In-Process Measurement of Thickness of Cured Resin in Evanescent-Wave-Based Nano-stereolithography Using Critical Angle Reflection

  • Deqing Kong
  • Masaki Michihata
  • Kiyoshi Takamasu
  • Satoru Takahashi
Original Article
  • 89 Downloads

Abstract

Stereolithography is one of the most powerful ways to fabricate complex three-dimensional polymeric-based structures layer-by-layer using optical power. Evanescent-wave-based nano-stereolithography using ultra-thin field distribution of evanescent wave to solidify photosensitive resin can provide a sub-micrometer vertical resolution of each layer. To meet strong demands for in-process thickness measurement of cured resin in evanescent-wave-based nano-stereolithography, a measurement method that utilizes variations of resin’s refractive index after polymerization and a high sensitivity of total internal reflection at the critical angle has been proposed. By launching the a measurement light from a substrate to resin at the critical angle and detecting reflections from the resin, slight change in refractive index and thickness of cured resin that have great influences on reflectivity can be in-process measured. This method has been firstly examined by simulation using the rigorous coupled wave analysis method. Here, we show that an increase in cured resin’s thickness induces a decrease in the reflectivity. In experiments, the largest reflectivity contrast between cured and uncured resins has been proved at the critical angle. In addition, the relationship between reflectivity drop and thickness has been calibrated and a linear relationship within a certain thickness range has been experimentally confirmed. Furthermore, the difference of curing process between continuous and discontinuous exposure has been investigated by using the proposed measurement method. At last, image subtraction and a median filter have been applied in imaging processing to remove influences of uneven illumination and background noises.

Keywords

Photosensitive resin Nano-stereolithography Evanescent light Critical angle In-process measurement RCWA method 

1 Introduction

Nowadays, manufacturing technologies for three-dimensional microcomponents play a vital role in many areas of modern technologies in the evolvement of functional applications such as microelectromechanical systems (MEMS), microoptical electronics systems (MOES), biochip, microfluidic devices, photonic crystal [1, 2]. Microstereolithography, as one of the most powerful techniques of manufacturing, has been developed to produce microsized three-dimensional structures layer by layer using optical power [3, 4, 5]. To date, there have been quite a number of and detailed researches on stereolithography. These stereolithography techniques with different apparatus of irradiation can be simply categorized as laser-scanning-based stereolithography [6, 7, 8] and projection-based stereolithography [9, 10, 11]. Evanescent-wave-based nano-stereolithography (EWNSL), as one of projection-based stereolithography, utilizes evanescent wave instead of propagating light to provide polymerization energy [12, 13, 14]. It can produce each layer of resin in the sub-micrometer resolution because of ultra-thin electric field distribution of the evanescent wave. The illustration of the evanescent wave and a fabrication process of EWNSL is shown in Fig. 1a, b, respectively. Remarkably, in this method each layer cured by the evanescent wave separates from the previous layer before an adhesion process. In contrast, conversional stereolithography methods directly fabricate a new layer on previous layers since cured layers are used to limit the region of curing. This means, in EWNSL, it is possible to measure production conditions of every single layer before adhesion processes. Preciously focusing on this point, we proposed an in-process measurement method in EWNSL to measure thickness of each layer of cured resin [15, 16, 17, 18].
Fig. 1

Schematic diagram of evanescent-wave-based nano-stereolithography. a Principle diagram of evanescent wave generated by total internal reflection. b Fabrication process of evanescent-wave-based nano-stereolithography

Significance of our investigation is based on an inevitable fact that some fabrications errors often appear in EWNSL. These unavoidable fabrication errors can be simply classified into projection errors that resulted from the scattering and diffraction, curing errors generated by environment disturbances in vulnerable photo initiation reactions, polymerization errors caused by chain reactions of monomer molecules and shrinkage errors due to post-curing of cured resin [19, 20, 21, 22, 23, 24]. All of above fabrication errors will finally lead to uneven thickness of each solidified layer, which as a serious problem directly influences the fabrication quality and limits the applications of EWNSL. Therefore, the in-process measurement on the thickness of the cured resin that detects the fabrication errors in time and assists us to further investigate origin of them in EWNSL is of great significance and value. In addition, the in-process measurement makes it possible to develop an intelligent fabrication system that uses measurement results as feedbacks to dynamically adjust irradiation, control the curing process and correct the fabrication errors.

Difficulties of the in-process measurement in EWNSL are mainly caused by following problems: Thickness of each layer is only several hundreds of nanometers; cured resin is submerged in uncured (liquid) resin in the fabrication process; there is gradient-refractive-index boundary between the cured and uncured resins due to uncompleted polymerization. Due to above problems, methods for thickness measurement based on interferometry cannot be used in EWNSL since the gradient boundary between the cured and uncured resins greatly weakens reflections. In addition, because curing of resin is a quick process, some thickness measurement methods based on a scanning probe that require a certain sweep time cannot be used. Other measurement methods like FTIR (Fourier transform infrared spectroscopy) [25, 26], differential scanning calorimetry (DSC) and Raman spectroscopy [27, 28] have been frequently used to monitor the curing process of resin; however, these methods are hard to obtain spatial information of the cured resin and therefore cannot meet the demands of thickness measurement. Differential interference contrast (DIC) microscopy has a theoretical potential to measure the thickness of cured resin in EWNSL, but it is hard to be applied as the in-process measurement in realistic productions. This is because measurement systems need to be supplemented in fabrication systems, and there is limited space for measurement systems. DIC needs to occupy both spaces over and under resin for detecting light transmitting through resin, which leads to a spatial conflict with the fabrication system.

In our previous investigation, surface plasmon resonance (SPR) [29, 30] has been applied for the in-process measurement in EWNSL [15, 16]. Based on a theory that the thickness of cured resin determines a resonance condition and significantly affects the reflectivity, the in-process measurement of the thickness has been achieved by detecting and analyzing the reflection. To generate SPR, resin has to be placed on a substrate with a metal filming layer, and a measurement light needs to be incident from the bottom of resins in a particular incident angle. On the downside, measurement accuracy is limited by a low lateral resolution in SPR measurement which is determined by propagation lengths of polaritons rather than optical diffraction limit.

In this research, we proposed a measurement method utilizing the high sensitivity of total internal reflection at the critical angle. Once resin is cured, the refractive index and the thickness change of resin will disturb the total internal reflection condition and result in the reflectivity drop. The in-process measurement can be simply achieved by detecting the reflection. Just as SPR measurement, only space below the substrate is needed and the measurement system shares a prism or an oil-immerging objective with the fabrication system. The difference is that in this method metal coating is not needed any more and the incident angle of measurement light is in the critical angle rather than the SPR angle.

The following contents of this paper consist three sections: theory, simulation and experiment. In the theory section, a study on the formation of resin layers will be briefly explained; after that, principles of our proposed method will be demonstrated. Its feasibility will be further discussed in the simulation section. In the experiment section, relationship between the reflectivity and the thickness of cured layer will be confirmed.

2 Theoretical Background

In EWNSL, field strength of the evanescent wave drops off exponentially away from the substrate; the closer the resin to the substrate, the higher exposure energy and therefore the faster curing speed [31]. As a result, the resin near the substrate will be cured firstly and the thicknesses of cured layer increase as exposure time expands. It is notable that, in the curing process, interfaces between the cured and the uncured resins are not clear. These gradient boundaries are made of half-cured resin in a state of uncompleted polymerization [22, 32]. As we mentioned in the Introduction section, the gradient boundary with low reflection brings some difficulties for the measurement. More seriously, due to the existence of the gradient boundary, the thickness of cured layers is finally determined after a washing and drying treatment. Therefore, it is impossible to measure absolute thickness in the fabrication since the absolute thickness does not exist before removing the uncured and half-cured resins. What we measure in the fabrication process are effective thicknesses of cured resin. They represent the thickness of cured layers that will probably appear after the cleaning process.

2.1 Curing Process

Essentially, the curing process of photosensitive resin is one of photo-polymerization reactions [22]. Reaction mechanism and synthetic process are shown in Fig. 2a. The uncured resin mainly contains photosensitive initiators and monomers. The curing process starts from initiators break into reactive species when resin is exposed by light in particular wavelength. The reactive species may be a free radical, cation or anion, determined by the type of initiator. In following reactions, the reactive species add to monomer molecules to form excited monomer as new radical, cation or anion centers, as the case may be. The process is repeated as the monomer molecules are successfully connected to the continuously propagating reactive centers, which finally results in the generation of high molecular weight polymers [33].
Fig. 2

Curing process of photosensitive resin. a Schematic diagram of resin’s reaction; b effective thickness against exposure energy

Curing rate of resin is decided by many factors including the types of initiator and monomer, the concentration of initiators and oxygen in resin, temperature and the exposure conditions. For a given composition of resin, reaction rate can be controlled by the exposure intensity and time. In the case of evanescent-wave-based nano-stereolithography, the exposure intensity as a function of a distance from a substrate along normal direction is defined by [13]:
$$ I_{z} = \tau I_{0} \exp^{ - (\alpha + 2\beta )z} $$
(1)
where I0 is the intensity of incident light. α and β are the attenuation coefficient of absorption and evanescent field. τ is the intensity coefficient of evanescent field in a certain incident angle. α, β and τ are defined as Eqs. (2)–(5).
$$ \alpha = [P]\varepsilon $$
(2)
$$ \beta = \frac{{2\pi n_{\text{R}} \sqrt {\sin^{2} \theta_{\text{I}} - ({{n_{\text{P}} } \mathord{\left/ {\vphantom {{n_{\text{P}} } {n_{\text{R}} }}} \right. \kern-0pt} {n_{\text{R}} }})^{2} } }}{\lambda }z $$
(3)
$$ \tau_{\text{P}} = 4\cos^{2} \frac{{\delta_{\text{s}} }}{2} $$
(4)
$$ \tau_{\text{S}} = 4\left( {\frac{{n_{1} }}{{n_{2} }}} \right)^{4} \sin^{2} \theta_{\text{I}} \cos^{2} \frac{{\delta_{\text{p}} }}{2} + 4\cos^{2} \theta_{\text{I}} \sin^{2} \frac{{\delta_{\text{p}} }}{2} $$
(5)
where [P] is the molar concentration of light-absorbing photoinitiator. ε is the absorption coefficient of P and varies with concentration, wavelength and temperature. This equation is obtained from Beer–Lambert law. θ is the incident angle of light, λ is the wavelength of exposure light, and nP and nR are the refractive index of prism and resin. τ is determined by polarization and the incident angle. Equations (4) and (5) show the expression of τ in s- and p-polarization, respectively. δP and δS are the phase variation of reflection in the two polarizations. According to the above definitions, the exposure intensity drops off exponentially away from a substrate. As a result, for curing process, the closer resin to the substrate, the higher exposure energy and faster curing speed.
For the curing thickness, based on the above fact that the curing rate decreases with the distance from the substrate, resin lying further from the substrate will be cured after accumulating enough exposure energy, and therefore, the thickness of cured resin keeps on increasing with the expansion of exposure time. However, the exact thickness of the cured resin is not only determined by the curing process but also a washing and the drying treatment after fabrication. As we mentioned above, the gradient boundary is made of incompletely polymerized resin, where the resin’s curing degree continuously changes from maximum to zero. In the washing and drying treatment, the resin in low curing degree will be dissolved by organic solvent and removed. The curing degree that determines whether resin is removed or not is defined as the critical curing degree. The exposure power corresponding to the critical curing degree of resin, determined by characteristic of resin and dissolvent, is defined as the critical exposure energy. For a given ingredient of resin and dissolvent, analytical computations can be done to get its thickness by bringing the critical exposure energy into Eq. (1). It can be denoted by the following expression:
$$ T_{\text{Eff}} = \frac{1}{\alpha + 2\beta }\ln \left( {\frac{{\tau U_{0} }}{{U_{\text{c}} }}} \right) $$
(6)
where U0 is the exposure energy defined as multiplying the intensity of incident light (I0) by the exposure time. UC is the critical exposure energy. Curing will be considered complete if the exposure energy is larger than the critical energy. Figure 2b plots the effective thickness of cured resin as a function of the exposure energy when nP = 1.78, nR = 1.47, θI = 65°, λ = 488 nm, UC = 100 mJ/cm2, and in p-polarization. Because α is far less than β under general conditions, α was neglected in the calculation. It is shown that the thickness of the cured resin is in a sub-micrometer scale and increases with the exposure energy in the logarithm way.

2.2 In-Process Measurement

A schematic diagram of our measurement method is shown in Fig. 3a. In this figure, besides the exposure light for fabrication, another beam of light in a larger wavelength was launched from the bottom of the resin used as the measurement light. It is notable that photosensitive resin only be cured by light in a certain wavelength range. Therefore, effects of the measurement light on curing process can be minimized by properly selecting a wavelength of the measurement light. The exposure light for fabrication is set around 70° (much larger than the critical angle) which could guarantee effectiveness of total internal reflection. The incident angle of the measurement beam is fixed at the critical angle. Reflection distribution of the measurement light is detected and used to calculate effective thickness of the cured resin.
Fig. 3

Principle of proposed measurement method. a Schematic diagram of in-process measurement using reflection at critical angle; b reflectivity as a function of incident angle in case of cured and uncured resins

The in-process measurement on cured resin is based on three significant principles: increase in resin’s refractive index in the curing process, abrupt reflectivity change at the critical angle, and interference of reflection from bottom and top of the cured resin. The increase in refractive index is an accompanying phenomenon in the curing process caused by a decrease in intermolecular distances. Increments of refractive index vary from types of resin and are normally in a range from 0.01 to 0.05 refractive index units (RIU) [34, 35, 36, 37]. As the cured resin has larger refractive index than the uncured resin, when light incident from a high-refractive-index prism to resin, as shown in the insert figure of Fig. 3b, the critical angle (θI) defined by Eq. (7) becomes larger with the increase in refractive index. Besides, according to Eq. (8), not only the critical but also reflectivity of light in p-polarization (rP) changes in the curing process.
$$ \theta_{\text{C}} = \arcsin (n_{\text{R}} /n_{\text{P}} ) $$
(7)
$$ r_{\text{P}} = \left| {\frac{{n_{\text{P}} \cos \theta_{\text{T}} - n_{\text{R}} \cos \theta_{\text{I}} }}{{n_{\text{P}} \cos \theta_{\text{T}} + n_{\text{R}} \cos \theta_{\text{I}} }}} \right|^{2} $$
(8)
where θT is the refractive angle and defined as θT = arcsin (sin(θI)nP/nR). Figure 3b shows the reflectivity as a function of the incident angle calculated from Eq. (8) when nP = 1.78 and nR = 1.47 and 1.51 representing the uncured and the cured resins, respectively. It is shown that the angular spectrum of reflectivity positively shifts after curing of resin. Most importantly, at the critical angle of the uncured resin, the reflectivity between the uncured and the cured resins differs greatly, which means it is possible to directly observe the horizontal shape of the cured resin by detecting reflection distribution at the critical angle. The thickness measurement of the cured resin is based on interference generated by reflections from the down and up boundaries of the cured resin. The interference is determined by the distance between two reflecting surface, and therefore, the reflection is greatly affected by the thickness of the cured resin. It is noteworthy that the bottom boundary of the cured resin is an interface between the substrate and the cured resin; its refractivity decreases significantly when resin’s refractive index increases. The top boundary of the cured resin is the gradient boundary mentioned above, which has a character of anti-reflection. In this research, because the measurement light is transmitted from the substrate to the uncured resin in the critical angle of these two medium, total internal reflection occurs when the light transmits to the uncured resin. Therefore, all of the light that transmits though the cured resin will be reflected from the gradient boundary. In a word, the reflectivity in the above system should change periodically with the increase in the thickness of the cured resin. The theoretical model has been clearly explained; however, it is hard to clarify a mathematical relation between the reflectivity and the thickness with the existence of the gradient boundary and potential multi-reflection. Therefore, a simulation was done to confirm our demonstration and study near-field optical response of the cured resin, and after that, the relation between the thickness and the reflectivity was experimentally measured.

3 Simulation on Optical Response in Critical Angle Illumination

To confirm that an increasing thickness of resin leads to the change in reflectivity and study near-field optical response when measurement light is launched at the critical angle, a simplified equivalent structure was simulated by using the rigorous coupled wave analysis (RCWA) method. Simulation model is shown in Fig. 4a. The refractive index of prism, uncured and cured resins was 1.78, 1.47 and 1.51, respectively. The wavelength of measurement light was 638 nm. The measurement light was launched at the critical angle (55.7°). The width of cured resin was fixed at 50 μm. Harmonics of RCWA calculation was set as 100. Calculation area was 200 μm in the direction parallel to the substrate (x axis) and 3 μm in the direction normal to the substrate (z axis). Boundary above and below the calculation field was PML (perfect matching layer); the boundary on two sides of calculation field was periodic boundary. A monitor was put below the interface to detect the intensity distribution of the reflection.
Fig. 4

Simulation on optical response of cured resin using RCWA method. a Simulation diagram; b intensity distribution of reflection in various thicknesses of cured resin. c Intensity distribution of E field in whole simulation field

Figure 4b plots computed results recorded by the monitor when the thickness of cured resin increases from 0.3 to 0.6 μm. It is shown that the reflectivity drops below cured resin enlarged obviously with the thickness increase. This confirmed the feasibility of thickness measurement using the proposed method. In addition, we found that the reflection distribution below the cured resin was not as flat as the top surface of cured resin; and at region that was not directly below the cured resin (in the area of x > 25 μm, right of cured resin) also appeared reflectivity drop. These unexpected oscillations were enhanced with the thickness increase as well. In our point of view, they were caused by multiple reflection in the cured resin. The measurement light that is reflected into substrate after interference will reflect again from the up surface of cured resin to substrate. Therefore, except leaking from the edge on right sides of cured resin, all of the light was finally reflected in substrate. The oscillations of reflection curve were caused by the supersession of multi-reflection.

To confirm this point, time-averaged intensity distribution of E field was plotted when a cured resin was in a thickness of 0.4 μm, as shown in Fig. 4c. It can be seen that the field distribution near the up surface of cured resin increases from left to right showing the accumulation of multi-reflection. In the consideration of experiment, a lot of factors including gradient boundary, uneven surface and non-uniform distribution of refractive index of cured resin will destroy the susceptible total internal reflection on up surface. As a result, the multi-reflection should be greatly weakened and there will be no obvious oscillations in physical experiments. In a word, the impact of thickness on reflectivity has been proved; the relation between thickness and reflectivity needs to be measured by actual experiments.

4 Experiments

4.1 Experiment Setup

A whole experiment system is shown in Fig. 5. It includes the fabrication and the measurement section. Two beams of light in the wavelength of 405 and 638 nm were used as the curing (fabrication) light and the measurement light, respectively. As we mentioned above, resin can be cured by light in particularly wavelength range. In this experiment, operation of measurement did not impact the fabrication as wavelength of the measurement light was out of curing wavelength of resin used in experiment. In the fabrication section, light was delivered by optical fiber and transmitted through collimator and polarizer. A shutter was used to control the exposure time. In order to fabricate cured resin in a smaller width seeking the real size of production in micro/nano-stereolithography, the curing light was focused by lens before being launched to prism. In the measurement section, the polarized laser light in the wavelength of 638 nm was delivered by the left arm of and reflected from the interface between resin and substrate. The reflected light propagated into imaging section and was collected by an 8-bit CMOS camera. In experiment operation, the two arms were, respectively, fixed on the rotation stage centered on the prism. The incident angle of the fabrication light was fixed at 65°, and the angle of measurement was determined by the direction of two measurement arms. Urethane–acrylate-based resin (KC1162) was used as sample. Its refractive index increases from 1.47 to 1.51 after being totally cured. Prism was in a refractive index of 1.78. In order to avoid the damage on prism, resin was put on a substrate that in the same refractive index with prism.
Fig. 5

Diagram of experiment setup

Regarding the performance of the measurement system, few points are worth discussing. First of all, it is hard to clarify imaging resolution of the current equipment since resin was measured in a large viewing angle. In our future work, sub-microresolution can be achieved by introducing oil immersion objective in a high numerical aperture. However, considering the measurement light has a larger wavelength than the fabrication light, detail loss of the measurement on cured resin is unavoidable in case of the fabrication and the measurement section shares a same objective lens. Furthermore, measurement range of thickness is not specifically defined; the intensity of reflection will change in a periodic of 450 nm and the multi-reflection will be largely enhanced when thickness keeps on increasing. Considering the cured resin produced by the evanescent-wave-based stereolithography is normally in a thickness within 500 nm, the measurement range of thickness is enough for our investigations. In addition, measurement rate of current system, mainly determined by the CMOS camera, is 90 frames per second (FPS). This rate is sufficient for our experiment as laser used in the experiment was weak (about 0.5 mW) and generated relatively low curing speed. The measurement rate can be further improved by using a high-speed camera to match fast fabrication speed in industrial production.

4.2 Measurement of Bulk Sample

Several experiments have been done. In the first experiment, to confirm the refractive index difference between uncured and cured resins and evaluate the experiment system, bulk cured resin that was submerged in uncured resin was observed in various incident illumination angles. (Fabrication section was not used in this experiment.) Figure 6a–c shows the images obtained by a CMOS camera when incident angle was in 54.1°, 55.7° and 61.2°. Cured resins were in the center of the image. Around the cured resin, there was some dark spot whose gray value did not change with incident angle. They were background noise caused by the dust. According to the refractive of prism and resin, the critical angle of uncured and cured resins was 55.7° and 58.1°, respectively. The relationship between critical angle and incident angle in Fig. 6a–c is shown in Fig. 6d. Figure 6a shows the image taken in case of θI (incident angle) < θCUR (critical angle of uncured resin) <  θCCR (critical angle of cured resin). Since both uncured resin and cured resins provided low reflectivity in such an incident angle, the brightness of resin’s both states was low, and there was only a slight variation between cured resin and uncured resin shown in this figure. Figure 6b shows the image gained when θCUR < θI < θCCR. In this case, the total internal reflection occurred at the boundary between uncured resin and substrate, which leads to the highest intensity of reflected light from uncured resin, while total internal reflection did not occur at the bottom of cured resin since current incident angle was smaller than the critical angle of cured resin. As a result, the brightness of both uncured and cured resins varies largely; the position and the shape of cured resin can be easily observed. This result confirmed the feasibility of our method when incident angle is at the critical angle. Figure 6c shows the image when θCUR <  θCCR <  θI. It is obvious that two states of cured resin cannot be distinguished as total internal reflection occurred at bottom of both and insensitivity of reflected light from resin in both states reach maximum.
Fig. 6

Experiment on bulk sample in various incident angles. Intensity distribution of reflected light launched in incident angle of a 54.1°, b 57.1° and c 61.2°, d relation between incident angle and critical angle, e cross section of gray value near boundary region between cured and uncured resins, f experimental gray contrast and theoretical reflectivity contrast as a function of incident angle

The cross section of gray value near the boundary region stretches from cured to uncured resin in the various incident angles is plotted in Fig. 6e. The gray value of image that directly indicates the intensity of reflection was used to evaluate the reflectivity. In order to remove the noise of images, every curve in these figure are averaged from ten lines stretching a same distance in cured and uncured resins. It is obvious that the gray values of cured resin and uncured resin vary with incident angle. The reflectivity contrast became maximum when the incident light was near the critical angle. At the boundary, there were small peaks that show the increments of reflectivity, which was caused by the scattering at the boundary. The reflectivity drop of cured resin was calculated by dividing averaged gray value drop by the gray value in total internal reflection condition (around 96 gray value units). The reflectivity drop obtained by experiment was plotted with the theoretical value gained by calculating the reflectivity difference between cured and uncured resins, as shown in Fig. 6f. It shows that the experiment results well agree with the theoretical value, which confirms the refractive index increase of resin and also proves the feasibility of our experiment equipment.

4.3 Measurement of the Relation Between Thickness of Cured Resin and Reflectivity

In this experiment, in order to confirm the influence on reflectivity caused by thickness variation and experimentally find the relationship between thickness and reflectivity, cured resin in a thickness of sub-micrometer was cured by evanescent light and measured by the proposed method.

Figure 7 illustrates the experiment results. Three samples exposed in 1.0, 2.0 and 3.0 s were measured, respectively. The reflection distribution of three samples detected by a CMOS camera is shown in Fig. 7a–c. Shape of cured resin can be directly observed according to the gray value deduction caused by the reflectivity drop. Besides, it is obvious that the gray value of cured resin in three figures was different; the gray value of cured resin decreases through the exposure time. This is well agreed with our assumption that a longer exposure time leads to a larger thickness of cured resin and finally results in stronger reflectivity drops. For comparison, the three samples were observed by optical microscopy after washing and drying process. It is notable that in the proposed method the measurement light was oblique incidence (55.7°), and an inclined projection results in the distortion of image. Distortion problem will be bypassed in our future work by applying immersion objective lens to generate the oblique incidence at the critical angle in multi-direction.
Fig. 7

Experiment results using proposed method. The distribution of gray value obtained by proposed method when sample was exposed by a 1, b 2 and c 3 s. df Corresponding image observed by optical microscopy after drying process. g Thickness measured by AFM and reflectivity drop obtained by in-process measurement as a function of position. h Reflectivity drop as a function of thickness

In order to find the relation between the reflectivity drop in in-process measurement and thickness of cured resin, the thickness of sample was measured by AFM (atomic force microscope) after drying process. Figure 7g plots the thickness and the reflectivity drop distribution in same cross section of cured resin. It is obvious that the reflectivity drop shows the same tendency with the thickness of cured resin. The slope of the reflectivity distribution was not as gentle as the curve of thickness. This is because in the in-process measurement, interference and dust caused uneven distribution of measurement light which directly influenced the measurement results. The uneven distribution can be removed by making subtraction between the gray value distributions obtained before and after curing. The imaging process and its results will be illustrated in the next experiment. In addition, the reflectivity drop varied linearly with the thickness of cured resin according to the measurement results. For this reason, the reflectivity-position curve was coinciding with the thickness position curve after adjusting the range of the two vertical axes in Fig. 7g. To confirm this point, the reflectivity drop as a function of the thickness is plotted in Fig. 7h. A linear relation between them can be clearly seen. In the experiments, the relation between the reflectivity drop and thickness was caused by many factors including profile of the gradient boundary, flatness of the cured layer and the refractive index of cured resin. The linear relation that was even out of our expectation might not be a standard situation. To explain the linear relation between the reflectivity drop and the thickness, the mathematical relation will be investigated in the future by simulating the curing process and modeling the gradient boundary. Compared with the simulation results, the obvious reflectivity drop on one side of cured resin was not shown in the experiment. This is because the shape of cured resin in the experiment is different from ideal rectangular shape in the simulation, and therefore, multi-interference between the top and bottom boundaries of cured resin was largely weakened.

4.4 In-process Measurement and Imaging Processing

In this experiment, in-process measurement was applied to study the curing process when resins were continuously and discontinuously exposed. Figure 8a shows reflection distribution of resin in an increasing exposure time of 0, 1, 2, 3 and 4 s, respectively. Figure 8b shows a series of reflection distribution of resin cured by discontinuous exposure. In this case, the shutter was closed for 1 s after each time of exposure in a duration of 1 s. The reflection distribution was measured immediately after each exposure. Comparing Fig. 8a and b, even the total exposure time of both was same, resin cured by continuous exposure showed lower reflectivity than discontinuous exposure, which means resin had a larger thickness when it was cured by continuous exposure time. This is an interesting phenomenon and was caused by the difference of average reaction rate. In the continuous exposure, the curing rate of resin keeps on increasing to maximum until all initiators were activated, but in discontinuous exposure, curing rate dropped once illumination was blocked. Therefore, in the same exposure time, resin cured continuously showed relatively larger thickness and led to a lower reflection. This phenomenon also proves the necessity of fast measurement since operation of blocking the exposure and waiting measurement will influence the fabrication process.
Fig. 8

Experiment results using proposed method and imaging processing. Original gray distribution for a continuous exposure and b discontinuous exposure. Processed image for c continuous exposure and d discontinuous exposure

Furthermore, the image processing techniques were applied to remove the uneven distribution of measurement light. It was achieved by the subtraction between the reflection distribution before and after exposure. The imaging filter was adopted after subtraction to remove the background noise. The processed images are shown in Fig. 8c, d, representing the continuous and discontinuous exposure condition corresponding to Fig. 8a, b. In processed image, the variation of shape and reflectivity drop of cured resin in the curing process are more obvious than in the original image; the difference between continuous and discontinuous exposure as we discussed above can also be clearly observed. Obviously, imaging process benefits the in-process measurement at lot; however, the usage of filter results in some loss of image information and impacts the measurement. More detailed research of the types and parameters of image filter will be done in our future work.

Figure 9a, c shows images of cured resin observed by optical microscopy after washing and drying treatment when resin was given 4-s continuous and discontinuous exposure. The profile marked by white lines was measured by AFM. The measurement results are plotted with the effective thickness obtained by in-process measurement, as shown in Fig. 9b, d. The blue point shows the raw data of thickness calculated from the reflectivity after subtraction. The gray line shows the processed date by applying bilateral and median filter to remove noise. Red line shows the thickness of cured resin measured by AFM after washing and drying treatment. Compared with the absolute thickness measured by AFM, effective thicknesses measured by proposed method were close to the absolute thickness, especially in the center of cured resin, where the difference was smaller than 20 nm. However, in the two sides of cured resin, effective thickness was larger and changed more steeply than the absolute thicknesses. The maximum difference between the effective and the absolute thickness reached 150 nm. The reason for this difference might be many factors. Most probably, the steepness of cured resin impacts the relationship between reflectivity and thickness, which results in the linear relation is not applicable for the following experiment. In addition, the washing and drying process influences the absolute thickness largely. To repeat a previous point, the absolute thickness appears after the washing and drying treatment, and therefore, in-process measurement is applied to predict thickness in fabrication process, which means we cannot provide a totally right measurement on thickness but can only give a tendency of thickness. According to above results, the maximum thickness and rough size of cured resin were successfully measured. In our future work, distribution of refractive index and multi-reflection will be introduced into calculation process; the gradient boundary problem will be calculated and simulated. More accuracy measurement of three-dimensional shape of cured resin will be aimed for.
Fig. 9

Image observed by microscopy after washing and drying process for a continuous exposure and c discontinuous exposure. Thickness of cross section measured by proposed method and AFM after washing and drying process for b continuous exposure and d discontinuous exposure

5 Conclusion

In conclusion, we proposed the in-process measurement in evanescent-wave-based nano-stereolithography to monitor curing process and measure effective thickness of cured resin. The measurement method using the refractive index increase of resin and the high sensitivity of total internal reflection when incident angle is at the critical angle was proposed to develop a suitable in-process measurement system in nano-stereolithography. The feasibility of proposed measurement method was firstly proved by simulation using the RCWA method. The influence of thickness of cured resin on reflectivity was confirmed. The experiment system including both evanescent wave curing and measurement sections was built to experimentally examine our method. There experiments were done using the developed system. In the first experiment, by measuring a bulk sample, the variation of resin’s refractive index and the reliability of experiment was confirmed. In the second experiment, relationship between reflectivity drop and effective thickness was measured. The linear relationship was experimentally found. In the third experiment, sample in the conditions of continuous and discontinuous exposure was in-process measurement and compared. The image processing that extracts the variation of resin and removes the background noise was demonstrated by this experiment. The difference between continuous and discontinuous exposure was clearly observed. It also effectively identified our method as a powerful way of in-process measurement in nano-stereolithography.

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Copyright information

© International Society for Nanomanufacturing and Tianjin University and Springer Nature 2018

Authors and Affiliations

  • Deqing Kong
    • 1
  • Masaki Michihata
    • 2
  • Kiyoshi Takamasu
    • 3
  • Satoru Takahashi
    • 2
  1. 1.Department of Advanced Interdisciplinary StudiesThe University of TokyoTokyoJapan
  2. 2.Research Center for Advanced Science and TechnologyThe University of TokyoTokyoJapan
  3. 3.Department of Precision EngineeringThe University of TokyoTokyoJapan

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