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Design of thermal imprinting system with uniform residual thickness
Microelectronic Engineering 86 (2009) 2222–2227
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Design of thermal imprinting system with uniform residual thickness Won-Ho Shin Factory Automation, Manufacturing Technical Research Center, Samsung Electro-Mechanics, 314, Maetan-Dong, Yeongtong-Gu, Suwon, Gyunggi-Do 443-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 December 2008 Accepted 6 March 2009 Available online 16 March 2009 Keywords: Thermal imprinting system Patterned circuit boards Uniformity of residual thickness
a b s t r a c t A new thermal imprinting system for the printed circuit boards (PCBs) with both large areas and fine conducting lines was developed adopting hot airs with a high pressure. Several small nickel stamps were used to cover the large area, and the stamps were replicated from an electroforming process, in addition, a vacuum jig was utilized to avoid bubbles captured in resins or imprinted interfaces. Stefan’s equation was used to estimate residual thicknesses of the imprinted resins, and effects of imprinting conditions on the residual thickness were investigated from numerical analyses to confirm process profiles and specifications of the developed equipment. The results show that the developed imprinting system can remarkably improve the uniformity of the residual thickness after imprinting, as compared with those of the conventional press, in spite of the thickness difference between the used stamps. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Finer become the surface-mount packaging used for integrated circuits (IC) such as ball grid array (BGAs), land grid array (LGAs) and pin grid arrays (PGAs) in accordance with the development of IC chips. The package substrates with 14/14 lm of line/space have been already commercialized, and the demands on less than 10/10 lm may increase extremely within 5 years. The current process for making copper patterns of the printed circuit boards (PCBs) is a conventional photo-lithography with subsequent the copper plating and the chemical wet etching. However, the conventional photo-lithography process is insufficient for the fine patterns less than 10/10 lm in as much as the undercut failures induced by the poor adhesion between a dielectric substrate and a plated copper layer. Hence, imperative is the new alternative lithography for preventing the undercut failures and forming such the fine patterns. The imprint lithography may be one of the most prospective candidates for the substitute lithography since the abilities to make the fine patterns below micron have been already proved in the semiconductor industries for a decade. Additionally, the patterns embedded in the dielectric substrate are not only preserved from the undercut failure. The imprint process of PCBs is different from that of the semiconductors in the view of the several points. First, the imprinted recesses are not adopted as an etching resist for reactive ion etching process but as the traces filling the conductive material and the dielectrics between patterned layers which consist of a multi-layer board (MLB). And then the only thermosetting resins can be used as an imprinted layer for enduring the copper plating process un-
E-mail address: [email protected] 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.03.077
der the high temperature conditions. Finally, necessary is a large stamp with 510 405 mm2 area which is selected for mass-production of PCBs. However, it is impossible to make such a huge stamp with fine patterns less than 10/10 lm, and difficult to apply the step and repeat method due to the long cycle time of the hot embossing and the uneven residues among steps. Generally, nickel is adopted for the daughter stamp of the thermal imprint due to its durability and its thermal resistance. After the seed layer is deposited on the master using both evaporation and sputtering, the replication of the stamp is implemented by nickel electroforming [1]. Since the replication of the nickel stamp is performed by electroforming, the roughness of the patterned surface is the similar level to the glass substrate, but the other surface has a poor surface roughness due to unstrained boundaries as shown in Fig. 1, in addition, the maximum thickness of the nickel stamp is limited up to the vicinity of 300 lm in as much as the electro-forming method, and then the global warpage of the nickel stamp may occur frequently due to adhesive forces between the resins and nickel stamp after de-molding process. These negative factors aggravate the conformal contact between the patterned surface of the stamp and the resins when a conventional press is adopted for the imprinting process. Consequently, it deteriorates both the fidelity of the transferred patterns and the uniformity of the residues. Additionally, the photo-resist mold inclines to be destructed after the replication because of its brittleness as well as its adhesion to nickel, and the disposable mold can be used to make only one nickel stamp. Since the several stamps are required to imprint simultaneously the large area, the distributions among the duplicated stamps must be solved for high fidelity pattern transferring. The roughness and the global warpage in a stamp can be solved by using the cushion pad [2], the roughness differences among stamps is much bigger than the allowable ability of the
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W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
Fig. 1. Electro-forming procedures for the replication of the nickel stamp.
viscosity of GX13
10000
7500
Viscosity of GX-13 (cP)
Minimum Viscosity (cP)
8000
5000
2500
6000
4000 o
Tenv=90 C o
2000
Tenv=110 C o
Tenv=130 C o
Tenv=140 C
0
0 90
100
110
120
130
0
140
200
400
600
800
1000
1200
Exposed Time (s)
o
Temperature ( C)
(b) viscosity histories
(a) minimum viscosity
Fig. 2. Viscosity histories of GX-13 according to the environmental temperature.
cushion pad. Hence, the new imprinting system becomes necessary to overcome these problems. In present study, the imprinting system for a large area with 10/ 10 lm of line/space is developed using the several 160 130 mm2 nickel stamps. The nickel stamps were duplicated by using the patterned photo-resist on the glass substrate which is made by a photo-lithography using an ultraviolet radiation. Additionally, the silane based the self assembled monolayer (SAM) coating was implemented on the surface of the nickel stamp for the easy detachment after imprinting.
where g, pðtÞ, h0 , t are the viscosity of the polymer as set by the process temperature, the pressure, and the initial laminated polymer layer and the imprint time, respectively; additionally, seff and hðt; TÞ are the effective length used for the imprinting process and the thickness of the polymer after imprinting. The GX-13 build-up film (Ajinomoto Co.) is adopted as the resin for imprinting analyses and its glass transition temperature (Tg) is distributed from 90 to 140 °C [5]. Additionally, its viscosity varies in accordance with the operating temperature and the exposed time as portrayed in Fig. 2. Though the higher pressure and the lower viscosity makes
2. Design of imprint systems
Temperature profiles Pressure profiles
2.1. Specifications of the imprint system
t¼
gðt; TÞs2eff ðtÞ 2pðtÞ
(
1 2
h ðt; TÞ
1 2
h0
) ð1Þ
Curing Pimprint
Timprint
Pressure (MPa)
o
Adopted is the autoclave using a hot air with a high pressure for the delivery of a uniform pressing force instead of a conventional hydraulic press. The fidelity of the imprinted patterns is mainly decided by the uniformity of pressing and heating. Hence, the time profiles of the imprinting temperature and pressure are the significant factors for imprint process. The imprinting process is considered as the squeezed flow of a molten resin layer, and the required time of the imprinting is obtained from Stefan-equation [3] under the assumption of an incompressible Newtonian fluid of a given viscosity and a lateral geometry [4]
Temperature ( C)
Tcure
Imprinting
Troom
Proom
Time (min) Fig. 3. Temperature and pressure profiles of the imprinting process.
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W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
the filling time shorten, the viscosity is the property of the material’s own, and it is limited to enforce the pressure by the pressing apparatus. Hence, it is crucial to design the proper operating temperature, pressure and time for the efficient imprinting system as shown in Fig. 3. The imprint system can increase the temperature of the resins up to 180 °C for the thermal curing of GX-13, and its chamber is able to endure the high internal pressure above 10 bar for the quick imprint. Fig. 4 evinces the cross section of the nickel stamp used in the imprint process, and it has two-level structures to simultaneously imprint the patterns and via hole, in addition, the two-level structure can be made through the multi-step exposure double development (MEDD) process [6]. The role of via hole is to electrically connect the imprinted pattern to the lower conductive layer, and a thickness of a residual layer is zero. The residual lay, however, is inevitable from Eq. 1 though the imprint is implemented
in the extremely high pressure and temperature. Therefore, the post process (desmear) for the removal of the residue is required, and the remains up to 3 lm thickness can be swept by using a soft etching through the post process. The thickness of the residual layer during the imprinting process can be estimated by the numerical methods, and the nonlinear ordinary difference equation is derived from Eq. 1.
( hiþ1 ¼
1
2pðt i ÞDt þ 2 2 hi gðT; t i Þseff ðt i Þ
)1=2 ð2Þ
Fig. 5 indicates the estimated thickness of the residual resins according to the various imprinting temperature and the thicknesses of build-up films where the imprinting area, the imprinting pressure, and the settling time of the pressure are 160 130 mm2, 2 MPa and 5 min, respectively. Fig. 5a denotes the estimated re-
Fig. 4. Cross-sections of the nickel stamp.
40
40
tbuild-up=37µm o
tbuild-up=25 µm o
Timprint = 90 C
35
Timprint = 90 C
35
o
Timprint = 110 C
30
o
Timprint = 130 C o
25
Timprint = 140 C
20
Complete Filling
15 10
Timprint = 110 C
30
o
Timprint = 130 C o
25
Timprint = 140 C
20 15 10
Complete Filling 5
5 0
Residual Thickness (µm)
Residual Thickness (µm)
o
Permitted Residue 0
200
0
400
600
800
Imprinting Time (sec)
(a) 37 µm build-up film
1000
1200
Permitted Residue 0
200
400
600
800
1000
Imprinting Time (sec)
(b) 25 µm build-up film
Fig. 5. Histories of the residual thicknesses according to the imprinting temperature during the imprinting procedure.
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W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
vacuum fitting valve
filing time is required to reduce the residual thickness up to the allowed level (<3 lm), and the shortest time for the imprinting process is 328 s at Timp = 140 °C. In the case of using the 25 lm film, Fig. 5b evinces that it takes a longer time than that of the previous cases to flawlessly fulfill the patterns, and the rapidest filling time is 104 s at Timp = 130 °C. In contrast, the pressing time which it takes to diminish the residue up to the permitted thickness is just 299 s at Timp = 140 °C. These results reveal that the thickness of the build-up film is able to significantly affect the filling time and the total imprinting time. Therefore, the thick resins may be suggested for the fast filling, and the thin film can be a good choice to reduce the total imprinting time. Additionally, the effect of the resin recovery must be also taken into account after the de-molding process for the accurate estimations. However, adapted was the PMMA polymer for the resist which has the higher viscosity (>105 Pa s at T = 180 °C) than that of GX-13 resins, the amount of the resin recovery was negligibly small above 1600N imprinting force [7]. In this study, the recoveries of the resins were not considered since the imprint was implemented for the more fluid resins with the higher imprinting force.
pin hole
thermocouple
sealant
gasket
knob
cover soft silicon pad
aluminum
body High Pressure & Temperature Air nickel stamp resin
cushion pad
2.2. Design of imprinting equipment
substrate
Generally, the imprinting process is performed in vacuum condition to improve the squeeze of resins and to prevent capturing of void. As opposed to the conventional hydraulic press, the pneumatic press cannot be used directly used in the vacuum condition since the imprinting force is implemented by the air pressure. Hence, the specially designed imprint jig, as depicted in Fig. 6, is necessary to imprint in vacuum using the pneumatic press. The imprinting jig designed for the pneumatic imprinting system consists of a body and a cover, and its body is primarily made of an aluminum alloy. Canals and pin holes are machined for the formation
Vacuum line
Fig. 6. Designed jigs for the imprint process in the vacuum environment.
sults of the residual thickness when using the build-up film with 37 lm thickness and it elucidates that it takes the imprinting time less than a 100 s to completely fill the patterns for all of the operating temperature, and the fastest filling time is just 26 s at Timp = 130 °C. However, the operating time much longer than the
air cooler
fin heater
duct
motor cooler
sirocco fan motor
Air Circulation
samples
jig
tray
insulation
frame
(a) the schematic design of the pneumatic imprint system insulation
duct tray
Front
Rear
frame
(b) the machined pressure chamber and the air tank Fig. 7. Pressure chambers with heating and circulating systems for imprint.
Air Tank
W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
Pressure (Pa)
Set Pressure
6
1.5x10
Quarterly Opened Valve Pressure of Chamber 6
1.0x10
Pressure of Air Tank
Half Opened Valve Pressure of Chamber Pressure of Air Tank
5
5.0x10
Fully Opened Valve Pressure of Chamber Pressure of Air Tank
0.0 0
2
4
6
8
10
12
14
16
18
20
Time (minute) Fig. 8. Pressure profiles of the imprinting chamber and the air tank according to the control of the pressure switch.
of the vacuum inside the imprint jig, and the thermocouple is embedded to the body to measure the temperature of the imprinted sample. The cover is composed with window frames made of an aluminum alloy and a soft silicon pad, and it is assembled into the body by the eight bolts with gaskets. Additionally, the rubber ring is inserted into the interface of two parts to prevent the air inflow from chamber. The imprinted samples are placed between the body and the cover, and the designed jig is able to accommodate three samples with 160 130 mm2 simultaneously. The prepared jig is inserted into the pressure chamber with the hot air for the imprint process, and the bulk of the pressure chamber significantly affects the manufacturing throughput as well as the settling time of the controlled temperature and pressure. The larger the selected chamber is, the more the products are able to imprint at once. However, the expansion of the chamber volume consequently induces the increment of compressor- and heatercapabilities required for the set profile. As shown in Fig. 7, the pressure chamber is designed as a cylindrical shell type, and its inner diameter and length are 600 and 1200 mm able to synchronously treat 10 jigs, respectively. Additionally, the 15-kW air compressor is selected to increase the internal pressure of the chamber. Though the adopted compressor is able to support the 3-MPa pressure, it is impossible for internal pressure to attain to 2-MPa within 5 min by itself. To reduce the settling time of the pressure, the air tank with 2000L is added between the compressor and the pressure chamber, and the tank was fulfilled with 3-MPa air by using the compressor before performing the imprinting process. Fig. 8 portrays the pressure profiles of the imprinting chamber and the air tank according to the opened area of the control valve, and the chamber is pressurized by the pressure difference between
3. Results and discussion The imprinting processes were performed using the conventional hydraulic press as well as the developed imprinting system to compare the uniformity of the residual resins in the same imprinting condition where the imprinting temperature, the imprinting pressure, the curing temperature and the curing pressure adopted in the present study were 80 °C, 0.6 MPa, 180 °C and 0.6 MPa, respectively. Additionally, the imprinting time and curing time were 20 and 60 min.
70
250
Thickness of GX-13
1.2
60 200 50
0.9
0.6
150
40 30
100
0.3
20
Set Temperature Set Pressure 10 Measured Temperature Measured Pressure
50
0
Thickness ( µm)
6
2.0x10
o
6
2.5x10
the chamber and the tank, in addition, the pressures of the air tank and the chamber were measured by using the pressure gauges. The pressure of the chamber increases almost linearly up to 2-MPa pressure, and then converges to an asymptotic line with 2.15MPa for all experimental tests. The minimum settling time is 237 s when the control valve is fully opened, and the carbon steel with 10 mm-thickness is used for the external shell of the chamber to endure the imprinting pressure above 2 MPa. Thirty-kW fin heaters are adopted to heat the internal air up to 180 °C with 5 °C/min, and the powers of heaters are controlled by silicon controlled rectifiers (SCRs), in addition, the Pt100-ohm is installed to measure the internal temperature of the chamber which is used in feedback control of the chamber temperature. The heated air is transferred from the heaters to the imprinted samples by a centrifugal fan with 75-kW inverter motor, and the chiller is set up to prevent the overheating of the fan motor as well as to curtail the cooling time for the de-molding process after imprinting as denoted in Fig. 7a. Additionally, the chamber is insulated to maintain its external temperature below 40 °C by the rock wool with 60-mm thickness for the safety of operators.
Temperature ( C)
6
3.0x10
Pressure (MPa)
2226
0 0
50
0 100
Time (minute) Fig. 10. Temperature and pressure profiles of the imprint process according to time.
Fig. 9. Experimental setup for the imprinting process.
W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
2227
Fig. 11. Imprinted results of the conventional hydraulic press and the developed imprinting system.
Fig. 9 depicts the experimental setup prior to the imprinting process using the developed system. Two imprinted sets which consist of nickel stamps, resins and substrates are inserted into a same vacuum jig as shown in Fig. 6. There are thickness differences between the used nickel stamps with 160 130 mm2-area of two imprinted sets to evaluate the conformal contact between stamps and resins according to the flatness of operation surfaces. The nickel stamp with 400 lm-thickness is used in set A, and the other with 200 lm-thickness is adopted in set B. GX-13 films were adopted as resin layers, and its area and thickness are 160 130 mm2 and 60 lm. The thick films were selected to remain the instead of 37 lm-thickness films to emphasis the difference between the uniform pressing performances of two imprinting systems, in addition, the copper clad layers were used as substrates. Fig. 10 illustrates the process profiles and the measured profiles of the developed imprinting system in this study, and it elucidates that the measured temperature and pressure of the imprinting system have good agreements to the set conditions except the temperature overshooting of the early curing period. Additionally, the thickness of the resin layer according to the imprinting time can be estimated at the given profiles as mentioned in the previous section, and the calculated residual thickness is 21.48 lm as portrayed in Fig. 10. Fig. 11 denotes that the imprinted patterns from the hydraulic press and the developed system in the present study, and the desmear and the chemical copper plating processes for the imprinted trenches were implemented to easily observe the cross sections of imprinting results using microscopes. Fig. 11a indicates the imprinted results using the conventional press, and the average thickness of residual layers is 8.71 lm in the case using the nickel stamp with 400 lm-thickness, in addition, that of the stamp with 200 lm-thickness is 49.4 lm. There are remarkable differences between the residual thicknesses of two cases due to the thickness discrepancy of the used stamps though the imprinting process was performed in the same vacuum jig and the same operating condition. Additionally, the residual thicknesses of the experimental results quite differ from the estimate thickness. Fig. 11b depicts
the imprinted results using the developed imprinting system, and the average residue of the stamp with 400 lm-thickness is 21.33 lm. The average residue of the resins with the 200 lmthickness stamp is 21.77 lm, and it has a similar to that of the stamp with 400 lm-thickness as opposed to the imprinted results using the conventional press. Additionally, those residual thicknesses also agree with the simulated result. These results evince that the developed imprinting equipment adopting the pneumatic system can significantly improve the uniform residue, and it is able to maintain the conformal contact between the stamps and the resins up to 200 lm-thickness difference. 4. Conclusion In present study, developed was the imprinting system for the large area using the several small stamps with 160 130 mm2. A hot air with a high pressure was adopted to apply the uniform pressing force instead of the conventional hydraulic press, and the vacuum jig was used to prevent the void captured between stamps and resins. The present results indicate that the air pressing system can remarkably improve the residue uniformity of the imprinted product as well as the conformal contact between stamps and resins during imprinting process as compared with those of the conventional press, and they depict the feasibility about the application of the imprint process for the PCB industries where it is difficult to adopt the imprint lithography due to the limited size of stamps. References [1] [2] [3] [4] [5] [6] [7]
L.J. Heyderman, J. Gobrecht, et al., Microelectron. Eng. 57–58 (2001) 357–380. S. Ra et al., Curr. Appl. Phys. 8 (2008) 675–678. J. Stefan, Akad. Wiss. Math. Natur. 69 (1874) 713. N. Bogdanski et al., Microelectron. Eng. 78–79 (2005) 598–604. S. Ra, H. Lee, J. Solid State Phenom. 112–123 (2007) 681–684. J. Yoon et al., SPIE Symp. Micromach. Microfab. 3512 (1998) 358–366. L. Ressier et al., Microelectron. Eng. 71 (2004) 272–276.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Design of thermal imprinting system with uniform residual thickness Won-Ho Shin Factory Automation, Manufacturing Technical Research Center, Samsung Electro-Mechanics, 314, Maetan-Dong, Yeongtong-Gu, Suwon, Gyunggi-Do 443-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 December 2008 Accepted 6 March 2009 Available online 16 March 2009 Keywords: Thermal imprinting system Patterned circuit boards Uniformity of residual thickness
a b s t r a c t A new thermal imprinting system for the printed circuit boards (PCBs) with both large areas and fine conducting lines was developed adopting hot airs with a high pressure. Several small nickel stamps were used to cover the large area, and the stamps were replicated from an electroforming process, in addition, a vacuum jig was utilized to avoid bubbles captured in resins or imprinted interfaces. Stefan’s equation was used to estimate residual thicknesses of the imprinted resins, and effects of imprinting conditions on the residual thickness were investigated from numerical analyses to confirm process profiles and specifications of the developed equipment. The results show that the developed imprinting system can remarkably improve the uniformity of the residual thickness after imprinting, as compared with those of the conventional press, in spite of the thickness difference between the used stamps. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Finer become the surface-mount packaging used for integrated circuits (IC) such as ball grid array (BGAs), land grid array (LGAs) and pin grid arrays (PGAs) in accordance with the development of IC chips. The package substrates with 14/14 lm of line/space have been already commercialized, and the demands on less than 10/10 lm may increase extremely within 5 years. The current process for making copper patterns of the printed circuit boards (PCBs) is a conventional photo-lithography with subsequent the copper plating and the chemical wet etching. However, the conventional photo-lithography process is insufficient for the fine patterns less than 10/10 lm in as much as the undercut failures induced by the poor adhesion between a dielectric substrate and a plated copper layer. Hence, imperative is the new alternative lithography for preventing the undercut failures and forming such the fine patterns. The imprint lithography may be one of the most prospective candidates for the substitute lithography since the abilities to make the fine patterns below micron have been already proved in the semiconductor industries for a decade. Additionally, the patterns embedded in the dielectric substrate are not only preserved from the undercut failure. The imprint process of PCBs is different from that of the semiconductors in the view of the several points. First, the imprinted recesses are not adopted as an etching resist for reactive ion etching process but as the traces filling the conductive material and the dielectrics between patterned layers which consist of a multi-layer board (MLB). And then the only thermosetting resins can be used as an imprinted layer for enduring the copper plating process un-
E-mail address: [email protected] 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.03.077
der the high temperature conditions. Finally, necessary is a large stamp with 510 405 mm2 area which is selected for mass-production of PCBs. However, it is impossible to make such a huge stamp with fine patterns less than 10/10 lm, and difficult to apply the step and repeat method due to the long cycle time of the hot embossing and the uneven residues among steps. Generally, nickel is adopted for the daughter stamp of the thermal imprint due to its durability and its thermal resistance. After the seed layer is deposited on the master using both evaporation and sputtering, the replication of the stamp is implemented by nickel electroforming [1]. Since the replication of the nickel stamp is performed by electroforming, the roughness of the patterned surface is the similar level to the glass substrate, but the other surface has a poor surface roughness due to unstrained boundaries as shown in Fig. 1, in addition, the maximum thickness of the nickel stamp is limited up to the vicinity of 300 lm in as much as the electro-forming method, and then the global warpage of the nickel stamp may occur frequently due to adhesive forces between the resins and nickel stamp after de-molding process. These negative factors aggravate the conformal contact between the patterned surface of the stamp and the resins when a conventional press is adopted for the imprinting process. Consequently, it deteriorates both the fidelity of the transferred patterns and the uniformity of the residues. Additionally, the photo-resist mold inclines to be destructed after the replication because of its brittleness as well as its adhesion to nickel, and the disposable mold can be used to make only one nickel stamp. Since the several stamps are required to imprint simultaneously the large area, the distributions among the duplicated stamps must be solved for high fidelity pattern transferring. The roughness and the global warpage in a stamp can be solved by using the cushion pad [2], the roughness differences among stamps is much bigger than the allowable ability of the
2223
W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
Fig. 1. Electro-forming procedures for the replication of the nickel stamp.
viscosity of GX13
10000
7500
Viscosity of GX-13 (cP)
Minimum Viscosity (cP)
8000
5000
2500
6000
4000 o
Tenv=90 C o
2000
Tenv=110 C o
Tenv=130 C o
Tenv=140 C
0
0 90
100
110
120
130
0
140
200
400
600
800
1000
1200
Exposed Time (s)
o
Temperature ( C)
(b) viscosity histories
(a) minimum viscosity
Fig. 2. Viscosity histories of GX-13 according to the environmental temperature.
cushion pad. Hence, the new imprinting system becomes necessary to overcome these problems. In present study, the imprinting system for a large area with 10/ 10 lm of line/space is developed using the several 160 130 mm2 nickel stamps. The nickel stamps were duplicated by using the patterned photo-resist on the glass substrate which is made by a photo-lithography using an ultraviolet radiation. Additionally, the silane based the self assembled monolayer (SAM) coating was implemented on the surface of the nickel stamp for the easy detachment after imprinting.
where g, pðtÞ, h0 , t are the viscosity of the polymer as set by the process temperature, the pressure, and the initial laminated polymer layer and the imprint time, respectively; additionally, seff and hðt; TÞ are the effective length used for the imprinting process and the thickness of the polymer after imprinting. The GX-13 build-up film (Ajinomoto Co.) is adopted as the resin for imprinting analyses and its glass transition temperature (Tg) is distributed from 90 to 140 °C [5]. Additionally, its viscosity varies in accordance with the operating temperature and the exposed time as portrayed in Fig. 2. Though the higher pressure and the lower viscosity makes
2. Design of imprint systems
Temperature profiles Pressure profiles
2.1. Specifications of the imprint system
t¼
gðt; TÞs2eff ðtÞ 2pðtÞ
(
1 2
h ðt; TÞ
1 2
h0
) ð1Þ
Curing Pimprint
Timprint
Pressure (MPa)
o
Adopted is the autoclave using a hot air with a high pressure for the delivery of a uniform pressing force instead of a conventional hydraulic press. The fidelity of the imprinted patterns is mainly decided by the uniformity of pressing and heating. Hence, the time profiles of the imprinting temperature and pressure are the significant factors for imprint process. The imprinting process is considered as the squeezed flow of a molten resin layer, and the required time of the imprinting is obtained from Stefan-equation [3] under the assumption of an incompressible Newtonian fluid of a given viscosity and a lateral geometry [4]
Temperature ( C)
Tcure
Imprinting
Troom
Proom
Time (min) Fig. 3. Temperature and pressure profiles of the imprinting process.
2224
W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
the filling time shorten, the viscosity is the property of the material’s own, and it is limited to enforce the pressure by the pressing apparatus. Hence, it is crucial to design the proper operating temperature, pressure and time for the efficient imprinting system as shown in Fig. 3. The imprint system can increase the temperature of the resins up to 180 °C for the thermal curing of GX-13, and its chamber is able to endure the high internal pressure above 10 bar for the quick imprint. Fig. 4 evinces the cross section of the nickel stamp used in the imprint process, and it has two-level structures to simultaneously imprint the patterns and via hole, in addition, the two-level structure can be made through the multi-step exposure double development (MEDD) process [6]. The role of via hole is to electrically connect the imprinted pattern to the lower conductive layer, and a thickness of a residual layer is zero. The residual lay, however, is inevitable from Eq. 1 though the imprint is implemented
in the extremely high pressure and temperature. Therefore, the post process (desmear) for the removal of the residue is required, and the remains up to 3 lm thickness can be swept by using a soft etching through the post process. The thickness of the residual layer during the imprinting process can be estimated by the numerical methods, and the nonlinear ordinary difference equation is derived from Eq. 1.
( hiþ1 ¼
1
2pðt i ÞDt þ 2 2 hi gðT; t i Þseff ðt i Þ
)1=2 ð2Þ
Fig. 5 indicates the estimated thickness of the residual resins according to the various imprinting temperature and the thicknesses of build-up films where the imprinting area, the imprinting pressure, and the settling time of the pressure are 160 130 mm2, 2 MPa and 5 min, respectively. Fig. 5a denotes the estimated re-
Fig. 4. Cross-sections of the nickel stamp.
40
40
tbuild-up=37µm o
tbuild-up=25 µm o
Timprint = 90 C
35
Timprint = 90 C
35
o
Timprint = 110 C
30
o
Timprint = 130 C o
25
Timprint = 140 C
20
Complete Filling
15 10
Timprint = 110 C
30
o
Timprint = 130 C o
25
Timprint = 140 C
20 15 10
Complete Filling 5
5 0
Residual Thickness (µm)
Residual Thickness (µm)
o
Permitted Residue 0
200
0
400
600
800
Imprinting Time (sec)
(a) 37 µm build-up film
1000
1200
Permitted Residue 0
200
400
600
800
1000
Imprinting Time (sec)
(b) 25 µm build-up film
Fig. 5. Histories of the residual thicknesses according to the imprinting temperature during the imprinting procedure.
1200
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W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
vacuum fitting valve
filing time is required to reduce the residual thickness up to the allowed level (<3 lm), and the shortest time for the imprinting process is 328 s at Timp = 140 °C. In the case of using the 25 lm film, Fig. 5b evinces that it takes a longer time than that of the previous cases to flawlessly fulfill the patterns, and the rapidest filling time is 104 s at Timp = 130 °C. In contrast, the pressing time which it takes to diminish the residue up to the permitted thickness is just 299 s at Timp = 140 °C. These results reveal that the thickness of the build-up film is able to significantly affect the filling time and the total imprinting time. Therefore, the thick resins may be suggested for the fast filling, and the thin film can be a good choice to reduce the total imprinting time. Additionally, the effect of the resin recovery must be also taken into account after the de-molding process for the accurate estimations. However, adapted was the PMMA polymer for the resist which has the higher viscosity (>105 Pa s at T = 180 °C) than that of GX-13 resins, the amount of the resin recovery was negligibly small above 1600N imprinting force [7]. In this study, the recoveries of the resins were not considered since the imprint was implemented for the more fluid resins with the higher imprinting force.
pin hole
thermocouple
sealant
gasket
knob
cover soft silicon pad
aluminum
body High Pressure & Temperature Air nickel stamp resin
cushion pad
2.2. Design of imprinting equipment
substrate
Generally, the imprinting process is performed in vacuum condition to improve the squeeze of resins and to prevent capturing of void. As opposed to the conventional hydraulic press, the pneumatic press cannot be used directly used in the vacuum condition since the imprinting force is implemented by the air pressure. Hence, the specially designed imprint jig, as depicted in Fig. 6, is necessary to imprint in vacuum using the pneumatic press. The imprinting jig designed for the pneumatic imprinting system consists of a body and a cover, and its body is primarily made of an aluminum alloy. Canals and pin holes are machined for the formation
Vacuum line
Fig. 6. Designed jigs for the imprint process in the vacuum environment.
sults of the residual thickness when using the build-up film with 37 lm thickness and it elucidates that it takes the imprinting time less than a 100 s to completely fill the patterns for all of the operating temperature, and the fastest filling time is just 26 s at Timp = 130 °C. However, the operating time much longer than the
air cooler
fin heater
duct
motor cooler
sirocco fan motor
Air Circulation
samples
jig
tray
insulation
frame
(a) the schematic design of the pneumatic imprint system insulation
duct tray
Front
Rear
frame
(b) the machined pressure chamber and the air tank Fig. 7. Pressure chambers with heating and circulating systems for imprint.
Air Tank
W.-H. Shin / Microelectronic Engineering 86 (2009) 2222–2227
Pressure (Pa)
Set Pressure
6
1.5x10
Quarterly Opened Valve Pressure of Chamber 6
1.0x10
Pressure of Air Tank
Half Opened Valve Pressure of Chamber Pressure of Air Tank
5
5.0x10
Fully Opened Valve Pressure of Chamber Pressure of Air Tank
0.0 0
2
4
6
8
10
12
14
16
18
20
Time (minute) Fig. 8. Pressure profiles of the imprinting chamber and the air tank according to the control of the pressure switch.
of the vacuum inside the imprint jig, and the thermocouple is embedded to the body to measure the temperature of the imprinted sample. The cover is composed with window frames made of an aluminum alloy and a soft silicon pad, and it is assembled into the body by the eight bolts with gaskets. Additionally, the rubber ring is inserted into the interface of two parts to prevent the air inflow from chamber. The imprinted samples are placed between the body and the cover, and the designed jig is able to accommodate three samples with 160 130 mm2 simultaneously. The prepared jig is inserted into the pressure chamber with the hot air for the imprint process, and the bulk of the pressure chamber significantly affects the manufacturing throughput as well as the settling time of the controlled temperature and pressure. The larger the selected chamber is, the more the products are able to imprint at once. However, the expansion of the chamber volume consequently induces the increment of compressor- and heatercapabilities required for the set profile. As shown in Fig. 7, the pressure chamber is designed as a cylindrical shell type, and its inner diameter and length are 600 and 1200 mm able to synchronously treat 10 jigs, respectively. Additionally, the 15-kW air compressor is selected to increase the internal pressure of the chamber. Though the adopted compressor is able to support the 3-MPa pressure, it is impossible for internal pressure to attain to 2-MPa within 5 min by itself. To reduce the settling time of the pressure, the air tank with 2000L is added between the compressor and the pressure chamber, and the tank was fulfilled with 3-MPa air by using the compressor before performing the imprinting process. Fig. 8 portrays the pressure profiles of the imprinting chamber and the air tank according to the opened area of the control valve, and the chamber is pressurized by the pressure difference between
3. Results and discussion The imprinting processes were performed using the conventional hydraulic press as well as the developed imprinting system to compare the uniformity of the residual resins in the same imprinting condition where the imprinting temperature, the imprinting pressure, the curing temperature and the curing pressure adopted in the present study were 80 °C, 0.6 MPa, 180 °C and 0.6 MPa, respectively. Additionally, the imprinting time and curing time were 20 and 60 min.
70
250
Thickness of GX-13
1.2
60 200 50
0.9
0.6
150
40 30
100
0.3
20
Set Temperature Set Pressure 10 Measured Temperature Measured Pressure
50
0
Thickness ( µm)
6
2.0x10
o
6
2.5x10
the chamber and the tank, in addition, the pressures of the air tank and the chamber were measured by using the pressure gauges. The pressure of the chamber increases almost linearly up to 2-MPa pressure, and then converges to an asymptotic line with 2.15MPa for all experimental tests. The minimum settling time is 237 s when the control valve is fully opened, and the carbon steel with 10 mm-thickness is used for the external shell of the chamber to endure the imprinting pressure above 2 MPa. Thirty-kW fin heaters are adopted to heat the internal air up to 180 °C with 5 °C/min, and the powers of heaters are controlled by silicon controlled rectifiers (SCRs), in addition, the Pt100-ohm is installed to measure the internal temperature of the chamber which is used in feedback control of the chamber temperature. The heated air is transferred from the heaters to the imprinted samples by a centrifugal fan with 75-kW inverter motor, and the chiller is set up to prevent the overheating of the fan motor as well as to curtail the cooling time for the de-molding process after imprinting as denoted in Fig. 7a. Additionally, the chamber is insulated to maintain its external temperature below 40 °C by the rock wool with 60-mm thickness for the safety of operators.
Temperature ( C)
6
3.0x10
Pressure (MPa)
2226
0 0
50
0 100
Time (minute) Fig. 10. Temperature and pressure profiles of the imprint process according to time.
Fig. 9. Experimental setup for the imprinting process.
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Fig. 11. Imprinted results of the conventional hydraulic press and the developed imprinting system.
Fig. 9 depicts the experimental setup prior to the imprinting process using the developed system. Two imprinted sets which consist of nickel stamps, resins and substrates are inserted into a same vacuum jig as shown in Fig. 6. There are thickness differences between the used nickel stamps with 160 130 mm2-area of two imprinted sets to evaluate the conformal contact between stamps and resins according to the flatness of operation surfaces. The nickel stamp with 400 lm-thickness is used in set A, and the other with 200 lm-thickness is adopted in set B. GX-13 films were adopted as resin layers, and its area and thickness are 160 130 mm2 and 60 lm. The thick films were selected to remain the instead of 37 lm-thickness films to emphasis the difference between the uniform pressing performances of two imprinting systems, in addition, the copper clad layers were used as substrates. Fig. 10 illustrates the process profiles and the measured profiles of the developed imprinting system in this study, and it elucidates that the measured temperature and pressure of the imprinting system have good agreements to the set conditions except the temperature overshooting of the early curing period. Additionally, the thickness of the resin layer according to the imprinting time can be estimated at the given profiles as mentioned in the previous section, and the calculated residual thickness is 21.48 lm as portrayed in Fig. 10. Fig. 11 denotes that the imprinted patterns from the hydraulic press and the developed system in the present study, and the desmear and the chemical copper plating processes for the imprinted trenches were implemented to easily observe the cross sections of imprinting results using microscopes. Fig. 11a indicates the imprinted results using the conventional press, and the average thickness of residual layers is 8.71 lm in the case using the nickel stamp with 400 lm-thickness, in addition, that of the stamp with 200 lm-thickness is 49.4 lm. There are remarkable differences between the residual thicknesses of two cases due to the thickness discrepancy of the used stamps though the imprinting process was performed in the same vacuum jig and the same operating condition. Additionally, the residual thicknesses of the experimental results quite differ from the estimate thickness. Fig. 11b depicts
the imprinted results using the developed imprinting system, and the average residue of the stamp with 400 lm-thickness is 21.33 lm. The average residue of the resins with the 200 lmthickness stamp is 21.77 lm, and it has a similar to that of the stamp with 400 lm-thickness as opposed to the imprinted results using the conventional press. Additionally, those residual thicknesses also agree with the simulated result. These results evince that the developed imprinting equipment adopting the pneumatic system can significantly improve the uniform residue, and it is able to maintain the conformal contact between the stamps and the resins up to 200 lm-thickness difference. 4. Conclusion In present study, developed was the imprinting system for the large area using the several small stamps with 160 130 mm2. A hot air with a high pressure was adopted to apply the uniform pressing force instead of the conventional hydraulic press, and the vacuum jig was used to prevent the void captured between stamps and resins. The present results indicate that the air pressing system can remarkably improve the residue uniformity of the imprinted product as well as the conformal contact between stamps and resins during imprinting process as compared with those of the conventional press, and they depict the feasibility about the application of the imprint process for the PCB industries where it is difficult to adopt the imprint lithography due to the limited size of stamps. References [1] [2] [3] [4] [5] [6] [7]
L.J. Heyderman, J. Gobrecht, et al., Microelectron. Eng. 57–58 (2001) 357–380. S. Ra et al., Curr. Appl. Phys. 8 (2008) 675–678. J. Stefan, Akad. Wiss. Math. Natur. 69 (1874) 713. N. Bogdanski et al., Microelectron. Eng. 78–79 (2005) 598–604. S. Ra, H. Lee, J. Solid State Phenom. 112–123 (2007) 681–684. J. Yoon et al., SPIE Symp. Micromach. Microfab. 3512 (1998) 358–366. L. Ressier et al., Microelectron. Eng. 71 (2004) 272–276.