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Advancing Microelectronics • Volume 29, No. 1 • January/February, 2002

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Printed Glass for Anodic Bonding - A Packaging Concept for MEMS and System On a Chip

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Leif Bergstedt and Katrin Persson, The Imego Institute, Aschebergsgatan 46, SE-411 33 Göteborg, Sweden, Phone: +46 31 7501800, Fax: +46 31 7501801, E-mail: leif.bergstedt@imego.com

Abstract

MEMS packaging is a multidisciplinary field where both the assembly and integration process have to be considered. To continue the miniaturization of microelectronics, efforts have to be made towards increased packaging density substrates and minimized area engaged by passive components. LTCC technology offers a potential solution for future applications. For testability, reliability and batch production reasons it is preferred to have a hermetic packaging method directly on the wafer level. It is also preferred to have a packaging system that can interact with the functions on the chip. Added to that, in many cases having passives buried in the chip carrier is better if the interconnection path is short.

This paper describes the work performed at Imego of attaching screen printable glass on LTCC carriers, which then is used for anodic bonding directly to silicon wafers providing a hermetic encapsulation. This opens up new dimensions by using LTCC as an interposer for CSPs in MEMS applications and later on in more common electronic applications.

Introduction

Recently, the interest for Low Temperature Co-fired Ceramic (LTCC) technology has increased. The automotive industry and telecommunications are areas where the technology has been used. The technology is said to have a great potential in RF-telecom products and also for optoelectronics and sensor packaging [1, 2]. The coefficient of thermal expansion (CTE) for LTCC materials is typically around 5 -7 ppm/K, which makes the substrate compatible for flip chip components. The technology has, however, one significant drawback and that is the shrinkage of the material, which is about 15% during the firing process. The thermal conductivity of the LTCC material can be improved by thermal vias [1].

By screen-printing glass suitable for anodic bonding directly on LTCC substrates and using anodic bonding to attach the LTCC to a silicon wafer, small hermetic packages can be created that are ready for dicing. An important advantage with this approach is that the dicing, handling and encapsulation issues can be avoided [3]. Wafer level bonding in this case results in chip structures each with their own protective cap. One drawback with the technique is that it may add cost to the device fabrication. However, because of the reduced packaging and testing costs, the overall manufacturing cost may decrease [4].

There are several wafer-bonding techniques available for silicon wafers [5]. Wafer bonding is the joining of two materials where only the properties of the materials themselves provide the bonding. Surface roughness and the chemical termination of the surfaces are important factors that determine the result of the bonding. The waviness and form of the wafers, the stress in the materials and the thickness of the wafers are also of great importance [6].

Anodic bonding of silicon to glass

Anodic bonding between silicon and glass (Pyrex 7740) is a well-known process [7]. Anodic bonding can be used both on plain glass wafers and on etched glass with cavities. A typical thickness for the silicon-wafer and the Pyrex-glass is about 500 µm each. It is a low temperature process for sealing glass to silicon where the silicon is biased positively. The voltages are in the range of 400 – 1000 V and the temperatures used are about 400 ºC. Sodium ions within the glass, move through the glass to the cathode forming a depletion layer close to the anode. The result is a very high electrical field and this causes the motion of oxygen anions, which lead to the formation of an oxide layer at the metallic interface. Published results indicate that a minimum thickness of the glass layer is required to ensure good bonding. The minimum thickness of the glass layer depends on the concentration of alkali ions in the glass [8].

There are some products on the market that use anodic bonding, for example [9]:

· Pressure sensors — Bosch

· Accelerometers — Sensonor

· Gyroscopes — British Aerospace

· Micropumps — Debiotech

The classical arrangement for an anodic bonding process is given below in Fig.1.

Anodic bonding of silicon wafers to glass-coated silicon wafers

The glass could be attached to the silicon by either spinning methods or by sputtering. This technique is mostly used for stacking wafers or for using silicon wafers as lids. Spin-on glasses are used in microelectronics for the deposition of silicon oxide. Work has been reported on a low-cost technique for preparation of glass layers suitable for anodic bonding of two silicon wafers [8].

Glass-frit bonding

This is a method used to stack silicon wafers with “thermo-compressed melted glass.” There is a possibility that this method may cause stress in the die if the glass used has a CTE that is different from silicon [4].

All these three above-mentioned methods give little possibility of interposing to a component carrier on a higher level as a PCB.

Selective deposited glass for anodic bonding

The basic idea with this concept developed at Imego is to develop a chip attach method from an established component carrier system, which has a technical environment that is close to the chip technology.

LTCC, chosen for this application, is a multi-layer glass composite with a CTE similar to that of silicon. This technology provides a complete infrastructure with conductors, vias and buried passives. A screen-printed or photo-imageable glass pattern for anodic bonding on a LTCC wafer forms an interface media in the LTCC-program that gives possibilities to direct attach to silicon wafers.

This gives the following possibilities:

- In MEMS applications part of the MEMS-function could be maintained in the LTCC carrier.

- For biological and chemical sensor chips the LTCC carrier can provide gas or liquid interface to the sensor chip.

- Full electrical interconnection possibility to any point of the chip.

- The LTCC carrier gives full mechanical and electrical interposer capability to PCBs, etc.

The glass has been screen-printed onto LTCC wafers, Fig 2., that have been anodically bonded to silicon wafers. There is a problem due to waviness of the LTCC substrates and also the surface roughness of the glass paste after sintering is not satisfying.

A lot of work is going on to find a polishing method to reduce the surface roughness and making the surface more suitable for anodic bonding. First trials have been performed with screen-printed glass paste directly on Pyrex-glass wafers, which then have been anodically bonded to silicon wafers with successful result.

Examples of the experiments can be seen below in Figs. 3 and 4, showing both the glass and the silicon side of two separated joints. The first one is screen-printed glass paste on a glass wafer, which was anodically bonded to a silicon wafer. The second one is from a glass patterned LTCC wafer bonded to a silicon wafer.

Defined cavities on LTCC

In standard LTCC technique a completely flat top surface of the LTCC is preferred. This flatness is maintained by glossy steel-sheets during the vacuum press cycle in the process flow. It is possible to make defined pressmarks in the LTCC-surfaces by having profiled steel-plates during the pressing operation. The pressmarks considered are about 50-60 µm in depth. There are a number of applications for these cavities. One example could be stand-off for microwave flip-chip where conductors on the substrate should not be too close to the chip surface. For sensor applications a defined cavity could be an active part of the sensor function. One example of this is capacity surfaces to accelerator MEMS chip, shown in Fig. 5.

The position of the accelerator element is measured by the capacitor value.

Tube connections to LTCC

One of the main technical problems for industrializing biological and chemical sensor-chips is the connection of liquids and gases to cavities in the sensor. In plain words: there is a lack of “plumbing” methods. LTCC provided with a glass pattern for anodic bonding opens up a complete toolbox for this kind of application. An example is shown in Fig. 6 below.

As LTCC shrinks about 15 % during the firing process it is possible to have a gas-tight connection to a tube, if the dimension is correct. However, the tubes must be e.g., gold-plated because of the high firing temperatures. With other metal tubes there is a risk of oxidation of the tube itself. Some initial tests have been performed, which among other things have shown that the LTCC must not be too thin.

Thin glass bonding to LTCC

At normal anodic bonding processes an electric field is put through 500 mm of glass, which is a common thickness of glass wafers. This means levels of several hundred volts. Sputtered glass on silicon wafers allows lower voltage since the silicon is more conductive than glass. The high voltage and temperature is a disadvantage when combining MEMS and IC devices on a single chip.

The thin layer of glass provides a limited supply of Na+ and O2- ions available in the glass film [9]. Sintef has studied the sodium distribution in thin-film anodic bonding [10]. The bond strength was found increased when bonding in nitrogen instead of a vacuum.

As it is possible to have good electrical distribution in all directions in a LTCC carrier; it is possible to have controlled distributed network for the anodic bonding. This might give the possibility of having anodic bonding of LTCC wafers to standard CMOS-wafers. Below are two figures showing thin glass screen-printed on top of AgPd conductors from a top view, Fig. 7 and a schematic drawing of printed glass on a distributed network on a LTCC wafer with a connection point for the anodic bonding possibility, Fig. 8.

Conclusions

The printed glass technology described in this paper opens up the possibility of using LTCC as a substrate to join MEMS wafers to. Today, such wafers are joined to glass or other silicon wafers before dicing, to avoid contamination problems. The prospect of LTCC as a substrate allows for cheaper chip scale packages, which in turn can become an essential factor in bringing MEMS products to a mass-market, due to the lower total cost.

The research characterization process is known and the focus onwards will be on technology and industrialization.

Acknowledgments

The authors would like to express their appreciation to

· Anders Engdahl and Colin Pickering, DuPont Microcircuit Materials which is supporting the project with LTCC knowledge and screen-printable glass for anodic bonding which is compatible with the Dupont Green Tape program.

· Kari Kautio, VTT Oulu who has supported us with LTCC-samples and industrial LTCC knowledge.

· “The Swedish Glass Research Institute” which has performed SEM-analysis of re-flowed glass and also for their basic glass knowledge.

References

1. J. Lenkkeri, “LTCC Technology Development,” The 2001 European Systems Packaging Workshop, January 22-24, Barcelona, 2001.

2. K. Kautio, “Multilayer ceramic process development,” Research activities in optoelectronics and electronics manufacturing 1998, VTT Electronics.

3. Patent pending WO0129890 A, 2001-04-26, “Method relating to Anodic Bonding,” L. Bergstedt, G. Andersson and B. Ottosson.

4. A. P. Malsche, C. O’Neal, S. B. Singh, W. D. Brown, W. P. Eaton and W. M. Miller, “Challenges in the Packaging of MEMS,” The International Journal of Microcircuits and Electronic Packaging, Volume 22, No. 3, pp. 233-241, 1999.

5. http://www.eecs.uic.edu/~peter/ eecs449/le/Wafer_Bonding.html

6. Doctoral thesis, “Wafer Bonding — Problems and Possibilities,” M. Bergh, Chalmers University of Technology, 1998.

7. G. T. Caldas, E. G. Rodrigues and R. Furlan, “Study of Anodic Bonding between Silicon and Pyrex 7740.”

8. H.J. Quenzer, A.V. Schulz, T. Kinkopf and T. Helm, “Anodic Bonding on Glass Layers prepared by a Spin-On Glass Process: Preparation Process and Experimental Results,” Transducers ’01, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, June 10-14, 2001, pp. 230-233.

9. T. Rogers, “Anodic Bonding,” course given by Applied Microengineering Ltd, FSRM Training in Microsystems, Didcot, England, 26th March 2001.

10. M.M. Visser, S. Weichel, P. Storås, R. de Reus and A.B. Hanneborg, “Sodium distribution in thin-film anodic bond-ing,” Sensors and Actuators A 2979 (2001) 1-6.

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