- Email: [email protected]
Application of low-temperature substrate bonding in fabrication of reusable micro-fluidic devices
ARTICLE IN PRESS
Vacuum 80 (2006) 488–493 www.elsevier.com/locate/vacuum
Short communication
Application of low-temperature substrate bonding in fabrication of reusable micro-fluidic devices Marshal Dhayal Biological Research Center of Industrial Accelerators & Center of Micro Device Systems, Dongshin University, 252 Daeho-dong, Naju, Chonnam, South Korea Received 10 April 2005; received in revised form 1 June 2005
Abstract The plasma polymerised (PP) films were deposited on silicon substrates and used to bond the substrates at a low temperature (130 1C). Different types of monomers were used to deposit PP films on m-electrode and m-channel of micro-fluidic devices (MFD) to tailor the surface properties. To confirm the PP film deposition on the substrates the surface chemistry was characterized using X-ray photoelectron spectroscopy (XPS). The bond strength of about 100 nm PP acrylic acid, p-xylene, styrene, 1-vinyl-2-pyrrolininne and allylamine films were measured more than 2 Mpa. The bonding strength was also tested before and after passing the fluid in MDF and no significant change was observed. Generally, no change in the structure of m-electrode was observed by the bonding, using a separating and cleaning process. Therefore, this bonding process is independent of the type of thin film deposited and the bonding can be easily carried out by me in the laboratory and the surface properties can be tailored for different applications. It also enables one to recycle and reuse the devices in production. This process allows the devices to be recycling and/or reusable for a better and cleaner global environment. r 2005 Elsevier Ltd. All rights reserved. Keyword: Plasma bonding; Plasma polymerization; Reusable organic micro-fluidics; Low-temperature bonding; Environment-friendly process
1. Introduction The substrate bonding technique is important in MEMS, bio-MEMS, micro/nano-fluidic devices, optoelectronics [1]. In the last few years, a number of different process and techniques were used such as Tel.: +82 104 620 1307; fax: 82 61 330 2825.
E-mail address: [email protected].
anodic bonding, direct wafer bonding and intermediate layer bonding [2–8]. The anodic bonding is most widely used only for glass substrate bonding to other conductive and semi-conductive materials [3–6] by applying an external voltage hence, this bonding process is substrate specific. The direct silicon wafer bonding requires a higher annealing temperature (800 1C) and this can cause unwanted changes for the materials. Vacuum wafer bonding
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.06.003
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
was also used as one of the promising approaches, which can reduce the temperature from about 800 to 400 1C in the case of silicon wafer [2,7,8]. The polymer-based micro devices have shown potential advantages in the bonding of devices at low temperatures [9]. Recently, Tan et al. [10] used a sol–gel intermediate layer silicon-to-silicon wafer bonding at a low temperature (100 1C) and reported the bonding strength depends on surface smoothness, porous intermediate layer and high density of OH groups whereas Dhayal et al. [11] have used a plasma polymerisation (PP) process for bonding of different substrates at low temperature. All above mentioned bonding processes, either substrate or process specific, require high annealing temperatures or loads and the devices bonded needed specific arrangement in the laboratory for the bonding. The devices were not recycled or reused. In this paper, the application of low-temperature substrate bonding in fabrication of recyclable and reusable micro-fluidic devices (MFDs) has been reported. The PP films were deposited for on silicon substrates and used for silicon–silicon wafer bonding at low temperature. The bond strength of different 100 nm PP films of acrylic acid, p-xylenne, styrene, 1-vinyl-2-pyrrolininne and allylamine on silicon were tested. This process has an advantage in the micro/nano-devices applications in biology with controlled surface properties. These controlled surface chemical and physical properties have advantages in bio-MEMS/NEMS, organic micro/ nano-fluidic devices, lab-on-a-chip etc. applications [9,11–13]. The plasma bonding process has advantages in the bonding of any type of substrate materials and thin PP films. The bonding can be easily carried out by me in the laboratory and the devices can be recycled and reused in production. This process allows the devices to be recycling and/ or reusable for a better and cleaner global environment. The present bonding process is environment friendly material processing. 2. Experimental 2.1. Plasma polymerisation process A schematic diagram of an inductively coupled RF PP reactor is shown in Fig. 1. This plasma
O2
Sample Holder
489
RF electrode (coil)
Turbo Pump Substrate
Rotary Pump Monomer Matching Unit
RF Power Supply
Fig. 1. A schematic diagram of plasma polymerisation reactor.
reactor is made from a 2-way cruciform glass vessel (10 cm diameter and 30 cm in length) and is pumped to a base pressure of order of 10 3 Pa using a turbo-molecular pump backed by a rotary pump. Both sides of the glass vessel were closed using stainless-steel grounded flanges. The main plasma was sustained through 13.56 MHz RF excitation. The RF driver coil was attached to the RF power supply via a matching unit. The matching unit was tuned manually and a reflected power was controlled less than 5%. In this study, 20 W RF power and 5.3 Pa monomer vapour pressure (for all deposition) has been used to deposit the thin films on glass and silicon substrates placed on a grounded sample holder. Different monomers were contained in the glassware mounted on one flange of the glass reactor through a micro leveler manually controlled gas (monomer) flow regulator. In this study, five different types of monomers (acrylic acid, p-xylene, styrene, 1-vinyl-2-pyrrolininne and allylamine) were used to deposit the thin PP films on the silicon substrates. The acrylic acid monomer was degassed using a freeze–thaw cycle for three times before admitting it to the chamber for the discharge. For p-xylene and allylamine PP films depositions, 5% argon gas was added in the chamber to stabilize and achieved uniform discharge conditions. The oxygen gas line was attached to the chamber through a gas flow controller system to the reactor and used to clean the reactor after each set of PP film deposition.
ARTICLE IN PRESS 490
M. Dhayal / Vacuum 80 (2006) 488–493
The surface properties of the PP films were characterised using X-ray photoelectron spectroscopy. A VG-Scientific ESCALAB 250 spectrometer with monocromatised Al ka X-ray source was used at the Korean Basic Science Institute. 2.2. Bonding process The silicon substrates were treated using low plasma ion energy (less than 5 eV) and ion flux (1018 m 2 s 1) [14–18] in the argon RF discharge. This substrate pre-treatment was carried out at 20 W RF power, 20 mTorr pressure RF discharge for 120 s to improve the adhesive properties of PP films on silicon. On these plasma treated silicon substrates, 100 nm PP films were deposited and two substrates were placed together, aligned and placed on a clean hot plate (temperature 130 1C). A load of 1 kg (on 1 1 cm2 surface area) was applied on the substrates during the bonding process of 5 min. By increasing the applied load there is a possibility of increasing the bond strength. However, that may also damage the micro-electrode structure in MFD. Therefore, a low load was applied in this study. After 5 min, the load was lifted off from the substrates and these were carefully removed from the hot plate and placed in a cool, dry place (at room temperature 25 1C) for 12 h before bonding strength measurements were carried out. A schematic diagram of surface modification, PP films deposition and physical bonding process is shown in Fig. 2.
3. Results and discussions The PP films were deposited on plasma pretreated silicon substrates and the surface chemistry was characterized using XPS to confirm the thin film deposition on the substrates. A wide scan XPS spectrum of all PP films deposited in this study is shown in Fig. 3 with a comparison of different elemental concentration variation at the surface. The acrylic acid, allylamin and 1-vinyl 2-pyrrolininne PP films showed the functionalities such as carboxylic, carbonyl, hydroxy, ether, amine etc. groups attachment on the substrates, whereas in pxylene and styrene the PP film has other advan-
Micro/nano system on silicon and glass Plasma surface modification Micro/nano system with plasma surface modification Thin film deposition by plasma polymerisation Micro/nano system with plasma coating
Load 1 Kg
Cooling at 25°C
Hot Plate (130°C)
Fig. 2. A schematic diagram of bonding process in micro/nano devices.
tages such as hydrophobic nature of surfaces. Therefore, these PP films have advantages in controlling the surface nature such as hydrophilic, hydrophobic and cell adhesive as well as surface functionalities on the same silicon substrate. These different types of coatings can have different applications in reusable MFD fabrication with a surface selectivity. The applied force per cm2 (pressure in MPa) on silicon-to-silicon bonded substrates is plotted with different times in Fig. 4 for different PP films. Results showed the bond strength of silicon-tosilicon substrate was 2 MPa and can be increased by increasing the applied load on the substrates during the bonding process. The results also showed same bonding strength for all different PP films. This is because most of polymers have Tg values between 70 and 140 1C so all these different intermediate layers can be bonded at a heating temperature of 130 1C. This bonding technique has usefulness to control the surface properties of micro/nano devices and can easily applied to bond the substrates. The bonding was also tested for applying a 20 N force for 24 h continuously by changing the room temperature between 25 and 35 1C. No significant change in bond strength was seen when increasing the film thickness by more
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
1.5
491
0.9 1-vinyl 2-pyrrolininne
allylamine
CPS (105)
CPS (105)
1.2 N1s
0.9 O1s 0.6
C1s
0.6 N1s
O1s
C1s
0.3
0.3 0.0 800
600
400
200
0.0 800
0
600
Binding Energy (eV)
400
200
Binding Energy (eV) 1.8
1.5
styrene
p-xylene 1.5 CPS (105)
CPS (105)
1.2 0.9 O1s 0.6
C1s
1.2 O1s
0.9
C1s
0.6
0.3 0.0 800
0
0.3 0.0 600
400
200
0
800
600
Binding Energy (eV)
400
200
0
Binding Energy (eV) 100 1-vinil 2-pyrrolininne
1.5 acrylic acid
allylamine
80
p-xylene Conc at. / %
CPS (105)
1.2 O1s
0.9 0.6
C1s
styrene
60
acrylic acid 40 20
0.3 0.0
0 800
600
400
200
Binding Energy (eV)
0
C
O
N
Element
Fig. 3. XPS elemental analysis of plasma polymerised films.
than 100 nm. However, when reducing the film thickness by less than 40 nm, the bond strength was not consistent. This change could be related to the boding uniformity, surface roughness or sur-
face adhesive properties. Therefore, for all the experiments the film thickness chosen was approximately 100 nm with a very smooth surface and uniformity.
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
492 2.4 A (Si-ppAc-Si) B (Si-p-xylene-Si) A C (Si-styrene-Si) D (Si-1-vinyl-2-pyrrolinine-Si) E (Si-allylamine-Si)
Bond strength (Mpa)
2.0
1.6
B
C
D
E
1.2
0.8
0.4
0.0
0
20
40
60 80 Time (minute)
100
120
Fig. 4. Plot of applied force per cm2 (pressure in MPa) on silicon-to-silicon bonded substrates with time for different intermediate plasma polymerised thin layers.
The most important application of this bonding process is the fact that the devices can be reused and recycled. For this, PP acrylic acid films were deposited on silicon-based m-electrode and mchannel and bonded to fabricate MFD. After the bonding, water (used as a fluid) was passed through the m-channel of MFD. The bonding strength was again measured after two hours of the experiment (Fig. 5 shows a trace of water flow in the m-channel with a cross-sectional view of micro channel). No change was measured in the bonding strength before and after the experiment. After using MDF, the m-channel and m-electrode were separated by placing the MFD device again on a hot plate (100 1C) and the m-channel and melectrode were carefully separated. After separating, these were washed using water, iso-propanol and the PP acrylic acid coatings were cleaned using oxygen plasma. The m-channel and m-electrode structures were examined in a microscope before and after the bonding and separating these. In most cases (about 90%), no destruction in the micro electrode was observed by bonding, separating and cleaning the two parts of the micro devices (as shown in Fig. 5). Therefore, the same mchannel and m-electrode can be used again. Hence, this plasma bonding process has advantages in the bonding of any type of substrate materials at low temperatures, and can easily be carried out by me in the laboratory. It also enables the reuse and recycling of the devices in the laboratory. The
Fig. 5. The water fluid trace in m-channel of micro fluid device after bonding of micro electrode and micro channel in MFD (top). The microscopic picture of m-electrodes is shown for two conditions (bottom).
recycle and/or reuse are essential for a better global environment. The present bonding process is environment friendly material processing [19].
4. Conclusion The plasma polymerized (PP) films deposited on silicon substrates were used to bond the substrates at a low temperature (130 1C). Different types of monomers were used to deposit PP films on melectrode and m-channel of MFD to control the surface properties. The surface chemistry was characterized using X-ray photoelectron spectroscopy (XPS) to confirm the PP film deposition on the substrates. A 2 MPa bond strength was measured for 100 nm PP Acrylic acid, p-xylenne, styrene, 1-vinyl-2-pyrrolininne and allylamine films deposited silicon wafer bonding. The bonding was also tested before and after using the MDF and no significant change was measured. No change in the structure of m-electrode was observed by the bonding, using, separating and cleaning process. Therefore, this bonding process is independent of substrate materials, PP thin films deposited on the substrates. The bonding can be easily carried out by me in the laboratory and surface properties can be tailored for different
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
applications. This bonding technique also enables these devices to be recycled and reused, not only in the laboratory but also production. This process allows the devices to be recycling and/or reusable for a better and cleaner global environment.
Acknowledgment This study was supported by COMDS and BRCIA at Dongshin University. Authors would like to thanks Dr. H. G. Jeong (BRCIA) for useful discussions and Mr. M. K. Ha (Korean Basic Science Institute) for helping in obtaining XPS data. References [1] Ko WH, Suminto JT, Yeh GJ. Bonding techniques for micro sensors: micromachining and micropackaging for transducers. Amsterdam, The Netherlands: Elsevier; 1985. [2] Yu WB, Tan CM, Wei J, Deng SS, Nai SML. Sensors Actuators A 2004;115:67. [3] Rogers T, Kowal J. Sensors Actuators A 1995;46-47:113. [4] Wallis GD, Pomerantz DI. J Appl phys 1969;40:3946. [5] Wei J, Nai SML, Wong CK, Lee LC. Thin Solid Films 2004;462-463:487.
493
[6] Lee DJ, Lee YH, Jang J, Ju BK. Sensors Actuators A 2001;89:43. [7] Gosele U, Stenzel H, Martini T, Steinkirchner J, Conrad D, Scheerschmidt K. Appl Phys Lett 1995;67(24):3614. [8] Tong QY, Kim WJ, Lee TH, Gosele U. Electrochem Solid State Lett 1998;1(1):52. [9] Anderson JR, Chiu DT, Jackman RJ, Cherniavskaya O, McDonald JC, Wu H, Whitesides SH, Whitesides GM. Anal Chem 2000;72(14):3158. [10] Tan CM, Deng SS, Wei J, Yu WB. J Phys. D: Appl. Phys. 2005;38:1308. [11] Dhayal M, Jeong HG, Choi JS. Use of plasma polymerisation process for fabrication of bio-MEMS for microfludic devices. Appl Surf Sci 2005; in press. [12] Hiratsuka A, Muguruma H, Lee KH, Karube I. Biosensors Bioelectron 2004;19:1667. [13] Bouaidat S, Winther JB, Christensen SF, Jonsmann J. Sensors Actuators A 2004;110:390. [14] Dhayal M, Forder D, Short RD, Barton D, Bradley JW. Surf Coat Technol 2003;162(2-3):294. [15] Marshal; Plasma treatment and deposition of polymeric materials with applications in life science. Ph.D. thesis, Department of Physics, UMIST, Manchester, England; 2003. [16] Dhayal M, Bradley JW. Surf Coat Technol 2004; 184(1):116. [17] Dhayal M, Parry KL, Short RD, Bradley JW. J Phys Chem B 2004;108(37):14000. [18] Dhayal M, Forder D, Short RD, Bradley JW. Vacuum 2003;70(1):67. [19] Wasa K. Bull Mater Sci India 1995;18(8):937.
Vacuum 80 (2006) 488–493 www.elsevier.com/locate/vacuum
Short communication
Application of low-temperature substrate bonding in fabrication of reusable micro-fluidic devices Marshal Dhayal Biological Research Center of Industrial Accelerators & Center of Micro Device Systems, Dongshin University, 252 Daeho-dong, Naju, Chonnam, South Korea Received 10 April 2005; received in revised form 1 June 2005
Abstract The plasma polymerised (PP) films were deposited on silicon substrates and used to bond the substrates at a low temperature (130 1C). Different types of monomers were used to deposit PP films on m-electrode and m-channel of micro-fluidic devices (MFD) to tailor the surface properties. To confirm the PP film deposition on the substrates the surface chemistry was characterized using X-ray photoelectron spectroscopy (XPS). The bond strength of about 100 nm PP acrylic acid, p-xylene, styrene, 1-vinyl-2-pyrrolininne and allylamine films were measured more than 2 Mpa. The bonding strength was also tested before and after passing the fluid in MDF and no significant change was observed. Generally, no change in the structure of m-electrode was observed by the bonding, using a separating and cleaning process. Therefore, this bonding process is independent of the type of thin film deposited and the bonding can be easily carried out by me in the laboratory and the surface properties can be tailored for different applications. It also enables one to recycle and reuse the devices in production. This process allows the devices to be recycling and/or reusable for a better and cleaner global environment. r 2005 Elsevier Ltd. All rights reserved. Keyword: Plasma bonding; Plasma polymerization; Reusable organic micro-fluidics; Low-temperature bonding; Environment-friendly process
1. Introduction The substrate bonding technique is important in MEMS, bio-MEMS, micro/nano-fluidic devices, optoelectronics [1]. In the last few years, a number of different process and techniques were used such as Tel.: +82 104 620 1307; fax: 82 61 330 2825.
E-mail address: [email protected].
anodic bonding, direct wafer bonding and intermediate layer bonding [2–8]. The anodic bonding is most widely used only for glass substrate bonding to other conductive and semi-conductive materials [3–6] by applying an external voltage hence, this bonding process is substrate specific. The direct silicon wafer bonding requires a higher annealing temperature (800 1C) and this can cause unwanted changes for the materials. Vacuum wafer bonding
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.06.003
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
was also used as one of the promising approaches, which can reduce the temperature from about 800 to 400 1C in the case of silicon wafer [2,7,8]. The polymer-based micro devices have shown potential advantages in the bonding of devices at low temperatures [9]. Recently, Tan et al. [10] used a sol–gel intermediate layer silicon-to-silicon wafer bonding at a low temperature (100 1C) and reported the bonding strength depends on surface smoothness, porous intermediate layer and high density of OH groups whereas Dhayal et al. [11] have used a plasma polymerisation (PP) process for bonding of different substrates at low temperature. All above mentioned bonding processes, either substrate or process specific, require high annealing temperatures or loads and the devices bonded needed specific arrangement in the laboratory for the bonding. The devices were not recycled or reused. In this paper, the application of low-temperature substrate bonding in fabrication of recyclable and reusable micro-fluidic devices (MFDs) has been reported. The PP films were deposited for on silicon substrates and used for silicon–silicon wafer bonding at low temperature. The bond strength of different 100 nm PP films of acrylic acid, p-xylenne, styrene, 1-vinyl-2-pyrrolininne and allylamine on silicon were tested. This process has an advantage in the micro/nano-devices applications in biology with controlled surface properties. These controlled surface chemical and physical properties have advantages in bio-MEMS/NEMS, organic micro/ nano-fluidic devices, lab-on-a-chip etc. applications [9,11–13]. The plasma bonding process has advantages in the bonding of any type of substrate materials and thin PP films. The bonding can be easily carried out by me in the laboratory and the devices can be recycled and reused in production. This process allows the devices to be recycling and/ or reusable for a better and cleaner global environment. The present bonding process is environment friendly material processing. 2. Experimental 2.1. Plasma polymerisation process A schematic diagram of an inductively coupled RF PP reactor is shown in Fig. 1. This plasma
O2
Sample Holder
489
RF electrode (coil)
Turbo Pump Substrate
Rotary Pump Monomer Matching Unit
RF Power Supply
Fig. 1. A schematic diagram of plasma polymerisation reactor.
reactor is made from a 2-way cruciform glass vessel (10 cm diameter and 30 cm in length) and is pumped to a base pressure of order of 10 3 Pa using a turbo-molecular pump backed by a rotary pump. Both sides of the glass vessel were closed using stainless-steel grounded flanges. The main plasma was sustained through 13.56 MHz RF excitation. The RF driver coil was attached to the RF power supply via a matching unit. The matching unit was tuned manually and a reflected power was controlled less than 5%. In this study, 20 W RF power and 5.3 Pa monomer vapour pressure (for all deposition) has been used to deposit the thin films on glass and silicon substrates placed on a grounded sample holder. Different monomers were contained in the glassware mounted on one flange of the glass reactor through a micro leveler manually controlled gas (monomer) flow regulator. In this study, five different types of monomers (acrylic acid, p-xylene, styrene, 1-vinyl-2-pyrrolininne and allylamine) were used to deposit the thin PP films on the silicon substrates. The acrylic acid monomer was degassed using a freeze–thaw cycle for three times before admitting it to the chamber for the discharge. For p-xylene and allylamine PP films depositions, 5% argon gas was added in the chamber to stabilize and achieved uniform discharge conditions. The oxygen gas line was attached to the chamber through a gas flow controller system to the reactor and used to clean the reactor after each set of PP film deposition.
ARTICLE IN PRESS 490
M. Dhayal / Vacuum 80 (2006) 488–493
The surface properties of the PP films were characterised using X-ray photoelectron spectroscopy. A VG-Scientific ESCALAB 250 spectrometer with monocromatised Al ka X-ray source was used at the Korean Basic Science Institute. 2.2. Bonding process The silicon substrates were treated using low plasma ion energy (less than 5 eV) and ion flux (1018 m 2 s 1) [14–18] in the argon RF discharge. This substrate pre-treatment was carried out at 20 W RF power, 20 mTorr pressure RF discharge for 120 s to improve the adhesive properties of PP films on silicon. On these plasma treated silicon substrates, 100 nm PP films were deposited and two substrates were placed together, aligned and placed on a clean hot plate (temperature 130 1C). A load of 1 kg (on 1 1 cm2 surface area) was applied on the substrates during the bonding process of 5 min. By increasing the applied load there is a possibility of increasing the bond strength. However, that may also damage the micro-electrode structure in MFD. Therefore, a low load was applied in this study. After 5 min, the load was lifted off from the substrates and these were carefully removed from the hot plate and placed in a cool, dry place (at room temperature 25 1C) for 12 h before bonding strength measurements were carried out. A schematic diagram of surface modification, PP films deposition and physical bonding process is shown in Fig. 2.
3. Results and discussions The PP films were deposited on plasma pretreated silicon substrates and the surface chemistry was characterized using XPS to confirm the thin film deposition on the substrates. A wide scan XPS spectrum of all PP films deposited in this study is shown in Fig. 3 with a comparison of different elemental concentration variation at the surface. The acrylic acid, allylamin and 1-vinyl 2-pyrrolininne PP films showed the functionalities such as carboxylic, carbonyl, hydroxy, ether, amine etc. groups attachment on the substrates, whereas in pxylene and styrene the PP film has other advan-
Micro/nano system on silicon and glass Plasma surface modification Micro/nano system with plasma surface modification Thin film deposition by plasma polymerisation Micro/nano system with plasma coating
Load 1 Kg
Cooling at 25°C
Hot Plate (130°C)
Fig. 2. A schematic diagram of bonding process in micro/nano devices.
tages such as hydrophobic nature of surfaces. Therefore, these PP films have advantages in controlling the surface nature such as hydrophilic, hydrophobic and cell adhesive as well as surface functionalities on the same silicon substrate. These different types of coatings can have different applications in reusable MFD fabrication with a surface selectivity. The applied force per cm2 (pressure in MPa) on silicon-to-silicon bonded substrates is plotted with different times in Fig. 4 for different PP films. Results showed the bond strength of silicon-tosilicon substrate was 2 MPa and can be increased by increasing the applied load on the substrates during the bonding process. The results also showed same bonding strength for all different PP films. This is because most of polymers have Tg values between 70 and 140 1C so all these different intermediate layers can be bonded at a heating temperature of 130 1C. This bonding technique has usefulness to control the surface properties of micro/nano devices and can easily applied to bond the substrates. The bonding was also tested for applying a 20 N force for 24 h continuously by changing the room temperature between 25 and 35 1C. No significant change in bond strength was seen when increasing the film thickness by more
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
1.5
491
0.9 1-vinyl 2-pyrrolininne
allylamine
CPS (105)
CPS (105)
1.2 N1s
0.9 O1s 0.6
C1s
0.6 N1s
O1s
C1s
0.3
0.3 0.0 800
600
400
200
0.0 800
0
600
Binding Energy (eV)
400
200
Binding Energy (eV) 1.8
1.5
styrene
p-xylene 1.5 CPS (105)
CPS (105)
1.2 0.9 O1s 0.6
C1s
1.2 O1s
0.9
C1s
0.6
0.3 0.0 800
0
0.3 0.0 600
400
200
0
800
600
Binding Energy (eV)
400
200
0
Binding Energy (eV) 100 1-vinil 2-pyrrolininne
1.5 acrylic acid
allylamine
80
p-xylene Conc at. / %
CPS (105)
1.2 O1s
0.9 0.6
C1s
styrene
60
acrylic acid 40 20
0.3 0.0
0 800
600
400
200
Binding Energy (eV)
0
C
O
N
Element
Fig. 3. XPS elemental analysis of plasma polymerised films.
than 100 nm. However, when reducing the film thickness by less than 40 nm, the bond strength was not consistent. This change could be related to the boding uniformity, surface roughness or sur-
face adhesive properties. Therefore, for all the experiments the film thickness chosen was approximately 100 nm with a very smooth surface and uniformity.
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
492 2.4 A (Si-ppAc-Si) B (Si-p-xylene-Si) A C (Si-styrene-Si) D (Si-1-vinyl-2-pyrrolinine-Si) E (Si-allylamine-Si)
Bond strength (Mpa)
2.0
1.6
B
C
D
E
1.2
0.8
0.4
0.0
0
20
40
60 80 Time (minute)
100
120
Fig. 4. Plot of applied force per cm2 (pressure in MPa) on silicon-to-silicon bonded substrates with time for different intermediate plasma polymerised thin layers.
The most important application of this bonding process is the fact that the devices can be reused and recycled. For this, PP acrylic acid films were deposited on silicon-based m-electrode and mchannel and bonded to fabricate MFD. After the bonding, water (used as a fluid) was passed through the m-channel of MFD. The bonding strength was again measured after two hours of the experiment (Fig. 5 shows a trace of water flow in the m-channel with a cross-sectional view of micro channel). No change was measured in the bonding strength before and after the experiment. After using MDF, the m-channel and m-electrode were separated by placing the MFD device again on a hot plate (100 1C) and the m-channel and melectrode were carefully separated. After separating, these were washed using water, iso-propanol and the PP acrylic acid coatings were cleaned using oxygen plasma. The m-channel and m-electrode structures were examined in a microscope before and after the bonding and separating these. In most cases (about 90%), no destruction in the micro electrode was observed by bonding, separating and cleaning the two parts of the micro devices (as shown in Fig. 5). Therefore, the same mchannel and m-electrode can be used again. Hence, this plasma bonding process has advantages in the bonding of any type of substrate materials at low temperatures, and can easily be carried out by me in the laboratory. It also enables the reuse and recycling of the devices in the laboratory. The
Fig. 5. The water fluid trace in m-channel of micro fluid device after bonding of micro electrode and micro channel in MFD (top). The microscopic picture of m-electrodes is shown for two conditions (bottom).
recycle and/or reuse are essential for a better global environment. The present bonding process is environment friendly material processing [19].
4. Conclusion The plasma polymerized (PP) films deposited on silicon substrates were used to bond the substrates at a low temperature (130 1C). Different types of monomers were used to deposit PP films on melectrode and m-channel of MFD to control the surface properties. The surface chemistry was characterized using X-ray photoelectron spectroscopy (XPS) to confirm the PP film deposition on the substrates. A 2 MPa bond strength was measured for 100 nm PP Acrylic acid, p-xylenne, styrene, 1-vinyl-2-pyrrolininne and allylamine films deposited silicon wafer bonding. The bonding was also tested before and after using the MDF and no significant change was measured. No change in the structure of m-electrode was observed by the bonding, using, separating and cleaning process. Therefore, this bonding process is independent of substrate materials, PP thin films deposited on the substrates. The bonding can be easily carried out by me in the laboratory and surface properties can be tailored for different
ARTICLE IN PRESS M. Dhayal / Vacuum 80 (2006) 488–493
applications. This bonding technique also enables these devices to be recycled and reused, not only in the laboratory but also production. This process allows the devices to be recycling and/or reusable for a better and cleaner global environment.
Acknowledgment This study was supported by COMDS and BRCIA at Dongshin University. Authors would like to thanks Dr. H. G. Jeong (BRCIA) for useful discussions and Mr. M. K. Ha (Korean Basic Science Institute) for helping in obtaining XPS data. References [1] Ko WH, Suminto JT, Yeh GJ. Bonding techniques for micro sensors: micromachining and micropackaging for transducers. Amsterdam, The Netherlands: Elsevier; 1985. [2] Yu WB, Tan CM, Wei J, Deng SS, Nai SML. Sensors Actuators A 2004;115:67. [3] Rogers T, Kowal J. Sensors Actuators A 1995;46-47:113. [4] Wallis GD, Pomerantz DI. J Appl phys 1969;40:3946. [5] Wei J, Nai SML, Wong CK, Lee LC. Thin Solid Films 2004;462-463:487.
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