ADVANCED LTCC PROCESSES USING PRESSURE ASSISTED SINTERING

Transcription

1 ADVANCED LTCC PROCESSES USING PRESSURE ASSISTED SINTERING Heiko Thust Michael Hintz, Arne Albrecht Technical University of Ilmenau Ilmenau, Thuringia, Germany ABSTRACT Low Temperature Cofired Ceramics (LTCC) offer a wide range of possibilities to realize multilayer circuits with an almost unlimited number of layers. It permits the integration of passive components like conductors, resistors, capacitors and inductors into the circuit carrier. Together with the low dielectric losses, good thermal conductivity and stability the LTCC technology is eminently qualified for many applications. However, freely sintered LTCC substrates are often limited in their dimensional accuracy. The overall x/y-shrinkage and tolerances during sintering might be responsible for problems at component assembly and complicates the specification of the electrical properties. Beside this, the integration of cofired components is limited to pastes with similar shrinkage behavior. In the last years zero- shrink techniques have been introduced more and more in LTCC- processes. These techniques use different constraining mechanisms to limit the x/y- shrinkage during sintering. Consequences are lower tolerances. One of these zero- shrink techniques is the Pressure Assisted Sintering (PAS). It also allows the cofired integration of full metal structures like foils or platings. The main problem of such inlays is the structuring or positioning on the unfired LTCC layers. Some various techniques to solve this problem were tested. The results are very promising. With high conductivity and excellent geometrical accuracy, these techniques offer new possibilities for many applications. Keywords: LTCC, PAS, cofired platings, multilayer INTRODUCTION At present the Unconstrained Sintering (UCS) is one of the most used LTCC processes. Despite many advantages the application field of freely sintered LTCC substrates is limited. One reason for this is the low dimensional accuracy. The resulting tolerances in shrinkage during sintering might be responsible for component assembly problems. Constrained sintering essentially eliminates almost completely the x/yshrinkage and reduces tolerances. Different constrained sintering processes are known such as the Self Constrained Sintering (SCS), the Pressureless Constrained Sintering (PLAS) and Pressure Assisted Constrained Sintering (PAS). Self Constrained Sintering is quite a new technique and works without external constraints [3]. The mechanism of this process is based on the glass ceramic composition of the tape itself. At present the choice of compatible pastes is limited. Especially resistor pastes are not available. PLAS and particularly the PAS process use a laminated release tape on the external faces of the module to constrain the LTCC during sintering. The release tapes are based on ceramic particles which sinter at higher temperatures than those used during the LTCC firing. So they do not shrink and mostly prevent the rest of the substrate from shrinking in x/ydirection. UCS Sintering Methods Constrained Sintering SCS PLAS PAS Figure 01. Sintering methods In most cases a removal process for the release tape is necessary after firing. Sand-, glassball-blasting or water- jet technique are usable for this action. It is helpful to combine it with a previous brushing. The release tape influences the features of cofired surface prints. So it is often necessary to add postfire prints or plating processes to keep the conductor pads solderable and bondable.

2 The Pressure Assisted Sintering process is the complexest method. During this process a uniaxial force assists the constraining effect of the release tapes. Therefore, a special hot pressure furnace is needed. PAS- PROCESS The PAS investigations were done with such a special furnace. The hot pressure furnace consists of a standard muffle furnace with an additional press tool for uniaxial pressures. It allows vertical forces between 0.6 and 35kN to be controlled and temperatures up to 1000 C during the sintering process. push rod muffle of furnace LTCC substrate porous setter distributor block substrate with release tape porous setter (SiC) SHRINKAGE BEHAVIOR The shrinkage behavior of freely sintered LTCC is not only influenced by the sintering parameters. All process steps before sintering can also influence the shrinkage of a LTCC- module. The following process steps are particularly critical: Tape casting Storage time Preconditioning Screen printing (interaction between thinner of pastes and tape Drying after screen printing Wet processes (for instance FODEL(TM)) Lamination Transportation (mechanical stress) Also the layout of the module influences the shrinkage. Here, the number of layers, areas of metallisations and cavities are important. That is why despite the strong control of all process parameters the tolerances of freely sintered LTCC modules are around 0.2%. With their limited x/y- shrinkage constrained sintered modules promise considerably lower tolerances. The investigation shows that with Pressureless Constrained Sintering the absolute shrinkage in x/y direction is restricted to around 0.3%, (material A / B). Pressure Assisted Sintering allows to controll the shrinkage between % (expansion) (material A) and % (material B). The optimal pressure depends on the material used. With some materials 0.0% shrinkage and also expansion of the modules are possible but in this case the tolerances rise rapidly. Best results regarding low tolerances are optained by pressures allowing a shrinkage of %. Material A Material B Figure 02. Principle of PAS For the investigation of the shrinkage behavior of the different materials, modules of mm fired thickness were processed. Circles of silver filled vias were placed around a center via in the top and in the middle layer of the modules to measure the shrinkage. After lamination and firing, the shift of the vias was measured with an x- ray microscope. Wherever possible all materials were processed as specified by the manyfacturer. Only the firing profile was adapted to the PAS process. The release tapes and the setter cover the surface and slow down the burn out of the organic binder materials. That is why a longer burn out phase during the firing process of the LTCC was used. During the burn out and the sintering part of PAS constant pressures were applied. Shrinkage x/y 0,4% 0,2% 0,0% -0,2% Sintering Pressure [psi] Figure 03. PAS- Shrinkage behaviour of different materials The Self constrained material shows a shrinkage of % (material C). The difference to shrinkage of <0.2% as specified by the manufacturer is founded in the lower used lamination pressure. Because technical limits, a lamination pressure of only 3000psi was used

3 (recommended: 4900psi). When freely sintered the shrinkage of the other materials was 12-14% (material A) and 18-21% (material B). The tolerances of the constrained processed modules are between 0.1% (PLAS) and 0.05% (PAS and SCS) [3][4]. This allows the use of LTCC materials in larger dimensions than in the UCS process without assembly problems. MECHANICAL BEHAVIOUR The covering of the surface during the PAS and also the uniaxial pressure during sintering influence the mechanical behavior of the substrate. Some investigations were carried out to show the effects of these inluences. The surface roughness determiation shows a lower roughness of PAS processed than freely sintered substrates (figure 04). strength of LTCC substrates is the high spread caused in the inhomogeneous composition of glas ceramic composites. In this case, PLAS and PAS processed structures show a slightly better behavior. Beside this, at PAS the flexural strength rises with higher pressures (figure 06). Flexural strength [N/mm²] psi 50 psi 150 psi Sintering pressure freely sintered PAS processed Figure 06. Flexural strength of modules processed at different sintering pressures (material A) Measurement 1 Ra = 0.65 Ra = 0.44 Measurement 2 Ra = 0.69 Ra = 0.40 Average Ra = 0.67 Ra = 0.42 Figure 04. Roughness (material A) CAVITY FORMATION Constrained sintering methods improve the accuracy in x/y- direction, but a problem especially of the PLASprocess is the formation of cavities. Vertical edges and the surrounding area show deformations and a shift of the inner layers. In figure 07, the size of the influenced area around cavityies is shown. It strongly depends on the thickness of the substrate. At a thickness of 2mm up to a lateral distance of more than 2mm, a shift of the inner layers is visible. 0,20 Figure 05. Surface of free sintered (left) and PASprocessed LTCC after sandblasting (material A) Releasetape and sintering pressure limit the growing of cristal needles on the surface (figure 05). Another effect comes from the needed sand blasting process of the PAS processed module. The lower roughness may be an important feature for using of thin film materials on the surface. The mechanical strength of PAS and PLAS processed modules was determined by a 3- point test. It is comparable to freely sintered modules but is also influenced by sand blasting. A problem of the breaking Shift [mm] 0,15 0,10 0,05 0, Distance to cavity [mm] Thickness 2,0 mm 1,6 mm 1,1 mm 0,7 mm Figure 07. Shift of inner layers near cavi ties in PLAS processed modules with different thickness PAS can avoid these problems and allows the edge geometry to be controlled. The shape of vertical edges is directly dependent on the applied sintering pressure. The optimal pressure depends on the LTCC material

4 and the number of layers. Stepped cavities, cantilever and blind holes are very critical. An advantage of the PLAS and PAS processes is the lower lamination pressure. This leads to lower tolerance after lamination. Beside this, the loading of edges is strongly sinking. This can lead to better results in realizing cavities or holes. Paste printing Realization of conductors Thinfilm Full metallic inlays Standard Photoimagable Direct on LTCC Temporary carrier POCC Foil structuring Figure 08. Edges of cavities after PLAS (left) and PAS (right) FULL METAL CONDUCTORS Today, the most discussed advantage of Pressure Assisted Sintering is the advanced dimensional accuracy. In addition, this process offers also some other very special advantages. The pressure during sintering allows cofired structures to be used with different TCEs and without shrinkage during sintering, such as full metallic inlays. At present, for standard LTCC and HTCC (High Temperature Cofired Ceramics) applications only screen printing is used for realizing conductors. With disadvantages like low conductivity, limited resolution and low edge accuracy, this process narrows the application field of cofired ceramics. Most of these disadvantages could be avoided by using full metallic structures. This are structures which are not based on sintered metal particles like pastes but on platings or metall foils. On the Technical University of Ilmenau (Germany) different methods of structuring of conductors were tested or developed, for instance a process called Plating on Cofired Ceramics (POCC) and metal foil structuring in the direct way or using a temporary carrier. One problem of such processes is the strong sensivity of the unfired LTCC tapes. For instance, the direct contact with liquids especially acids is critical. So only processes with non-aggressive liquids and contact times of some seconds are applicable.to realize plating processes without direct contact between critical liquids and unfired LTCC tape, two ways are possible. Figure 09. Possibilities of realization of cofired conductors The first one uses an organic or metallic cover layer [2]. It has to be easily removable after plating without stressing the unfired tapes or has to sublimate during sintering. This direct POCC method consists the following process steps: 1. Application of the cover layer 2. Application of the conductive starting layer 3. Resist coating 4. Exposure and development of the resist 5. Plating 6. Resist removing 7. Etching of the starting layer 8. Removing of the cover layer 9. Drying In an other way it is possible to perform the plating process on a temporary carrier. In this case it is favorable to use a carrier composed of sublimating materials. After metal structuring this carrier can be laminated between the LTCC tapes. During the PAS process the temporary carrier sublimates totally and the platings are positioned inside the substrate. Also subtractive processes like etching or laser structuring of metal foils are possible in the direct or indirect way. Figure 10 shows an x-ray image of laser-structured Ag- foils in the middle of a 6- layer module.

5 Via Ag- Foil 1mm 1mm The edges are rough and not vertical. This is a strong disadvantage for radio frequency applications. The POCC process allows this problem to be solved. It is possible to build structures with an excellent geometry of vertical edges in combination with a high thickness (figure 12). The shape of the edges is mainly determinated by the resist and is scarcely influenced by the lamination and sintering process. This is a good condition to realize conductors with low high frequency losses. Figure 10. X- ray images of innerlayer conductors based on Ag- foils No cracks or displacements are visible and the connection between Ag- foil and vias (Ag- paste) is excellent (figure 11). So it is also possible to combine full metal and screen printed structures. 25µm Ag- Paste Especially in the field of high frequency applications is a growing interest in higher resolutions of LTCC conductor structures. Therefore in the last years high resolution screens and photo- imageable pastes were developed. However, the resolution of printed pastes is mainly limited by the particle size of the ingredients and the low conductivity. At present the resolution of printed or photo imaged inner layer prints is limited to 30-50µm. Full metal conductors have no limitations caused in particle sizes. The main limitation for the resolution of plated or etched structures is the resistresolution. That is why very small structures with wides of under 20µm are possible. Ag- Foil Screen printing Photoimageable prints Full metal Resistivity per square (10µm thickness) 4-6 mohm (silver) 4-6 mohm (silver) 2mOhm (silver) Figure 11. Connection between Via (Ag- paste) and Ag- Foil The resistivity is around mohm/square. Standard Ag- pastes have a value of 5mOhm/square at a thickness of 10µm. At a comparable thickness the full metal conductors present more than double conductivity. Screen printed structures have typical misshaped edges (figure 12 right). Thickness of conductors Resolution Roughness of edges Geometry of vertical edges Manufacturing effort 5-50µm 5-30µm 2->80µm low 50-80µm high 30-50µm high < 20µm possible high low low bad bad good low medium high Figure 12. Plated lines with thickness of around 50µm (left) and typically screen printed line of 7µm thickness (right) Via crossover no problem critical no problem Figure 13. Features of different processed cofired conductors

6 The direct comparison between paste based conductors and full metal conductors shows the potential and the problems of the different technologies. CONCLUSION Today the reasons for using of Pressure Assisted Sintering are mainly the higher accuracy of the fired structures and the possiblity to realize full conductor areas. This often justifies the disadvantages like additional effort for release tape, its removal and hot pressure furnace and the critical behavior of cofired structures on the surface. However the investigations have shown that the PAS- process has a wide potential. So the Pressure Assisted Sintering- process leads to slighty better mechanical behavior of the ceramic body and a very special advantage is the possiblity to integrate cofired full metal conductor structures Different methods for realizing of such structures like plating, etching and laser structuring of foils are possible. Advantages of such full metal structures are high electrical and thermal conductivity. Furthermore structures with excellent edge geometry, low roughness, high aspect ratio and high resolutions are possible. In this way these technologies are qualified for new applications in the fields of radio frequencies and power devices. REFERENCES [1] Mikeska, K.R., Shaeffer, D.T., Jensen, R.H. Method for Reducing Shrinkage during Firing of Green Keramik Bodies, U.S. PAT No [2] Albrecht, A., Botiov, J., Fischer, M., Drüe, K.H., Hintz, M.Wurmus, Alternative Ansätze zur Herstellung hochstromtragfähiger Leiter in LTCC, IMAPS Germany 2003, Munich [3] Barnwell, P., Amaya, E., Lautzen HeraLock Self-constrained LTCC Tape, IMAPS Nordic 2002, Stockholm [4] Hintz, M., Thust, H., Polzer, E., Generic Investigation on 0- Shrinkage processed LTCC IMAPS Nordic 2002, Stockholm [5] DuPont Electronics LTCC Design and Layout, Guidline Green Tape (TM) System [6] Skurski, M. Photoimageable Silver Cofirable Conductor Compatible With 951 Green Tape (TM), International Journal of Microcircuits and Electronic Packaging