Ex) Article Title, Author, Keywords
Current Optics
and Photonics
Ex) Article Title, Author, Keywords
Curr. Opt. Photon. 2023; 7(1): 38-46
Published online February 25, 2023 https://doi.org/10.3807/COPP.2023.7.1.38
Copyright © Optical Society of Korea.
Nayoung Kim1, Do Won Kim2,3, Nam Su Park2,4, Gyeong Hun Kim5, Yang Do Kim6, Chang-Seok Kim1,2,5
Corresponding author: *ckim@pusan.ac.kr, ORCID 0000-0002-2811-8137
†These authors contributed equally to this work.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Optical Fourier-domain reflectometry (OFDR) sensors have been widely used to measure distances with high resolution and speed in a noncontact state. In the electroplating process of a printed circuit board, it is critically important to monitor the copper-plating thickness, as small deviations can lead to defects, such as an open or short circuit. In this paper we employ a phase-based OFDR sensor for in situ relative distance sensing of a sample with nanometer-scale resolution, during electroplating. We also develop an optical-path difference (OPD)-regulated sensing probe that can maintain a preset distance from the sample. This function can markedly facilitate practical measurements in two aspects: Optimal distance setting for high signal-to-noise ratio OFDR sensing, and protection of a fragile probe tip via vertical evasion movement. In a sample with a centimeter-scale structure, a conventional OFDR sensor will probably either bump into the sample or practically out of the detection range of the sensing probe. To address this limitation, a novel OPD-regulated OFDR system is designed by combining the OFDR sensing probe and linear piezo motors with feedback-loop control. By using multiple OFDR sensors, it is possible to effectively monitor copper-plating thickness in situ and uniformize it at various positions.
Keywords: Copper plating thickness uniformization, Electroplating process, In-situ sensing, Optical Fourier-domain reflectometry, Optical-path difference regulation
OCIS codes: (060.2370) Fiber optics sensors; (110.1650) Coherence imaging; (110.4500) Optical coherence tomography; (120.4630) Optical inspection; (280.4788) Optical sensing and sensors
Recently, the development of electronic products has been accelerated for various applications. Accordingly, electronic products are becoming smaller and faster, for better performance. Such advanced electronics require short wiring lengths and narrow line widths in printed circuit boards (PCBs). However, narrow lines and spacing can lead to serious defects, such as open or short circuits from copper-plating deviation [1]. Typically, copper-plating defects are caused by several factors, including electrolytes, plating conditions, external forces, and foreign materials. These types of copper-plating defects can be resolved during the electroplating processing. However, the resulting problems caused by the copper-plating deviation cannot be easily addressed after completion of the electroplating manufacturing process.
Currently, several instruments are used to measure copper-plating thickness, including the X-ray fluorescence spectrometer (XRF), eddy-current coating-thickness tester, electromagnetic coating-thickness tester, coulometric coating-thickness tester, optical microscopy (OM), and scanning electron microscopy (SEM) [2–5]. The principles of XRF are as follows: First, when a copper-containing sample is irradiated by X-rays, it emits its own fluorescent X-rays based on the amount of copper present in the sample. Then the XRF can detect the intensity of the fluorescent X-rays, which determines the plating thickness [2]. However, it is difficult to perform
Therefore, in practical processing applications it is extremely difficult to measure plating thickness via the conventional equipment mentioned above, because of various limitations, including sample contact, sample destruction, expense, and the time-consuming nature of the measurement methods, along with sensing problems. In the practical electroplating process of a PCB, the overall surface is on the order of several square meters. Therefore, multiple platinum-mesh electrodes are required for a rapid and wide electroplating process. For the regulation of plating deposition to satisfy the overall plating-thickness specification,
The OFDR system is similar to optical coherence tomography (OCT) in principle. It uses a wavelength-swept laser to scan the lasing wavelengths repeatedly at regular intervals, leading to optical interference between the partially reflected light from the sample surface and the remaining light passing along a given optical-path difference (OPD) of the interferometer. Given that the information regarding the distance is determined from this interference signal [6–8], the height variation in the sample surface can be directly analyzed at the given directional position. Typically, even tiny OPD can lead to phase differences in optical interference; Hence OFDR is capable of high-resolution distance at nanometer-scale resolution. Additionally, provided that a high-speed measurement of 100 kHz or higher is possible, the plating thickness is measured in real time, in a noncontact and nondestructive manner [9, 10].
Based on these principles, OCT and OFDR systems are widely used in various fields requiring distance measurement [11–14]. In practical applications, these systems are also widely used in biomedicine, including the measurement of corneal or tissue position during surgery [15, 16].
The optical-fiber probes for OFDR or OCT have multiple advantages, including small size, light weight, and high resistance to corrosive chemicals. The flat-tip designs based on the common-path (CP) interferometer feature simple schematics without a separate reference arm, convenient interchangeability, low cost, and robustness to external vibrations [17–20].
However, since the output beam of the flat-tip design has a divergent beam profile, signal loss increases with the distance between the fiber tip and sample. Meanwhile, if the distance between the sample and the fiber tip is too short, the intensity of the light reflected from the sample may exceed the saturation limit of the photodetector. Therefore, an optimal distance between sample and fiber tip must be carefully maintained for the maximum signal-to-noise ratio (SNR).
In this study, we demonstrate two OPD-regulated OFDR probes that can move vertically and regulate the preset distance from the sample via feedback control, by combining a linear piezo motor and an OFDR system. We implement the OPD-regulated system on each probe for two reasons: Optimal distance setting before the plating-thickness measurement, and optical-fiber protection to avoid bumping into the sample during motion. The proposed OPD-regulated sensing module enables accurate and repeatable distance regulation.
Furthermore, the OPD-regulated OFDR probe can prevent damage to the fragile fiber tip or the sample due to their unexpected contact during probe movement. The wide stroke of the piezo linear motor (30 mm) allows centimeter-range vertical movement with high speed and precision.
We also propose a real-time plating-thickness monitoring system with nanometer resolution based on the phase variation of the interference signal. We successfully demonstrate real-time uniformization of the plating thickness during electroplating. The copper-plating deviation was controlled by comparing the plating thicknesses at multiple positions on the sample.
Figure 1 shows the configuration of the proposed OFDR system, with two identical OPD-regulated sensing probes A and B to measure the plating thickness at multiple sample positions. Multiple OFDR sensors must be applied to measure the thickness deviation of electroplating depending on the locations upon a sample. We use two functionally and operationally identical probes in this experiment. The light source is a wavelength-swept laser (HSL-10/20; Santec, Aichi, Japan) with central wavelength of 1,310 nm, a −3 dB bandwidth of 80 nm, a coherence length of up to 18 mm, peak output power of approximately 50 mW, and a sweep rate of 100 kHz. The laser output is divided into the two paths of the sensing probes via a 50:50 1 × 2 optical coupler (OC) (TW1300R5A1; Thorlabs, NJ, USA). Each divided laser beam is guided into the first port of the circulator (CIR) (CIR1310; Thorlabs), with the second port attached to a sensing probe. Using the CP structure, interference is generated between the light reflected from the end-tip surface of the optical fiber (SMF28; Corning, NY, USA) and that reflected from the sample surface by OPD [21, 22]. Without an antireflection coating on the optical fiber, Fresnel reflection (~3.5% at the glass–air interface) occurs from the flat surface of the silica fiber’s end tip, providing the reference light. Each beam reflected from the sample surface and the reference surface creates an interference signal with a frequency proportional to the OPD between the two surfaces. For a single reflector at a distance
where
where
The third port of CIR is connected to a photodetector (PD) (PDB410; Thorlabs), which converts the optical interference from the sensing probe into an electrical signal. When the A-line trigger from the laser is on, a digitizer (ATS9350; Alazar Tech, Quebec, Canada) with a sampling rate of up to 500 MSPS starts acquiring the interferogram signal of a single A-line. For linear response to change in distance, as well as optimal sensitivity, the raw interferogram signal is transformed linearly in the wave-number domain using the interpolation method. Afterward, windowing, zero padding, and fast Fourier transform are performed on the interpolated A-line data. These processes produce complex A-line data (with real and imaginary parts), with information displayed on the frequency spectrum. Finally, the magnitude and phase of the complex data are used to calculate the distance
For OPD regulation, the sensing-probe module consists of an optical fiber integrated with a small rigid tube through the central hole, combined with a driving rod (DR); A linear piezo motor (PM) (LL1011A; Piezo Motor, Uppsala, Sweden) creating the linear movement of the DR, with a resolution of less than 1 nm, maximum speed of 15 mm/s, and stroke length of 30 mm; Along with an external housing to protect the module. The PM is coupled securely inside the housing and fixed to a stable external holder.
According to the measured OPD from the OFDR system, the optical fiber’s position is vertically shifted using the motion controller (PMD101; Piezo Motor) to regulate the distance between the sample surface and the tip of the optical fiber in real time. The magnitude of the position movement is calculated through a feedback-loop system based on optimized proportional-derivative control. For example, if the end of the fiber is closer than the preset distance, the linear PM moves backward. In the opposite case, the motor moves forward to regulate the preset distance, thereby preventing contact with the sample.
Figure 2 shows the schematic of the experiment, and results for the OPD-regulated OFDR system. Figure 2(a) shows the schematic of the OPD-regulation test on a multi-grooved metal sample with 2-mm intervals and a 0.6-mm dip. The OPD-regulated OFDR system is moved horizontally at a speed of 5 mm/s using a translation stage (MLS203; Thorlabs). As shown on the left in Fig. 2(b), when the OPD regulation is off, the distance of the grooved sample from the fiber tip end is calculated in real time for the first second, while the height of the sensing probe remains constant. As shown in Fig. 2(a), the preset distance is automatically maintained during the lateral movement of the sensing head, after initiating OPD-regulation mode. Hence, the fiber tip moves vertically to maintain the relative distance from the sample at 800 μm, which is set for OPD-regulated OFDR operation.
Without OPD regulation, when the sensing probe moves laterally over the electroplated sample of PCB, the OFDR sensing probe can bump the sample surface (as the slope of a flat sample is not easily controlled), thus leading to a potential crash. Conversely, with OPD regulation, the distance between the optical-fiber tip’s end and the sample surface is maintained stably, without the probability of a bump. The OPD-regulated OFDR system can be moved vertically with a minimum resolution of 10 μm at once, with a maximum movement range of 78 mm and a response speed of 1 kHz. Once the system is at a specific position to sense the electroplating thickness, the OPD-regulated OFDR operation must be turned off to measure the small change in plating thickness under stable conditions.
Figure 3 shows a schematic of the
To check the stability of this sensing system, the phase-based distance is measured under zero-potential conditions for 30 min. Figure 4(a) shows the result of
To measure the plating thickness in the electroplating process of a PCB, the growth of copper thickness must depend on the voltage applied to the Pt electrodes. Two OPD-regulated OFDR sensing probes are located opposite each other, 80 mm from the center of a CCL sheet with a length of 220 mm. To verify the plating thickness using cross-sectional SEM imaging after electroplating, an insulating tape that prevents further electroplating is attached next to each sensing point.
For the two experiments with different values of the output voltage of power supply A (
Figures 5(a) and 5(c) show the
Similarly, as shown in Fig. 5(c), under
The second experiment involves uniformizing the copper-plating thickness by monitoring two different positions in real time using OFDR sensing probes A and B, and controlling via proportional voltage tuning of the two power supplies. In this experiment, the voltage condition of power supply A is initially set to a fixed value in the potential mode. Then power supply B is simultaneously used to uniformize the plating thickness, by turning it on or off based on the
Figure 6 shows the results of enforcing plating-thickness uniformity during electroplating for 15 min with two OFDR-sensing probes on site. Initially
In this study, we have proposed an OPD-regulated OFDR sensor system to measure plating thickness in real time, and to implement uniform plating thickness across multiple positions. To avoid a collision between the OFDR-sensing probe and the CCL-sheet sample’s surface, and to regulate the optical distance for high-SNR phase-based distance sensing, the OFDR sensor was combined with feedback motion control via a linear PM. A high response frequency of 1 kHz and stroke length of 30 mm enabled the proposed sensing probe to move without collision, even on a centimeter-scale surface structure. During the copper-plating-thickness measurement via two OFDR sensing probes, the
Our OFDR sensor is based on a CP interferometer and a flat sensing tip; thus, the SNR is relatively low, compared to that of a conventional swept-source OCT. However, to improve the SNR a balanced detector can be used to suppress the DC envelope of an interferogram, by using a small portion of laser output through a fiber coupler. This method allows the digitizer to use the full-scale range (positive and negative voltages), thereby increasing the voltage resolution. Additionally, since most samples in our application will be metals and the sensing beam is divergent, the optical-power limit for the sample is not considered. When additional sensing probes or a higher SNR is needed, the output power of the laser can be amplified using a boosted optical amplifier [29].
Monitoring the phase-based distances at different sample positions in real time during electroplating makes it possible to successfully measure the plating thickness
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.
2-Year Research Grant of Pusan National University.
Curr. Opt. Photon. 2023; 7(1): 38-46
Published online February 25, 2023 https://doi.org/10.3807/COPP.2023.7.1.38
Copyright © Optical Society of Korea.
Nayoung Kim1, Do Won Kim2,3, Nam Su Park2,4, Gyeong Hun Kim5, Yang Do Kim6, Chang-Seok Kim1,2,5
1Department of Cogno-Mechatronics Engineering, Pusan National University, Busan 46241, Korea
2Department of Advanced Circuit Interconnection, Pusan National University, Busan 46241, Korea
3Department of Advance Package Business Team, Samsung Electronics, Suwon 16674, Korea
4Department of Electronic Component Design TF, Samsung Electro-Mechanics, Suwon 16674, Korea
5Engineering Research Center for Color-Modulated Extra-Sensory Perception Technology, Pusan National University, Busan 46241, Korea
6School of Materials Science and Engineering, Pusan National University, Busan 46241, Korea
Correspondence to:*ckim@pusan.ac.kr, ORCID 0000-0002-2811-8137
†These authors contributed equally to this work.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Optical Fourier-domain reflectometry (OFDR) sensors have been widely used to measure distances with high resolution and speed in a noncontact state. In the electroplating process of a printed circuit board, it is critically important to monitor the copper-plating thickness, as small deviations can lead to defects, such as an open or short circuit. In this paper we employ a phase-based OFDR sensor for in situ relative distance sensing of a sample with nanometer-scale resolution, during electroplating. We also develop an optical-path difference (OPD)-regulated sensing probe that can maintain a preset distance from the sample. This function can markedly facilitate practical measurements in two aspects: Optimal distance setting for high signal-to-noise ratio OFDR sensing, and protection of a fragile probe tip via vertical evasion movement. In a sample with a centimeter-scale structure, a conventional OFDR sensor will probably either bump into the sample or practically out of the detection range of the sensing probe. To address this limitation, a novel OPD-regulated OFDR system is designed by combining the OFDR sensing probe and linear piezo motors with feedback-loop control. By using multiple OFDR sensors, it is possible to effectively monitor copper-plating thickness in situ and uniformize it at various positions.
Keywords: Copper plating thickness uniformization, Electroplating process, In-situ sensing, Optical Fourier-domain reflectometry, Optical-path difference regulation
Recently, the development of electronic products has been accelerated for various applications. Accordingly, electronic products are becoming smaller and faster, for better performance. Such advanced electronics require short wiring lengths and narrow line widths in printed circuit boards (PCBs). However, narrow lines and spacing can lead to serious defects, such as open or short circuits from copper-plating deviation [1]. Typically, copper-plating defects are caused by several factors, including electrolytes, plating conditions, external forces, and foreign materials. These types of copper-plating defects can be resolved during the electroplating processing. However, the resulting problems caused by the copper-plating deviation cannot be easily addressed after completion of the electroplating manufacturing process.
Currently, several instruments are used to measure copper-plating thickness, including the X-ray fluorescence spectrometer (XRF), eddy-current coating-thickness tester, electromagnetic coating-thickness tester, coulometric coating-thickness tester, optical microscopy (OM), and scanning electron microscopy (SEM) [2–5]. The principles of XRF are as follows: First, when a copper-containing sample is irradiated by X-rays, it emits its own fluorescent X-rays based on the amount of copper present in the sample. Then the XRF can detect the intensity of the fluorescent X-rays, which determines the plating thickness [2]. However, it is difficult to perform
Therefore, in practical processing applications it is extremely difficult to measure plating thickness via the conventional equipment mentioned above, because of various limitations, including sample contact, sample destruction, expense, and the time-consuming nature of the measurement methods, along with sensing problems. In the practical electroplating process of a PCB, the overall surface is on the order of several square meters. Therefore, multiple platinum-mesh electrodes are required for a rapid and wide electroplating process. For the regulation of plating deposition to satisfy the overall plating-thickness specification,
The OFDR system is similar to optical coherence tomography (OCT) in principle. It uses a wavelength-swept laser to scan the lasing wavelengths repeatedly at regular intervals, leading to optical interference between the partially reflected light from the sample surface and the remaining light passing along a given optical-path difference (OPD) of the interferometer. Given that the information regarding the distance is determined from this interference signal [6–8], the height variation in the sample surface can be directly analyzed at the given directional position. Typically, even tiny OPD can lead to phase differences in optical interference; Hence OFDR is capable of high-resolution distance at nanometer-scale resolution. Additionally, provided that a high-speed measurement of 100 kHz or higher is possible, the plating thickness is measured in real time, in a noncontact and nondestructive manner [9, 10].
Based on these principles, OCT and OFDR systems are widely used in various fields requiring distance measurement [11–14]. In practical applications, these systems are also widely used in biomedicine, including the measurement of corneal or tissue position during surgery [15, 16].
The optical-fiber probes for OFDR or OCT have multiple advantages, including small size, light weight, and high resistance to corrosive chemicals. The flat-tip designs based on the common-path (CP) interferometer feature simple schematics without a separate reference arm, convenient interchangeability, low cost, and robustness to external vibrations [17–20].
However, since the output beam of the flat-tip design has a divergent beam profile, signal loss increases with the distance between the fiber tip and sample. Meanwhile, if the distance between the sample and the fiber tip is too short, the intensity of the light reflected from the sample may exceed the saturation limit of the photodetector. Therefore, an optimal distance between sample and fiber tip must be carefully maintained for the maximum signal-to-noise ratio (SNR).
In this study, we demonstrate two OPD-regulated OFDR probes that can move vertically and regulate the preset distance from the sample via feedback control, by combining a linear piezo motor and an OFDR system. We implement the OPD-regulated system on each probe for two reasons: Optimal distance setting before the plating-thickness measurement, and optical-fiber protection to avoid bumping into the sample during motion. The proposed OPD-regulated sensing module enables accurate and repeatable distance regulation.
Furthermore, the OPD-regulated OFDR probe can prevent damage to the fragile fiber tip or the sample due to their unexpected contact during probe movement. The wide stroke of the piezo linear motor (30 mm) allows centimeter-range vertical movement with high speed and precision.
We also propose a real-time plating-thickness monitoring system with nanometer resolution based on the phase variation of the interference signal. We successfully demonstrate real-time uniformization of the plating thickness during electroplating. The copper-plating deviation was controlled by comparing the plating thicknesses at multiple positions on the sample.
Figure 1 shows the configuration of the proposed OFDR system, with two identical OPD-regulated sensing probes A and B to measure the plating thickness at multiple sample positions. Multiple OFDR sensors must be applied to measure the thickness deviation of electroplating depending on the locations upon a sample. We use two functionally and operationally identical probes in this experiment. The light source is a wavelength-swept laser (HSL-10/20; Santec, Aichi, Japan) with central wavelength of 1,310 nm, a −3 dB bandwidth of 80 nm, a coherence length of up to 18 mm, peak output power of approximately 50 mW, and a sweep rate of 100 kHz. The laser output is divided into the two paths of the sensing probes via a 50:50 1 × 2 optical coupler (OC) (TW1300R5A1; Thorlabs, NJ, USA). Each divided laser beam is guided into the first port of the circulator (CIR) (CIR1310; Thorlabs), with the second port attached to a sensing probe. Using the CP structure, interference is generated between the light reflected from the end-tip surface of the optical fiber (SMF28; Corning, NY, USA) and that reflected from the sample surface by OPD [21, 22]. Without an antireflection coating on the optical fiber, Fresnel reflection (~3.5% at the glass–air interface) occurs from the flat surface of the silica fiber’s end tip, providing the reference light. Each beam reflected from the sample surface and the reference surface creates an interference signal with a frequency proportional to the OPD between the two surfaces. For a single reflector at a distance
where
where
The third port of CIR is connected to a photodetector (PD) (PDB410; Thorlabs), which converts the optical interference from the sensing probe into an electrical signal. When the A-line trigger from the laser is on, a digitizer (ATS9350; Alazar Tech, Quebec, Canada) with a sampling rate of up to 500 MSPS starts acquiring the interferogram signal of a single A-line. For linear response to change in distance, as well as optimal sensitivity, the raw interferogram signal is transformed linearly in the wave-number domain using the interpolation method. Afterward, windowing, zero padding, and fast Fourier transform are performed on the interpolated A-line data. These processes produce complex A-line data (with real and imaginary parts), with information displayed on the frequency spectrum. Finally, the magnitude and phase of the complex data are used to calculate the distance
For OPD regulation, the sensing-probe module consists of an optical fiber integrated with a small rigid tube through the central hole, combined with a driving rod (DR); A linear piezo motor (PM) (LL1011A; Piezo Motor, Uppsala, Sweden) creating the linear movement of the DR, with a resolution of less than 1 nm, maximum speed of 15 mm/s, and stroke length of 30 mm; Along with an external housing to protect the module. The PM is coupled securely inside the housing and fixed to a stable external holder.
According to the measured OPD from the OFDR system, the optical fiber’s position is vertically shifted using the motion controller (PMD101; Piezo Motor) to regulate the distance between the sample surface and the tip of the optical fiber in real time. The magnitude of the position movement is calculated through a feedback-loop system based on optimized proportional-derivative control. For example, if the end of the fiber is closer than the preset distance, the linear PM moves backward. In the opposite case, the motor moves forward to regulate the preset distance, thereby preventing contact with the sample.
Figure 2 shows the schematic of the experiment, and results for the OPD-regulated OFDR system. Figure 2(a) shows the schematic of the OPD-regulation test on a multi-grooved metal sample with 2-mm intervals and a 0.6-mm dip. The OPD-regulated OFDR system is moved horizontally at a speed of 5 mm/s using a translation stage (MLS203; Thorlabs). As shown on the left in Fig. 2(b), when the OPD regulation is off, the distance of the grooved sample from the fiber tip end is calculated in real time for the first second, while the height of the sensing probe remains constant. As shown in Fig. 2(a), the preset distance is automatically maintained during the lateral movement of the sensing head, after initiating OPD-regulation mode. Hence, the fiber tip moves vertically to maintain the relative distance from the sample at 800 μm, which is set for OPD-regulated OFDR operation.
Without OPD regulation, when the sensing probe moves laterally over the electroplated sample of PCB, the OFDR sensing probe can bump the sample surface (as the slope of a flat sample is not easily controlled), thus leading to a potential crash. Conversely, with OPD regulation, the distance between the optical-fiber tip’s end and the sample surface is maintained stably, without the probability of a bump. The OPD-regulated OFDR system can be moved vertically with a minimum resolution of 10 μm at once, with a maximum movement range of 78 mm and a response speed of 1 kHz. Once the system is at a specific position to sense the electroplating thickness, the OPD-regulated OFDR operation must be turned off to measure the small change in plating thickness under stable conditions.
Figure 3 shows a schematic of the
To check the stability of this sensing system, the phase-based distance is measured under zero-potential conditions for 30 min. Figure 4(a) shows the result of
To measure the plating thickness in the electroplating process of a PCB, the growth of copper thickness must depend on the voltage applied to the Pt electrodes. Two OPD-regulated OFDR sensing probes are located opposite each other, 80 mm from the center of a CCL sheet with a length of 220 mm. To verify the plating thickness using cross-sectional SEM imaging after electroplating, an insulating tape that prevents further electroplating is attached next to each sensing point.
For the two experiments with different values of the output voltage of power supply A (
Figures 5(a) and 5(c) show the
Similarly, as shown in Fig. 5(c), under
The second experiment involves uniformizing the copper-plating thickness by monitoring two different positions in real time using OFDR sensing probes A and B, and controlling via proportional voltage tuning of the two power supplies. In this experiment, the voltage condition of power supply A is initially set to a fixed value in the potential mode. Then power supply B is simultaneously used to uniformize the plating thickness, by turning it on or off based on the
Figure 6 shows the results of enforcing plating-thickness uniformity during electroplating for 15 min with two OFDR-sensing probes on site. Initially
In this study, we have proposed an OPD-regulated OFDR sensor system to measure plating thickness in real time, and to implement uniform plating thickness across multiple positions. To avoid a collision between the OFDR-sensing probe and the CCL-sheet sample’s surface, and to regulate the optical distance for high-SNR phase-based distance sensing, the OFDR sensor was combined with feedback motion control via a linear PM. A high response frequency of 1 kHz and stroke length of 30 mm enabled the proposed sensing probe to move without collision, even on a centimeter-scale surface structure. During the copper-plating-thickness measurement via two OFDR sensing probes, the
Our OFDR sensor is based on a CP interferometer and a flat sensing tip; thus, the SNR is relatively low, compared to that of a conventional swept-source OCT. However, to improve the SNR a balanced detector can be used to suppress the DC envelope of an interferogram, by using a small portion of laser output through a fiber coupler. This method allows the digitizer to use the full-scale range (positive and negative voltages), thereby increasing the voltage resolution. Additionally, since most samples in our application will be metals and the sensing beam is divergent, the optical-power limit for the sample is not considered. When additional sensing probes or a higher SNR is needed, the output power of the laser can be amplified using a boosted optical amplifier [29].
Monitoring the phase-based distances at different sample positions in real time during electroplating makes it possible to successfully measure the plating thickness
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.
2-Year Research Grant of Pusan National University.