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Advancing Microelectronics • Volume 29, No. 2 • March/April, 2002

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Photoimageable Thick-Films in Microwaves

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Barbara Dziurdzia, Department of Electronics, University of Mining and Metallurgy, PL 30-059 Cracow, Poland, 30 Al. Mickiewicza Str., Magorzata Jakubowska, Institute of Electronic Materials Technology, PL 01-919 Warsaw, Poland, 133 Wólczynska Str.

Abstract

Microwave circuits demands for high resolution patterning, rectangular cross-sections, sharp edges and smooth surfaces are met by the photoimageable thick-film technology successfully competing with traditional screen printing. The paper presents parameters of photosensitive pastes, describes the exposure and development units necessary for carrying out the photoimaging process, depicts manufacturing steps that lead by photoforming to fabrication microwave structures of parameters comparable to thin film ones. Edge-coupled filters of centre frequency 6, 12, 14 GHz built on microstrip resonators are the test samples. Transmission and reflection characteristics of filters are measured using vector network analyser and compared to the simulated characteristics.

Key words: thick films, photoimaging, microwaves

1. Introduction

Until now thin film technology has been most commonly used in manufacturing microwave hybrids. However, an often overlooked option for thin film technology is thick-film technology especially in a lower range of microwave spectrum. Thick-film technology offers improvement of circuit thermal dissipation and environmental robustness, low cost and feasibility to high volume production that is an important factor in the time of explosive growth of wireless communication. Moving the useful electromagnetic spectrum into higher regions of frequency implies searching for more precise method of patterning which results in enhanced circuit performance with dense interconnects, smooth surfaces, rectangular walls and sharp edges of microwave structures. This is beyond capabilities of traditional screen printing even when fine screens and advanced fritless pastes are used and that is the reason why photoimageable thick-film technology is emerging on the stage of modern technologies.

2. Photoimageable thick-film technology

2.1. Photoimaging process

The photoimageable thick-film technology is a combination of conventional thick-film technology with some processes typical for thin film technology. The first manufacturing step is the deposition of the photosensitive paste on a substrate by blank screen printing. The paste is levelled at room temperature and then dried at 800C. The dried layer is exposed to UV light through the negative photomask to form a latent image which afterwards is developed in a spray processor using appropriate development solution. At the end, the developed pattern is fired in the furnace typical for thick-films. Fig.1 shows processing sequence for photoimageable conductors in comparison to etching of fired thick-film conductors and reveals the simplicity of photoimaging.

In contrast to standard thick-film process, the photoimageable process separates printing and pattern generation allowing independent optimisation of these two manufacturing steps. Photosensitive pastes are designed to form a very smooth surface, free of pinholes and other defects after printing. That is the job of exposure and development process to provide high resolution and good quality of the pattern.

Photoimaging makes available 50µm lines and spaces with accuracy of 5µm and smooth, well-defined, near-vertical edge features, while dielectrics resolve 75µm vias on a125µm pitch. The standard resolution for traditional thick-film technology is 150µm line / 200µm space with accuracy 25µm and 250 µm via diameter on a 500µm pitch [1,2].

Photoimageable technology requires special thick-film materials. Photosensitive pastes contain a photoinitiator which during UV exposure absorbs light and starts polymerization of the acrylic monomers turning them into photopolymers. Photopolymers harden in the exposed areas while the unexposed areas of the layer are easily dissolved in the developing solution and removed under the delicate spray of it. Photoimageable thick film conductors and dielectrics have post-fired properties similiar to standard thick-film materials because all photosensitive ingredients are removed in the firing process.

2.2. Photoimageable conductors and dielectrics

Photoimageable conductors and dielectrics of several manufacturers such as Du Pont, ESL, Heraeus and Hybridas are available on the market.

Du Pont offers photoimageable gold conductors Fodel Au 5659 and Au 5956L, silver conductor Fodel Ag 4556 and dielectric Fodel 4060.

ESL offers two types of silver photoimageable conductors for fine-line applications: ESL 9904 and ESL 9904A. ESL 9904A is recommended for use on bare alumina while ESL 9904 only on top of 4911 or 4911-B dielectrics.

In all areas where Du Pont and ESL pastes are printed, dried, exposed and developed, the safe yellow light should be used to prevent accidental polymerization.

Hybridas offers photoimageable gold, silver, palladium silver and platinum conductors as well as resistive and dielectric pastes. Due to the low sensitivity of Hybridas materials to white light all technological operations can be performed under normal lightining conditions.

In experiments reported in this paper silver photoimageable conductor Fodel Ag 4065 was used and its basic properties are collected in Table 1.

2.3. Equipment

The photoimageable process requires a typical equipment for thick-films completed by the exposure and development units.

In experiments reported in this paper the exposure unit was adapted for thick-film purposes on the basis of the unit typical for processing thin films while the development unit is originally designed for the spray development of thick-films.

The exposure unit (Fig.2) is equipped in a high pressure mercury lamp of power 500 W with efficient cooling, ignition and shutter driving systems and an exposure timer. The lamp emits UV light in the wavelength range from 365nm to 400nm. The light beam collimated by the set of condenser lens travels in a horizontal path toward the exposure mirror where it is reflected down to asubstrate. The light intensity equal to 25mW/cm2 is achieved on the surface of the exposed substrate. The unit is provided with a mechanical adjusting system that enables the positioning the light source precisely in the focus of the condenser lens and the mask and substrate alignment system that permits setting the substrate in relationship to the mask under control of a split-field microscope. The condenser lens are protected by a very efficient infra red cut-off filter that is additionally cooled with a stream of decompressing nitrogen.

The development unit (Fig.3) enables developing of the exposed pattern by washing away the parts of the layer which aren’t polimerised under UV radiation. The substrate held horizontally rotates at a speed ranging between 250 and 400 rpm. Development solution, which is 1% Na2CO3, is delivered to the substrate surface through a narrow tube ended with a nozzle which atomises the liquid to the spray. The development is followed by a short deionised water rinse that is processed by the second nozzle actuated in the same way as the developing nozzle. At the end the spinning substrate is dried as the result of rotation. Development solution and water are pumped into the nozzles with miniature gear pumps.

It is important to expose the substrate printed with dry photosensitive paste by high intensity UV light of aproper spectrum in a maximum short time to prevent shrinkage of the layer and the edge curl. It is also valid to develop the pattern under constant carefully chosen pressure of development solution, otherwise non-hardened parts of the layer won’t be removed from the substrate. The time of exposure and development must be chosen as a result of a series of introductory experiments.

3. Experiments and results

Band-pass edge-coupled filters of centre frequency 6 GHz, 12 GHz and 14 GHz have been designed and performed with the use of photoimageable technology.

The design is a symetrical structure (Fig.4) that consists of six resonators of widths wi, lengths li and gaps between strips si. Table 2 consists geometrical dimensions of resonators for filters of centre frequencies 6 GHz (F061), 12 GHz (F122) and 14 GHz (F141). The narrowest gaps s1 between the strips of the first resonator are equal to 140 µm, 100 µm and 70 µm in filters F061, F122 and F141, respectively, and formation of these gaps is achallenge for traditional screen printing.

Silver photosensitive paste Fodel Ag 4065 was used for deposition on 96% Al2O3 substrates. Time of exposure of about 30 sec. and time of development of about 10 sec. were applied. The narrow- est gaps of filter structures were easily performed by the photoimaging process with accuracy of patterning +/- 5µm.

The thickness of the fired layer was 10+/-2 µm. The shrinkage after firing equal to 20 µm in X-Y direction was taken into account when negative photomasks were designed.

A characteristic edge curl (Fig 5, Fig.6, Fig.7) along the strips was observed however smooth, even edge definition (Fig.8) and high, vertical walls of the layer (Fig. 9) meet microwave requirements well.

Transmission and reflection characteristics were measured for all Fodel versions of filters using vector network analyser HP 8720C. Transmission characteristic describes forward transmission coefficient of a filter (S21) in function of frequency while reflection characteristic depicts the input reflection coefficient (S11) with the output matched to Z0 in function of frequency. The parameter S21 is commonly called attenua- tion of a filter while the parameter S11 is directly related to VSWR and impedance.

Fig.10a,b, Fig.11a,b, Fig.12a,b show the experimental transmission and reflection characteristics for the filters F061, F122 and F141 on the background of their com- puted characteristics. Table 3 collects filter parameters determined on the basis of these characteristics.

4. Conclusions

It has been shown that in the range up to 14 GHz photoimageable thick-film technology on alumina can offer an attractive solution for constructing microwave circuits by providing precise patterning of accuracy comparable to the one of thin films. When combined with an-other new advanced thick-film ceramic system - Low Temperature Co-Fired Ceramic (LTCC), photoimageable technology can reach an even much higher frequency limit even up to 45 GHz. This combination has become the subject of intensive investigations in many technological centres all over the world [3,4,5,6].

References:

1. Du Pont Data Sheets 2001.
   
2. Tredinnick M., Barnwell P., Malanga D., “Thick-film Fine Line Patterning — A Definite Discussion of the Alternatives,” 2001 International Symposium on Microelectronics, Baltimore, USA, Oct. 9-11, 2001, p.679-681.
   
3. Barnwell P., “Ceramic Circuitry — A Technology for the Future,” 38th IMAPS Nordic Conference, Oslo, 2001, p.84 – 88.
   
4. Bacher R.J., Wang Y.L., Skurski M.A., et al., “Next Generation Ceramic Multilayer System,” 2000 International Symposium on Microelectronics, Boston, USA, Sept. 20-22, 2000, p. 202-207.
   
5. Barnwell P., Zhang Weiming, Lebowitz Jeff, et al., “ An Investigation of the Properties of LTCC Materials and Compatible Conductors for their Use in Wireless Applications,” 2000 International Symposium on Microelectronics, Boston, USA, Sept. 20-22, 2000, p. 659-664.
   
6. Devlin L., Pearson G., Pittock J., “RF and Microwave Component Development in LTCC,” 38th IMAPS Nordic Conference, Oslo, 2001, p.96-110.
   

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