technique to security and anti-forgery, we plan to improve the scanning time and image resolution. Acknowledgments: This work was supported financially by the Main Research Program of the Korea Food Research Institute funded by the Ministry of Science, ICT & Future Planning (E0142105-04). Author Contributions: Gyeongsik Ok and Hee Jun Shin conceived and designed the concept of the experiments. Hee Jun Shin wrote the manuscript and analyzed the experimental results. SU-8 film samples using UV light were prepared by Min-Cheol Lim and Kisang Park. THz time-domain spectroscopy characteristics of SU-8 were analyzed by Hee Jun Shin, Sae-Hyung Kim and Sung-Wook Choi. All the authors participated in the discussion and the analysis of the results. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Chesak, C.E. Holographic counterfeit protection. Opt. Commun. 1995, 115, 429–436. [CrossRef] Janucki, J.; Owsik, J. A holographic method for document protection against counterfeit. Opt. Commun. 2003, 228, 63–69. [CrossRef] Aggarwal, A.K.; Kaura, S.K.; Chhachhia, D.P.; Sharma, A.K. Concealed moiré pattern encoded security holograms readable by a key hologram. Opt. Laser Technol. 2006, 38, 117–121. [CrossRef] Yoon, B.; Lee, J.; Park, I.S.; Jeon, S.; Lee, J.; Kim, J.M. Recent functional material based approaches to prevent and detect counterfeiting. J. Mater. Chem. C 2013, 1, 2388–2403. [CrossRef] Cui, Y.; Hegde, R.S.; Phang, I.Y.; Lee, H.K.; Ling, X.Y. Encoding molecular information in plasmonic nanostructures for anti-counterfeiting applications. Nanoscale 2014, 6, 282–288. [CrossRef] [PubMed] Huang, K.; Carulli, J.M.; Makris, Y. Counterfeit electronics: A rising threat in the semiconductor manufacturing industry. In Proceedings of the 44th IEEE International Test Conference, Anaheim, CA, USA, 6–13 September 2013. Deisingh, A.K. Pharmaceutical counterfeiting. Analyst 2005, 130, 271–279. [CrossRef] [PubMed] Lee, R.A. Micro-technology for anti-counterfeiting. Microelectron. Eng. 2000, 53, 513–516. [CrossRef] Zunze, H.; Hao, Z. A new digital anti-counterfeiting passport. In Proceedings of the 2010 2nd International Conference on Signal Processing Systems, Dalian, China, 5–7 July 2010. [CrossRef] Bae, H.J.; Bae, S.; Park, C.; Han, S.; Kim, J.; Kim, L.N.; Kim, K.; Song, S.-H.; Park, W.; Kwon, S. Biomimetic microfingerprints for anti-counterfeiting strategies. Adv. Mater. 2015, 27, 2083–2089. [CrossRef] [PubMed] Mills, A. Oxygen indicators and intelligent inks for packaging food. Chem. Soc. Rev. 2005, 34, 1003–1011. [CrossRef] [PubMed] Abargues, R.; Rodriguez-Canto, P.J.; Albert, S.; Suarez, I.; Martínez-Pastor, P. Plasmonic optical sensors printed from Ag—PVA nanoinks. J. Mater. Chem. C 2014, 2, 908–915. [CrossRef] Ergün, S.; Sönmez, S. Terahertz technology for military applications. J. Mil. Inf. Sci. 2015, 3, 13–16. [CrossRef] Federici, J.F.; Schulkin, B.; Huang, F.; Gary, D.; Barat, R.; Oliveira, F.; Zimdars, D. THz imaging and sensing for security applications—Explosives, weapons and drugs. Semicond. Sci. Technol. 2005, 20, S266. [CrossRef]

Sensors 2017, 17, 2825

15. 16. 17.

18. 19. 20. 21. 22. 23. 24.

25. 26.

8 of 8

Kemp, M.C.; Taday, P.F.; Cole, B.E.; Cluff, J.A.; Fitzgerald, A.J.; Tribe, W.R. Security applications of terahertz technology. Proc. SPIE 2003, 5070, 44–52. [CrossRef] Tribe, W.R.; Newnham, D.A.; Taday, P.F.; Kemp, M.C. Hidden object detection: Security applications of terahertz technology. Proc. SPIE 2004, 5354, 169. [CrossRef] Zhang, H.; Sfarra, S.; Saluja, K.; Peeters, J.; Fleuret, J.; Duan, Y.; Fernandes, H.; Avdelidis, N.; Ibarra-Castanedo, C.; Maldague, X. Non-destructive investigation of paintings on canvas by continuous wave terahertz imaging and flash thermography. J. Nondestruct. Eval. 2017, 36, 34. [CrossRef] Federici, J.; Moeller, L. Review of terahertz and subterahertz wireless communications. J. Appl. Phys. 2010, 107, 111101. [CrossRef] Song, Q.; Zhao, Y.; Redo-Sanchez, A.; Zhang, C.; Liu, X. Fast continuous terahertz wave imaging system for security. Opt. Commun. 2009, 282, 2019–2022. [CrossRef] Lorenz, H.; Despont, M.; Fahrni, M.; LaBianca, N.; Renaud, P.; Vettiger, P. SU-8: A low-cost negative resist for MEMS. J. Micromech. Microeng. 1997, 7, 121. [CrossRef] Shin, H.J.; Oh, S.J.; Kim, S.I.; Kim, H.W.; Son, J.-H. Conformational characteristics of β-glucan in laminarin probed by terahertz spectroscopy. Appl. Phys. Lett. 2009, 94, 111911. [CrossRef] Del Campo, A.; Greiner, C. SU-8: A photoresist for high-aspect-ratio and 3D submicron lithography. J. Micromech. Microeng. 2007, 17, R81–R95. [CrossRef] Ong, B.H.; Yuan, X.; Tjin, S.C. Adjustable refractive index modulation for a waveguide with SU-8 photoresist by dual-UV exposure lithography. Appl. Opt. 2006, 45, 8036–8039. [CrossRef] [PubMed] Salazar-Miranda, D.; Castillon, F.F.; Sanchez-Sanchez, J.J.; Angel-Valenzuela, J.L.; Marquez, H. Refractive index modulation of SU-8 polymer optical waveguides by means of hybrid photothermal process. Rev. Mex. Ing. Quím. 2010, 9, 85–90. Avila, L.F.; Nalin, M.; Cescato, L. Self diffraction holographic techniques for investigation of photosensitive materials. Proc. SPIE Hologr. Adv. Modern Trends III 2013, 8776, 87760G. [CrossRef] Wang, X.B.; Sun, J.; Chen, C.M.; Sun, X.Q.; Wang, F.; Zhang, D.M. Thermal UV treatment on SU-8 polymer for integrated optics. Opt. Mater. Express 2014, 4, 509–517. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).