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Available online at www.sciencedirect.com

Sensors and Actuators A 141 (2008) 213–216

Glass wafers bonding via Diels–Alderreaction at mild temperature

Minjie Zhang a,b, Jianying Zhao a, Lianxun Gao a,∗a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Science, Changchun, PR Chinab Graduate School of Chinese Academy of Sciences, Beijing, PR China

Received 31 January 2007; received in revised form 5 June 2007; accepted 1 August 2007Available online 6 August 2007

bstract

In this paper, we introduced a novel bonding method of glass wafers by Diels–Alder reaction at mild temperature. After standard hydroxylization

nd aminosilylation, two wafers were modified by 2-furaldehyde and maleic anhydride, respectively. Then they were brought into close contact andightly held with a clamping fixture. A strong bonding could be achieved by annealing for 5 h at 200 ◦C. Bonding strength is as high as 1.78 MPand sufficient for most application of microfluidic chips. This bonding process has the advantages of simple operation and good smoothness of theafer surface. Mild temperature can prevent the channel of microfluidic chip distorted or collapse and shorten the cooling time.

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2007 Elsevier B.V. All rights reserved.

eywords: Glass wafers; Bonding; Diels–Alder reaction; Mild temperature

. Introduction

Wafer bonding is one of the fundamental technologies for fab-ication of advanced microsystems [1–3]. Generally speaking,hree main bonding approaches can be adopted: direct bonding,nodic bonding and intermediate layer bonding. Direct bonding4–7] needs to expose wafers to an elevated temperature above000 ◦C, which can lead to the damage of microstructure andemperature-sensitive element on wafers. Compared with directonding, anodic bonding [8–15] has an advantage of much lowerrocessing temperature (<500 ◦C), but requires the aid of a highlectrical field in the region of 700–2000 V. It would have greatnfluence on the integrated circuits on wafers. As an alternativeonding approach to achieve at lower temperature and milderonditions, intermediate layer bonding [16–20] has recentlyttracted considerable interest. Many low-temperature-bondingrocesses have been developed [21–23]. Recently, our groupound a new method of bonding quartz wafers by atom transferadical polymerization at 300 ◦C [24]. The bonding strength was

s high as 1.5 MPa.

Diels–Alder reaction between maleimide and furan involves[4 + 2] cycloaddition reaction to generate a six-membered ring.

∗ Corresponding author. Tel.: +86 431 85262203; fax: +86 431 85685653.E-mail address: lxgao@ciac.jl.cn (L. Gao).

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924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2007.08.001

t can be achieved without catalyst by heating and no volatileolecular is produced during reaction [25]. Roucoules et al.

as demonstrated the possibility of solution-phased Diels–Aldereaction at alkene-functionalized surfaces [26]. In this paper,

novel bonding approach based on the formation of chemi-al covalent bond by Diels–Alder cycloaddition reaction waseported. In order to achieve bondable surfaces, two glass waferserminated by amine groups were modified by 2-furaldehydend maleic anhydride, respectively. Then they were piled care-ully and fixed with a clamp. Subsequent annealing treatment forh at 200 ◦C leaded to a Diels–Alder cycloaddition reaction to

orm C C covalent bond between wafers. The bonding strengthas as high as 1.78 MPa. Furthermore, effective bonding haseen achieved in common laboratory without the requirement oflean-room facilities or complicated cleaning processes. The for-ation of bondable surfaces and the bonding process are shown

n Scheme 1.

. Experiments

.1. Regents and materials

Anhydrous solutions were dried and distilled by standardethods. Aminopropyltriethoxysilane (Acros) was distilled

nder reduced pressure and stored under dry nitrogen. Maleic

214 M. Zhang et al. / Sensors and Actuators A 141 (2008) 213–216

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Scheme 1. Formation of bond

nhydride (TianJin Chemical Corporation) was sublimed beforese. Other regents were used without further purification.5 mm × 10 mm glass wafers and 50 mm × 10 mm quartzafers were bought from JingKe Optics Apparatus Corporation.V–vis spectra were recorded on a UV-2550 (Shimadzu) spec-

rophotometer. Scanning electron microscope (SEM) imagesere scanned by XL 30 ESEM FEG (FEI Company, USA). The

hear forces were measured by an electronic testing machineINSTRON1121, USA).

.2. Immobilization of maleic anhydride on wafers

Standard cleaning process and solution-phased aminosily-ation of wafers were achieved according to literature method24]. The prepared wafers were immersed into anhydrous DMAcolution (20 mL) containing 0.5 g maleic anhydride under nitro-en atmosphere for 5 h at 50 ◦C. Unreacted maleic anhydriden the surface was removed by washing wafers with methanol.fter being dried under vacuum, the wafers were put into

he mixture of 0.5 g 1,1′-carbonyldiimidazole (CDI), 0.05 g 4-dimethylamino) pyridine (DMAP) and 20 mL anhydrous THFor 5 h at 70 ◦C. Subsequently the wafers were sonicated threeimes (1 min each time) in THF and dried under vacuum.

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Fig. 1. UV–vis spectrum of the reaction on w

urfaces and bonding process.

he wafers were stored in argon atmosphere before furtherse.

.3. Immobilization of 2-furaldehyde on wafers

Immobilization of 2-furaldehyde was achieved by expos-ng the wafers terminated by amine groups to a solution of-furaldehyde (0.2 mL) in anhydrous methanol (20 mL) for 6 ht room temperature under nitrogen atmosphere. Then the wafersere taken out from the solution and sonicated three times (1 min

ach time) in methanol to remove deposited 2-furaldehyde. Aftereing dried under vacuum, the wafers were stored in argontmosphere before further use.

.4. Wafer bonding

Two wafers terminated by maleimide and furan, respectively,ere sonicated three times (2 min each time) in methanol prior toonding. After bringing the cleaned surfaces into close contact,

hey were placed in a clamping fixture and compressed by fas-ening clamping screws. The tightly held pairs were annealedor 5 h at 200 ◦C. Then they were cooled down to room tem-erature in 6 h and held for 5 h to reduce the residual stress.

afers (A) and the bonding process (B).

M. Zhang et al. / Sensors and Actuators A 141 (2008) 213–216 215

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inally, the clamp was removed and a strong bonding waschieved.

. Results and discussion

.1. Characterization by UV–vis spectroscopy

The reactions on surfaces were examined with UV–vis spec-rometer using quartz wafers instead of glass wafers as substratesecause of the strong ultraviolet absorption of the latter. Thepectrum of various surfaces are shown in Fig. 1A. The wideeak of 300 nm (line 4) and the absorption centering at 269 nmline 5) suggest the presence of maleimide and furan structures,espectively.

When we tried to bond the functional quartz wafers, onlyocal area was successfully bonded. The change of spectrometerefore and after bonding are shown in Fig. 1B. The movement ofharacteristic peak from 302 nm (line 7) to 283 nm (line 8) proveshat new structures have formed. Interestingly, some interferenceringes could be observed in non-bonded areas. The spectrom-ter of these areas (line 6) has great difference from lines 7 andand is similar to line 1 (blank wafer). This result suggests that

he required structures have been destroyed and the chemicalnteraction is not present on these areas. The reason why largereas bonding of quartz wafers are difficult to obtain is not clear.

.2. Strength of the bonding wafers

In this study, bonding strength was evaluated using ahear force testing technique described as literature [24]. For5 mm × 10 mm well-bonded wafers, the strength of 1.78 MPaecorded and did not decrease after storage for 40 days. Thetrength is sufficient for most application of microfluidic chips.

.3. Influence factors of bonding

Annealing temperature plays an important role during low-

emperature-bonding process. We tried to bond the functionalafers at lower temperature, such as 100 and 150 ◦C, howevernly weak bonding was achieved even prolonging the reac-ion time to 12 h. Obviously, no enough energy is provided to

iraB

ce (A) and non-bonded interface (B).

reak or form the covalent bond at lower temperature. Gen-rally speaking, higher temperature is energetically favorable.owever, the organic layers on surfaces could be destroyedrior to bonding at too high temperature. This is supported byhe experiment facts that the surfaces of wafers become brownnd the bonding is failed at the temperature of higher than00 ◦C.

Additionally, the successful bonding still depends on anotherrucial factor that is the cooling speed of bonded pairs. The fastooling would prevent from releasing the tension produced dur-ng bonding and crack bonded pairs. In our experiment, it wasound that the cooling completed in 6 h could fulfill the require-ents of bonding and not cause the aforementioned problem.

.4. SEM images of bonding wafers

Well-bonded pairs obtained by this approach have high trans-arence and good smoothness of wafer surface. In some cases,ocal area was not bonded successfully. Similar to quartz wafers,nterference fringes could be observed in non-bonded areas.canning electron microscope (SEM) images of different inter-aces of well-bonded area and non-bonded area are presentedn Fig. 2. For well-bonded area, the interface is indistinct. Inontrast, an obvious microgap can be seen in the interface ofon-bonded area (Fig. 2B). Two independent organic layers inicrogap separate the substrates. These phenomena reveal that

hemical interaction has occurred between well-bonded sub-trates.

We believe that the failure of bonding is caused by particu-ates contamination on the surfaces or trapped air at the bondingnterface. It should be noted that these causes are easily avoidedn the experiment and have little influence on the application ofhis method.

. Conclusions

In this paper, a simple and effective bonding method was

ntroduced. Two wafers terminated by maleimide and furan,espectively, were put together with a clamp and annealed for 5 ht 200 ◦C to form C C covalent bond by Diels–Alder reaction.onding strength of well-bonded pairs was as high as 1.78 MPa,

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Lianxun Gao is a professor in the State Key Laboratory of Polymer Physics

16 M. Zhang et al. / Sensors and

hich is sufficient for most applications of microfluidic chips.dditionally, effective bonding has been achieved in common

aboratory without the requirement of clean-room facilities oromplicated cleaning process. At the same time, more mild tem-erature can prevent the channel of microfluidic chip distortedr collapse and shorten the cooling time.

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iographies

injie Zhang is currently a PhD in the State Key Laboratory of Polymer Physicsnd Chemistry, Changchun Institute of Applied Chemistry, Chinese Academyf Sciences. Her current research interests are in the MEMS, �-TAS and organicynthesis.

ianying Zhao is currently an instructor in the College of Chemical Engineer-ng at Shandong University of Technology, China; she received her PhD inhangchun Institute of Applied Chemistry, Chinese Academy of Science in006. Her current research interests are in the MEMS, �-TAS.

nd Chemistry, Changchun Institute of Applied Chemistry, Chinese Academyf Sciences; he received his PhD degree from Faculty of Science at Hokkaidoniversity in 1992. His research has focused on the MEMS, �-TAS, chirality’srganic compounds and polymer materials synthesis.