Development and Processing of an Anodic Bondable LTCC Tape

Development and Processing of an Anodic Bondable LTCC Tape E. Müller, T. Bartnitzek, F. Bechtold, B. Pawlowski, P. Rothe, R. Ehrt, A. Heymel, E. Weiland, T. Schröter, S. Schundau, K. Kaschlik VIA electronic GmbH Hermsdorf, Thüringen, 07629, Germany Phone: +49 (0)36601 81529, Fax: +49 (0)36601 81530 E-mail : e.mueller@via-electronic.de; t.bartnitzek@via-electronic.de Abstract In this research project we developed and tested successfully a new LTCC tape for anodic bonding between LTCC and silicon. With a CTE = 3,4 x 10-6 /K it is matched to the CTE of silicon and close to silicon in its behaviour. Driving force was miniaturization and increasing integration density of an acceleration sensor. The targetted size reduction was 40%. The anodic bondable wafer had been substituted with the anodic bondable LTCC multilayer. The sensor, geometric size 4,5 x 7,2 x 1,5 mm 3, had successfully been designed and manufactured and qualified. After material development, the LTCC processability of the developed tape had been tested as well as material compatibility with gold thick-film-metallization. These LTCC substrates had been processed with thin film technology, with adapted material systems, as well. We have manufactured metallized anodic bondable LTCC multilayer wafers, size up to 4 inch, thickness 0,6mm suitable for wafer level packaging. The measured surface roughness of the polished LTCC substrates was Ra < 100nm, substrate flatness < 5µm/10mm. The anodic bonding tests with LTCC wafers had been carried out with applied bonding temperatures of 350-400, voltage 1,0-1,5 kv and demonstrated that the novel LTCC material is appropiate for use in silicon (MEMS) technology. Key words: LTCC, Anodic Bonding, Accelerometer, Miniaturization, Ceramic Packaging, MEMS 1. Introduction The worldwide demand of miniaturization in microelectronics and components and increasing packaging density was the main driving force for the development of this new material: the anodic bondable LTCC tape. LTCC technology offers a potential solution for hermetic packaging direct at wafer level. Goal in this research project was size reduction of an existing acceleration sensor of 40% by the fact that the wafer, which usually joins with silicon wafer at anodic waferbonding, was substituted with anodic bondable LTCC multlayer wafer. LTCC means Low Temperature Cofired Ceramic. The core of this technology is a low temperature sintering, flexible ceramic foil. Green tapes (= unsintered foils) are mechanically structured, printed with established thick film technology, laminated together and then sintered (cofired) at about 900. The result is a three dimensional wired high density multilyer board made of ceramic. The multilayer board is further processed with the well known interconnection technologies of screen printing, chip and wire bonding and sourface mount technology. Anodic bonding, the direct wafer bonding technology is a hermetic sealing technique suitable for sensor packaging. The anodic bonding of LTCC with silicon at wafer level with processing temperatures of 350 400 and U = 1,0 1,5 kv is a new concept of anodic bonding. This technique enables new functionalities and packaging and enhancement of component performances. Using LTCC technology for the anodic bondable substrates allows a series of advantages like: - multifunctionality with 3D interconnected multilayer substrate - ceramic multilayer packaging structure with hermetic packaging between chip and LTCC

- decreasing packaging size with direct interconnections between chip and package - use of standard thick film technology - possibility of thin film structuring - standard assembly and interconnection technologies of the LTCC modul - potential of passive element integration (to move passives from the expensive chip area into the LTCC area) - possibility for local bonded areas: size of the bonded area can be controlled by structuring the LTCC surface in green or sintered state - variable single layer thickness down to 50µm and polished down to 10-20µm - potential for MEMS products - LTCC standard wafer technology enables efficient low cost production Within this research project, the regional partners Hermsdorfer Institut für Technische Keramik, Hermsdorf (HITK), Institut für Fügetechnik und Werkstoffprüfung Jena (IFW), Siegert TFT, Hermsdorf and M & S, Jena worked together with VIA electronic, Hermsdorf and present the successful developed anodic bondable LTCC tape. 2. Requirements Direct wafer bonding puts high demands on wafers. Challenges are imposed by reducing or eliminating mechanical stress due to differences in the coefficients of thermal expansion of the package materials: LTCC substrate and silicon. The development of the anodic bondable LTCC tape includes several requirements and investigation aspects like: material composition development with anodic bonding properties and matched CTE to CTE of silicon fitting and optimization of tape casting process LTCC processability compatibility with thick film Au metallization system high surface quality: sufficiently flat 4 inch substrates and low sourface roughness adequate polishing technology for ceramic substrates of diameter 100mm and substrate thickness < 1 mm anodic bonding processability at 350-400 wafer level packaging Therefore, detailed scientific investigations had to be carried out. 3. Material Development A new modified ceramic composite tape has been developed. The tape casting process, the binder burnout and the firing were adapted to the new tape material. The composition of the -ceramic tape was optimized by partially substituting the key ingredients and borosilicate by CTE min -es, cordierite and Na + -es. The Na + - content is necessary to realized the ionic migration between silicon and -ceramic. The other materials are basic ingredients for the adaptation of the CTE. Several material combinations were tested as shown in the following fig. 1. cordierite CTE min cordierite CTE min Fig. 1: scheme of LTCC composition, partial substitution of basic ingredients by cordierite and CTE min -es The original -ceramic-tape is a composite between and borosilicate (fig. 1, above left). The is responsible for forming the composite and inhibits the firing process. The lowers the firing temperature. By partial substitution of the individual components it should be possible to lower the coefficient of thermal expansion of the tape to match that of silicon and to provide enough Na + for anodic bondability. In a first step the partial substitution of by CTE min - es (fig. 1 above right) was tested. Different Na + -containing es like Ceran and Sitag proofed ineffective. Therefore in a second step the was partially substituted by cordierite (fig. 1 below left). In the third test series, both substitution methods were combined. Alumina was partially substituted by cordierite and the by CTE min -. Finally, the coefficient of thermal expansion of the new composite tape is equal to that of silicon. To realize a tape thickness of about 100µm with high surface quality it was essential to use fine

grained raw materials. A procedure of powder pretreatment was used to obtain fine-grained powders with mean grain size of 600 nm 700 nm. Milling experiments with cordierite and different types of were performed. As an example, fig. 2 shows the particle size of a fine milled borosilicate, milled in a tumbling mill (left) or an attrition mill (right). The milling in an attrition mill results in grain sizes of 600-700 nm. The challenge consisted of processing this anodic bondable LTCC as wafer sized multilayer metallized stack with multi ups in 5 inch size (green) for realization of an 4 inch anodic bondable LTCC multilayer wafer. layer Stacked vias 100 80 Sedigraph 5100 Sympatec Helos, Trockendisp. 100 80 Sedigraph 5100 Symatec Helos, Naßdisp. BET: 16,09 m²/g 60 60 40 20 40 20 Fig. 3: Outline cross section of anodic bondable LTCC 8-layer substrate, with stacked vias, cross section, thickness 0,6 mm 0 0,1 µm 1 µm 10 µm Korngröße Feinmahlung Borofloat 33 (Trommelmühle) GV 36 - Naßmahlung (MEK) 96 Std. Fig. 2: Fine milling of borosilicate The processing of the raw materials is a key step in the preparation of the novel tapes. Different material combinations were tested for tape casting. Ceramic slurry formulations have been developed. A stress-free tape without drying shrinkage was obtained using an optimized drying technology, special carrier tapes and new solvent compositions. The new LTCC tape: borosilicate / Al 2 O 3 / cordierite composite was prepared with a coefficient of thermal expansion of 3,4 ppm/k and a Na + - content of 1,7 wt %. 4. LTCC Processing 1,03 µm 1,35 µm 0,1 µm 1 µm 10 µm Korngröße Several series of various developed LTCC tapes were tested and processed. To investigate their LTCC process compatibility, running through all LTCC-processing steps like sheet cutting, conditioning, punching, screen printing, stacking, laminating and sintering had been carried out, see fig. 3 and table 1. One LTCC tape with a CTE of 3,4 ppm/k at 400 fulfills LTCC processability and also best anodic bonding properties without inducing thermomechanical stress. 0 Feinmahlung Borofloat 33 (Attritor) GF 06 - sprühgetrocknetes Produkt 0,62 µm 1,18 µm Each process step required adapting of the processing parameters. Optimization of laminating and sintering technique was necessary. Sintering shrinkage with shrinking tolerances were evaluated. Table 1: Processability of anodic bondable LTCC tape LTCC processing step Processing Tape handling, 120 µm not critical thickness, sheet cutting, removing of carrier foil Via Punching, 200µm not critical Via filling not critical Conductor printing, not critical standard Stacking, positioning not critical Isostatic lamination not critical Sintering, 5 inch not critical, substrate, 0,6 mm after thickness optimization metallization compatible compatibility Au thick film pastes After several metallizing tests, a compatible metallization system in gold was found and used in manufacturing of the metallized LTCC wafer substrates, see fig. 4 and 5.

The principle bases on the Na + - diffusion and building of SiO2- bridges and oxide layer at the interface -silicon with tensile strenght of several MPa. Sourface roughness, waviness and chemical composition of the materials mainly determine bonding results. Fig. 4: Anodic bondable 4 inch LTCC wafer substrate, metallized, multi up, 200 µm Au-vias The anodic bonding properties of this new LTCC are based on its adequate component which contains the Na +. After bond tests of several LTCC tapes it was clear that only when essential aspects are taken into account, optimal bonding is possible: - CTE matched on that of silicon - CTE difference LTCC-Si from 25-400: < +/- 0,2 ppm/k - sufficient Na + -content : 1-2 wt % - minimal sourface roughness Ra < 100nm - Wafer flatness < 5 µm/10mm - Bonding temperature: 350 400 For obtaining a good bonding result it is very important to have high qualitative polished substrate surface. Therefore, polishing technique for ceramic substrates of diameter 100mm with substrate thickness < 1 mm and flatness < 5µm/10mm with metallization (vias on surface), Ra < 100nm represented a real challenge. Experience and optimization of the polishing process were necessary to fulfill all these requirements. Fig. 5: Anodic bondable LTCC wafer, 4 layer, wafer thickness 0,5 mm, with embedded metallization, polished Tested bonding parameters: - temperature: 300-400 - voltage: 1000-1500V (possibility to use lower voltage) - bonding time: 3-20min, see fig. 6 We obtained following LTCC substrate values: Sintering shrinkage S x, y = 16% +/- < 0,2 % (within the same tape lot, at 25 MPa) Sourface roughness Ra as fired 0,2 µm Substrate flatness as fired < 10µm/10 mm For big sized wafer processing, tolerance of sintering shrinkage +/- < 0,2 % will decrease yield substantially so, shrinkage tolerance of the LTCC substrates have to be improved further on. 5. Anodic Bonding Processing The standard anodic bonding process refers to bonding of Na + - with silicon, in direct contact, without adhesive, at high voltage U = 1,0 1,5 kv and temperature of 400. Fig.6: Anodic bonded LTCC test sample, diameter 50 mm on Si-wafer with diameter 100mm

Good bonding between this metallized LTCC material and Si-wafers over the whole 4 inch wafer surface had been obtained, see fig. 7. electrodes Silicon LTCC Silicon Glass Fig.9: Outline cross section of acceleration chip sensor and bonded layers LTCC Fig.7 : SEM, cross section of anodic bonded LTCC with Silicon The new LTCC with CTE of 3,4 ppm/k matches the CTE of silicon, see fig. 8. Bonding strenght tests (pulltest, Instron 4411) showed that not the LTCC-Si bonding area had been destroyed and broken but the silicon or LTCC material in the joined region. There was no bowing of the joined layer so that insignificant thermo-mechanical stress will be induced in the LTCC and silicon layer. The metallized LTCC wafer had been bonded simultaneously with the Si-wafer and wafer, a three layer stack as 4 inch multi up. The electrodes had been sputtered in aluminium on LTCC wafer connecting the Au-vias with thin film CrNi and Al-layer system. Singulation had been carried out by dicing saw, see fig. 10. CTE (ppm / K) 4,5 3 1,5 0 40 60 100 200 300 Temperature () LTCC with Au-vias LTCC unmetallized 400 500 Fig.10: Acceleration chip sensor, LTCC wafer bonded with Si-wafer. Left: before singularization, right: singularized 4,5 x 7,2 x 1,5 mm 3 Testing and functional measurements of the manufactured demonstrator were performed with success. Fig.8: CTE measurement on anodic bondable LTCC with and without metallization (vias) 6. Demonstrator The outline of the realized demonstrator, an acceleration chip sensor with differential capacitive method, is shown in fig. 9. 7. Conclusions In this research project we have obtained a very good result in material development and processing of a new functional LTCC tape, the anodic bondable LTCC, with anodic bonding properties and CTE of 3,4 ppm/k, closely matched to CTE of silicon. After material development, the LTCC processability of the anodic bondable tape had been demonstrated as well as material compatibility with gold thick-film-metallization. With this new LTCC tape and anodic bonding wafer technology we have manufactured successfully a miniaturized acceleration chip sensor. Its functionality had been proven.

Therefore, joining of metallized LTCC multilayer and silicon in a wafer process without any intermediar bonding layers is possible. Future chip packaging will include joining chip and LTCC, some functions will be in chip, some in LTCC. The possibility of using LTCC substrates for MEMS applications is given as well as the potential of application for biological and chemical sensor chips with fluid channels. Overall manufacturing cost may decrease because of the reduced packaging size. The good results in development of this new LTCC tape led to the contact and common activities with new partners. Also, ideas of new innovative packaging concepts and new products are in discussion. Acknowledgements The work carried out was supported by the German Ministry of Education and Research (BMBF) in the research project MABOGOS (FKZ 03WKF01B, FANIMAT). References [1] Gerlach A., Maas D., Seidel D., Bartuch H., Schundau S., Kaschlik K., Low-temperature anodic bonding of silicon to silicon wafers by means of intermediate layers, Microsystem Technologies, Band 5 (1999) H 3, pg. 144-149 [2] Müller E., Pawlowski B., Weiland E., Brode W., Heymel A., Kaschlik K., Buß W., Mikrostrukturierte und anodisch bondbare Glaskeramikfolien für optische und sensorische Anwendungen MABOGOS, Abschlußbericht, BMBF Projekt 03WKF01, 2004 [3] Bechtold, F.: Technologische Herausforderungen zur Verbesserung der Einsatzmöglichkeiten von LTCC, Symposium der DKG Verfahren der keramischen Mehrlagentechnik: Stand und Zukunftsperspektiven, Erlangen 2004 [4] MEMS packaging Microphone, VTT electronics, http://www.vtt.fi/ele/research/ope/references/me ms_microphone.htm [5] Hunt B., LTCC integration gets fired up, Microwave Engineering, June 2002 [6] Bergstedt L., Persson K., Printed Glass for Anodic Bonding - A Packaging Concept for MEMS and System on a Chip, Advancing Microelectronics, Vol. 29, No. 1, 2002