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Thin film, thick film microstrip band pass filter: a comparison and effect of bulk overlay
Microelectronics Reliability 42 (2002) 1953–1958 www.elsevier.com/locate/microrel
Thin film, thick film microstrip band pass filter: a comparison and effect of bulk overlay Sunit Rane a
a,*
, Vijaya Puri
b
Centre for Materials for Electronics Technology (C-MET), Panchawati, Dr. Bhabha Road, Pune 411008, India b Thick and Thin Film Device Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Received 6 November 2001; received in revised form 1 July 2002
Abstract This paper reports the comparison of two fabrication techniques (viz. thin and thick film) for microstrip broadband filter in the X-band. The effect of bulk Al2 O3 overlay (in pellet form and at different positions) on the characteristics of the broadband filter is also reported in this paper. The characteristics vary with the overlay position and thin film circuits seem to be more affected than thick film circuits. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Increased use of microwave hybrids for wireless communication systems has led to the development and acceptance of new technologies offering advanced circuit functions at low cost. Until recently, thin film technology dominated the microwave market. Traditional thick film technology was unable to compete with thin film technology for microwave applications due to the poor line resolution and high losses of thick films. But rapid development in new thick film materials systems and advanced thick film circuit patterning techniques have changed this view and proved that thick film technology can reach beyond its former limitations [1,2]. Thick film technology allows designers to combine microwave and digital functions on common high thermal conductivity alumina substrates and to incorporate the elements into the main microwave structures. Additionally thick film technology provides significant advantages such as low cost and feasibility for mass production [3]. Due to the increased use of multilayer microstrip transmission lines interspread with dielectric layers for high-density interconnection structures in modern multi-
*
Corresponding author. E-mail addresses: [email protected] (S. Rane), [email protected] (V. Puri).
chip modules, the performance of the dielectric layers at high frequency is very critical in the performance of the MCMs. Rane and Puri [4–8] have studied the dielectric overlay in different forms. The oxides are the most common material used for the overlays. The changes in the characteristics (i.e. pass band, midband frequency and midband transmittance) of the microstrip filter due to alumina cover has been reported by Rane and Puri [9]. The comparison between the thin and thick film microstrip band pass filter and effect of bulk overlay on microstrip band pass filter is reported in this paper.
2. Experimental The actual design configuration of the seven-section parallel-coupled microstrip band pass filter was taken from [10]. The bandwidth of the filter was 1000 MHz (10.0–11.0 GHz). The layout of the microstrip band pass filter is shown in Fig. 1. The thin film (Cu) filter was photolithographically delineated on 99% alumina substrate and the thick film (silver) filter on 96% alumina substrate by screen printing technique using different thick film pastes (SBR series and ESL). The same layout was used for both the fabrication techniques. Thick film circuits were fired at 750 °C peak firing temperature for a 45 min firing cycle in a three zone thick film firing
0026-2714/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 2 ) 0 0 1 2 9 - 4
1954
S. Rane, V. Puri / Microelectronics Reliability 42 (2002) 1953–1958
Fig. 1. Layout of the microstrip band pass filter.
Table 1 Data of actual geometrical sizes of thin and thick film circuits Circuit form Thin film copper Thick film (SBR3) Thick film (SBR4) Thick film (ESL) Design value
Width of resonator (sections) (lm)
Edge definition (lm)
1
2
3
4
5
6
7
8
620/419 600/417 590/336 653/436 635/427
422/628 393/590 370/575 400/620 427/635
620 600 611 600 635
623 598 580 640 635
623 606 595 650 635
623 617 600 645 635
625/412 600/395 590/355 600/390 635/427
416/627 380/625 380/603 427/660 427/635
furnace. The thickness of thin and thick film circuit was 5–6 lm and 10–12 lm respectively. Table 1 shows the data of actual geometrical sizes and edge definition of the thin and thick film circuits. For the overlay studies, the pellets of Al2 O3 were made of different thickness (1000–2500 lm) with a diameter of 1 cm. The size of the pellet was small so it did not cover all the sections of the filter simultaneously. This facilitated measurements at different positions (viz. input side, center and output side) on the filter. At each position, the pellet covers three coupling sections of the circuit. The microwave transmittance and reflectance were measured pointby-point using a setup consisting of an X-band signal generator, isolator, attenuator, directional coupler and RF detector. The microwave measurements were taken for each thickness increment and position of overlay.
3. Results
14 45 47 43 –
seen that the Cu thin film filter showed a bandwidth of 900 MHz (10.2–11.1 GHz) an almost flat response in this region. Beyond this, there is a gradual decrease in transmittance to zero at 9.6 and 12.0 GHz. The average transmittance is 0.60 at the pass band region. From the figure, it is seen that though the designed pass band was 10.0–11.0 GHz, the thick film circuit showed a shift in the pass band to the lower frequency side. The circuits prepared by three different thick film formulations have a bandwidth of 600 MHz (9.8–10.4 GHz) for SBR3, 300 MHz (9.8–10.1 GHz) for SBR4 and 600 MHz (9.8–10.4 GHz) for ESL formulation. The average transmittance is 0.36, 0.33 and 0.51 respectively. The transmittance of the filter with SBR3 and SBR4 paste are much lower than the thin film counterpart while the circuit with ESL paste show slightly lower transmittance. There is a gradual decrease in transmittance to almost zero at 9.0 and 11.1 GHz for all the cases. It is also seen that the bandwidth is similar for the circuit prepared with SBR3 and ESL paste.
3.1. Band pass filter without overlay 3.2. Band pass filter with bulk Al2 O3 overlay The frequency versus transmittance and reflectance characteristics of the seven-section parallel-coupled band pass filter is given in Fig. 2. From the figure, it is
Figs. 3–5 show the band width, midband transmittance and midband frequency versus thickness of
S. Rane, V. Puri / Microelectronics Reliability 42 (2002) 1953–1958
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Fig. 2. (a,b) Pass band transmittance and reflectance of thin and thick film band pass filter.
overlay for the various positions of overlay both on the thin and thick film circuits. The thickness of overlay Ô0Õ indicates no overlay situation. The thickness of pellet overlay starts from 1000 lm. The overlay thickness dependent effects are not observed in the reflectance measured. The midband reflectance is very low (0.01) and the overlay does not show any specific changes in the circuit characteristics hence the data is not included. 3.2.1. Effect on bandwidth From the Fig. 3a, it is seen that due to Al2 O3 pellet overlay at the input side, the band width is constant with respect to thickness of overlay for thin film circuit and all the three thick film circuits beyond a thickness of 1250 lm. It is slightly lower than without overlay for the thick film circuit but around 200 MHz for the thin film circuit. When the overlay is kept at the center (Fig. 3b) of the circuit with SBR4 and ESL composition, there is a constant band width of 500 and 400 MHz respectively for all thickness of overlay. The band width increased from 300 to 500 MHz for the SBR4 composition circuit due to the overlay. For the thick film circuit with SBR3 composition, the band width drops drastically from the no overlay situation (600 ! 100 MHz) when the overlay is kept at the center. As the overlay thickness increases, an oscillatory trend is observed. For the Cu thin film circuit, due to overlay at center and of thickness 1000 lm, the band
Fig. 3. (a–c) Bandwidth of microstrip band pass filter as a function of thickness of overlay at different positions.
width decreases drastically (800 ! 300 MHz). As the overlay thickness increases, the band width increases and becomes almost constant (500 MHz) beyond 2000 lm thickness. If the overlay is at the output side (Fig. 3c), the circuit shows a constant band width of 600 MHz for SBR3 and ESL thick film circuit irrespective of the thickness of the overlay and 400 MHz for all thicknesses of overlay for SBR4 thick film circuit, which is higher than for no overlay. For the thin film circuit when the pellet overlay is kept at the output side, the band width decreases drastically due to 1000 lm thick overlay. But as the thickness increases, the band width increases becoming almost 700 MHz. 3.2.2. Effect on midband transmittance Fig. 4a shows that with the pellet overlay at the input side, the thin film circuit shows higher transmittance compared to thick film circuits of all compositions for all thicknesses of overlay. In all cases, the value is lower
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Fig. 4. (a–c) Midband transmittance of microstrip band pass filter as a function of thickness of overlay at different positions.
than that of the no overlay situation. When the Al2 O3 pellet is kept at the center of the circuit as overlay (Fig. 4b), the midband transmittance decreased from no overlay situation for all the cases. When the overlay is kept on the Cu thin film circuit, initially up to 1500 lm thickness the transmittance decreases and then an increasing trend is observed. For the thick film circuits, there is almost no change of transmittance due to thickness of overlay. When the overlay is placed at the output side (Fig. 4c), the thin film circuit and SBR3 thick film circuit show an initial decrease in midband transmittance as compared to the no overlay case. For the other two thick film circuits there is almost no change in transmittance. 3.2.3. Effect on midband frequency From Fig. 5a, it is seen that when the pellet overlay is kept at the input side, all the circuits show approximately constant midband frequency except the thick film circuit of SBR3 composition.
Fig. 5. (a–c) Midband frequency of microstrip band pass filter as a function of thickness of overlay at different positions.
When the overlay is placed at the center position (Fig. 5b), there is a decrease in the midband frequency as compared to no overlay condition. The thin film circuit shows almost thickness independent characteristics except at a thickness of 1500 lm where the midband frequency decreases. For the circuit of SBR3 composition, the midband frequency decreases drastically from 10.05 to 9.85 GHz and then increases in the thickness of overlay. The circuit of SBR4 composition shows 9.85 GHz midband frequency for all thicknesses of overlay and that of ESL composition shows 10.0 GHz midband frequency for all thickness of overlay. For both these circuits there is no change in frequency due to increases in thickness of overlay. The thin film circuits show a gradual increase in midband frequency as the thickness of overlay increases though initially there is a large decrease in the midband frequency at 1000 lm compared to the no overlay situation due to the overlay at output position (Fig. 5c). The circuit with SBR3 and ESL composition shows a constant 10.1 GHz midband frequency for all the overlay
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thickness whereas for SBR4 composition the midband frequency is 10.0 GHz for all the overlay thickness. There is almost no change from no overlay condition. From the above results, it can be stated that the characteristics of the circuits vary when the overlay position is changed and if there is partial covering of the circuit. Thin film circuits seem to be affected more than the thick film circuits.
4. Discussion The band pass filter consists of a series of half wavelength resonators with the parallel side coupling along a distance of quarter wavelength. The characteristic impedance (Z0 ) affects the relative phase velocity of even and odd modes of the parallel-coupled microstriplines. Since the energy transfer has to take place seven times as the wave travels from the input to the output end, there is a complex field distribution involving the coupling regions spread by the field and radiative components. The geometrical sizes of the resonators (especially first and last two) of the SBR3 paste circuit are closer to the design values in comparison to SBR4 paste circuit. Also, the edge definition of the SBR3 circuit is slightly less than the SBR4 paste circuit. The geometrical sizes of the ESL paste circuit are slightly different than the design value. The thick film circuits show higher losses compared to the thin film circuits due to inferior edge definition, spreading of metallization and also due to binder content [11]. Since microstrip discontinuities have associated reactive elements, the losses are increased [12]. The leaky modes from each resonator length will also contribute in a complex way to the properties of the sevensection band pass filter. The circuit properties depends on the material used for the fabrication. In the present study different binder percentage was used for SBR series of pastes. The binder percentage of ESL paste was unknown. The variation in the binder content may be the reason for the different results with different materials. The reflectance data is not exactly complementary to the transmittance data for the thick film circuits, though the reflectance in band pass region of all the samples are less due to impedance matching. The thick film metallization does not create additional impedance mismatches though there are very negligible changes in the reflectance due to metallization [13]. The microstrip circuits overlayed with dielectric material improve the circuit performance [4–6]. Free et al. [14] reported the suitability of the basic thick film process for fabricating the microstrip circuits and also the effect of glaze on the relative permittivity by performing the simulation of a 30 GHz filter. They observed that the performance of the filter is not significantly affected over the range of dimensional error. The overlay is in pellet form and simply kept over the microstripline. The presence of air gap and consequent
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incomplete confining of the fringing field results in changes in the characteristic impedance of lines. Changes in the characteristic impedance of the order of 1–3 X for a 50 X microstripline due to air gap has been reported by Das and Pozar [15]. At high frequency existence of surface wave and transverse mode coupling have been reported [16]. Due to overlay in pellet form, where the thickness is greater than substrate thickness these surface wave phenomenon and dispersive effects may become predominant. Joshi et al. [17] have reported that for thick covers, a change in mode also occurs. Rane and Puri [7] reported that losses are dependent on thickness of overlay. Also, the dielectric constant of overlay material does not affect the properties of microstriplines. The coupling action of the coupled circuits relies on the proximity effect of two lines. In most of the circuits, the design is in such a way that the odd mode is used for coupling [18]. Similar to microstriplines, in the coupled circuits the fringing fields in air are present. All the coupled circuits have modal equivalent fringing field even mode (Dle ) and odd mode (Dlo ) lengths [19]. The changes in the equivalent fringing field lengths affect the odd mode and even mode propagation in the presence of overlay over the sample. The values of Dle are one order of magnitude larger than the values of Dlo . The fringing fields in the odd mode, which are mainly concentrated in the substrate, are spread less than the fringing fields in the even mode, which are mainly placed in the air. If the dielectric in the form of either thin film or thick film is deposited on the coupling section,the effective widths of the conducting strip increases both in the main line and the resonator section. Also the dielectric overlay is deposited in between the coupling gap. As the coupling gap decreases, the value of parallel capacitance (Cp ) slightly increases, the value series capacitance (Cs ) decreases and also the value of Dlo increases and that of Dle remains practically not affected [20]. The edge effect in the even mode is much more pronounced than the edge effects in the odd mode. This phenomenon might be influencing more the properties of thick film circuits than those of the thin film circuits. Whether it is fringing fields or edge effects, both the even and odd modes are affected due to the presence of a dielectric over the microstripline filter. Depending upon the type of filter and type of overlay, the effects are different. Metallization as well as thick film composition dependent behavior were observed for the band pass filter overlayed by thick film Al2 O3 [21]. When the pellet overlay is kept at different positions of the thin film circuit, due to a number of edges present, the overlay might be partially covering the edges and the coupled sections. There is a charge density singularity at the strip edges, which gets modified due to partial overlay. On the underside of the overlay, the fields are fairly strong. At the overlay edges, the field amplitudes are comparatively weak. When the wave is traveling
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S. Rane, V. Puri / Microelectronics Reliability 42 (2002) 1953–1958
either from an overlaid section to a non-overlaid section or vice versa, it meets a complex situation enroute to the output. The fact that the characteristics of the filter has not changed much due to thickness of the pellet or position of the pellet, indicates that since the thickness of overlay is very large, the fringing fields are absorbed by the overlay and results only in attenuation of the transmission curve. When the overlay is kept in the input side there is some thickness dependent changes. The situation due to pellet overlay on thick film circuit is different from thin film circuit. This might be due to the specific properties associated with thick film circuits such as edge definition, irregular surface morphology and comparatively large edge discontinuities, the separation of coupling gap gets affected more than the thin film circuits.
[2]
[3]
[4]
[5]
[6]
5. Conclusion It is seen that out of two indigenous pastes (SBR3 and SBR4), SBR3 paste is comparable to imported (ESL) paste. Also, it is seen that for circuits with more coupling structures, thick film technology is comparable to the conventional thin film technology even though there are slightly higher losses. For multicoupled circuits, just by shifting the position of overlay, tuning of the filter can be achieved. Since the pellet is physically placed over the circuit it does not react with the metallization or the substrate and can be removed very easily. It is felt that given the nature of overlay, proper choice of dielectric material for use as overlay over the microstrip circuits, will enable one to achieve highly accurate properties without costly and time consuming design processes. Dielectric overlay on microstrip circuits are of great significance to high-density large-scale integrated circuits. Use of bulk overlay can be reliable for modifying parameters such as coupling and cross talk, as and when required, since these are directly related to changes in the field distribution in the vicinity of the conductor.
[7]
[8]
[9]
[10] [11]
[12] [13]
[14]
[15]
Acknowledgements The work was carried out at Thick & Thin Film Device Lab, Department of Physics, Shivaji University, Kolhapur, India. The author S. Rane gratefully acknowledges for the same. Similarly, V. Puri acknowledges University Grants Commission, India for the award of ÔResearch Scientist BÕ. References [1] Barnwell P, Wood J, Reynolds Q. A microwave circuit fabrication technology using an advanced thick film
[16] [17]
[18] [19] [20] [21]
materials. In: Proceedings of Emerging Microelectronic Interconnection Technology (IMAPS), India, February 1998. p. 399–404. Kemppinen E, Mikkonen P, Collander P, Leppavuori S. Performance of Cu & Ag based microstrips up to mm wave frequencies. In: 11th European Microelectronics Conference, Venice Italy, May 1997. p. 100–8. Dziurdzia B, Ciez M, Nowak S, Gregorczyk W, Thust H, Polzer E. Thick film fabrication yields thin film performance. Microwaves & RF 2000:97–100. Rane S, Puri V. Behaviour of parallel coupled microstrip band-pass filter & simple microstripline due to thin film Al2 O3 overlay. Active & Passive Electron Compon 1996; 19:125–32. Rane S, Puri V. Moisture ambient effects on an Al2 O3 thin film overlaid parallel coupled broad band microstrip filter. Microelectron Int 1997;14(3):19–20, 30. Rane S, Puri V. Behaviour of thin film TiO2 overlayed k=2 microstrip rejection filter due to ageing of the overlay. Active & Passive Electron Compon 1998;21:297– 307. Rane S, Puri V. Study of Ku band properties of Al2 O3 and Nb2 O5 pellet ovelayed microstriplines of different widths. IETE Tech Rev 1999;16(1):129–33. Rane S, Puri V. Thick film microstrip rejection filter with improved Q using overlay. Microelectron Int 2001;18(1): 23–8. Rane S, Puri V. Influence of alumina overlay on the performance of thin and thick film microstrip band pass filter. Int J Microcirc Electron Packag 2001;24:263–72. Edward TC. Foundations of microstrip circuit design. John Wiley & Sons; 1983. Rane S, Puri V. Performance comparison of seven section parallel coupled microstrip filter using various Ag thick film pastes. In: Proceedings of Computers & Devices for Communication (CODEC) Calcutta, India, 1998. p. 445–7. Palmer K, Clebete J. In: SAIEE APRICON, 1996. p. 83–9. Rane S. Studies on thick and thin film overlayed microstripline passive components in X and Ku band. PhD thesis, Shivaji University, Kolhapur, India, 1999. Free C, Pitt K, Tian Z. The effects of overglazing on the performance of microwave thick film circuits in the frequency 8–18 GHz. In: International Microelectronics and Packaging Society (IMAPS), Poland, 2000. Das N, Pozar D. Generalized spectral domain green function for multilayer dielectric substrates with applications to multilayer transmission lines. IEEE Trans Microwave Theory & Tech 1987;25(2):326–35. Vendelin G. Limitations on stripline Q. Microwave J 1970:63–9. Joshi K, Pollard R, Postoyalko V. Microstrip with dielectric overlay: variational analysis and validation. IEE Proc Microwave Antenn & Propagat 1994;141(2): 138–40. Mullard technical communications, vol. 116, 1972. p. 177. Kirschings M, Jonsen R, Koster N. Electron Lett 1983; 19(10):377. Martel J, Boix R, Horno M. IEE Proc H 1992;139(3):239. Rane S, Puri V. Thick film dielectric overlay effects on thin and thick film microstrip band pass filter. Microelectron J 2001;32(8):649–54.
Thin film, thick film microstrip band pass filter: a comparison and effect of bulk overlay Sunit Rane a
a,*
, Vijaya Puri
b
Centre for Materials for Electronics Technology (C-MET), Panchawati, Dr. Bhabha Road, Pune 411008, India b Thick and Thin Film Device Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Received 6 November 2001; received in revised form 1 July 2002
Abstract This paper reports the comparison of two fabrication techniques (viz. thin and thick film) for microstrip broadband filter in the X-band. The effect of bulk Al2 O3 overlay (in pellet form and at different positions) on the characteristics of the broadband filter is also reported in this paper. The characteristics vary with the overlay position and thin film circuits seem to be more affected than thick film circuits. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Increased use of microwave hybrids for wireless communication systems has led to the development and acceptance of new technologies offering advanced circuit functions at low cost. Until recently, thin film technology dominated the microwave market. Traditional thick film technology was unable to compete with thin film technology for microwave applications due to the poor line resolution and high losses of thick films. But rapid development in new thick film materials systems and advanced thick film circuit patterning techniques have changed this view and proved that thick film technology can reach beyond its former limitations [1,2]. Thick film technology allows designers to combine microwave and digital functions on common high thermal conductivity alumina substrates and to incorporate the elements into the main microwave structures. Additionally thick film technology provides significant advantages such as low cost and feasibility for mass production [3]. Due to the increased use of multilayer microstrip transmission lines interspread with dielectric layers for high-density interconnection structures in modern multi-
*
Corresponding author. E-mail addresses: [email protected] (S. Rane), [email protected] (V. Puri).
chip modules, the performance of the dielectric layers at high frequency is very critical in the performance of the MCMs. Rane and Puri [4–8] have studied the dielectric overlay in different forms. The oxides are the most common material used for the overlays. The changes in the characteristics (i.e. pass band, midband frequency and midband transmittance) of the microstrip filter due to alumina cover has been reported by Rane and Puri [9]. The comparison between the thin and thick film microstrip band pass filter and effect of bulk overlay on microstrip band pass filter is reported in this paper.
2. Experimental The actual design configuration of the seven-section parallel-coupled microstrip band pass filter was taken from [10]. The bandwidth of the filter was 1000 MHz (10.0–11.0 GHz). The layout of the microstrip band pass filter is shown in Fig. 1. The thin film (Cu) filter was photolithographically delineated on 99% alumina substrate and the thick film (silver) filter on 96% alumina substrate by screen printing technique using different thick film pastes (SBR series and ESL). The same layout was used for both the fabrication techniques. Thick film circuits were fired at 750 °C peak firing temperature for a 45 min firing cycle in a three zone thick film firing
0026-2714/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 2 ) 0 0 1 2 9 - 4
1954
S. Rane, V. Puri / Microelectronics Reliability 42 (2002) 1953–1958
Fig. 1. Layout of the microstrip band pass filter.
Table 1 Data of actual geometrical sizes of thin and thick film circuits Circuit form Thin film copper Thick film (SBR3) Thick film (SBR4) Thick film (ESL) Design value
Width of resonator (sections) (lm)
Edge definition (lm)
1
2
3
4
5
6
7
8
620/419 600/417 590/336 653/436 635/427
422/628 393/590 370/575 400/620 427/635
620 600 611 600 635
623 598 580 640 635
623 606 595 650 635
623 617 600 645 635
625/412 600/395 590/355 600/390 635/427
416/627 380/625 380/603 427/660 427/635
furnace. The thickness of thin and thick film circuit was 5–6 lm and 10–12 lm respectively. Table 1 shows the data of actual geometrical sizes and edge definition of the thin and thick film circuits. For the overlay studies, the pellets of Al2 O3 were made of different thickness (1000–2500 lm) with a diameter of 1 cm. The size of the pellet was small so it did not cover all the sections of the filter simultaneously. This facilitated measurements at different positions (viz. input side, center and output side) on the filter. At each position, the pellet covers three coupling sections of the circuit. The microwave transmittance and reflectance were measured pointby-point using a setup consisting of an X-band signal generator, isolator, attenuator, directional coupler and RF detector. The microwave measurements were taken for each thickness increment and position of overlay.
3. Results
14 45 47 43 –
seen that the Cu thin film filter showed a bandwidth of 900 MHz (10.2–11.1 GHz) an almost flat response in this region. Beyond this, there is a gradual decrease in transmittance to zero at 9.6 and 12.0 GHz. The average transmittance is 0.60 at the pass band region. From the figure, it is seen that though the designed pass band was 10.0–11.0 GHz, the thick film circuit showed a shift in the pass band to the lower frequency side. The circuits prepared by three different thick film formulations have a bandwidth of 600 MHz (9.8–10.4 GHz) for SBR3, 300 MHz (9.8–10.1 GHz) for SBR4 and 600 MHz (9.8–10.4 GHz) for ESL formulation. The average transmittance is 0.36, 0.33 and 0.51 respectively. The transmittance of the filter with SBR3 and SBR4 paste are much lower than the thin film counterpart while the circuit with ESL paste show slightly lower transmittance. There is a gradual decrease in transmittance to almost zero at 9.0 and 11.1 GHz for all the cases. It is also seen that the bandwidth is similar for the circuit prepared with SBR3 and ESL paste.
3.1. Band pass filter without overlay 3.2. Band pass filter with bulk Al2 O3 overlay The frequency versus transmittance and reflectance characteristics of the seven-section parallel-coupled band pass filter is given in Fig. 2. From the figure, it is
Figs. 3–5 show the band width, midband transmittance and midband frequency versus thickness of
S. Rane, V. Puri / Microelectronics Reliability 42 (2002) 1953–1958
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Fig. 2. (a,b) Pass band transmittance and reflectance of thin and thick film band pass filter.
overlay for the various positions of overlay both on the thin and thick film circuits. The thickness of overlay Ô0Õ indicates no overlay situation. The thickness of pellet overlay starts from 1000 lm. The overlay thickness dependent effects are not observed in the reflectance measured. The midband reflectance is very low (0.01) and the overlay does not show any specific changes in the circuit characteristics hence the data is not included. 3.2.1. Effect on bandwidth From the Fig. 3a, it is seen that due to Al2 O3 pellet overlay at the input side, the band width is constant with respect to thickness of overlay for thin film circuit and all the three thick film circuits beyond a thickness of 1250 lm. It is slightly lower than without overlay for the thick film circuit but around 200 MHz for the thin film circuit. When the overlay is kept at the center (Fig. 3b) of the circuit with SBR4 and ESL composition, there is a constant band width of 500 and 400 MHz respectively for all thickness of overlay. The band width increased from 300 to 500 MHz for the SBR4 composition circuit due to the overlay. For the thick film circuit with SBR3 composition, the band width drops drastically from the no overlay situation (600 ! 100 MHz) when the overlay is kept at the center. As the overlay thickness increases, an oscillatory trend is observed. For the Cu thin film circuit, due to overlay at center and of thickness 1000 lm, the band
Fig. 3. (a–c) Bandwidth of microstrip band pass filter as a function of thickness of overlay at different positions.
width decreases drastically (800 ! 300 MHz). As the overlay thickness increases, the band width increases and becomes almost constant (500 MHz) beyond 2000 lm thickness. If the overlay is at the output side (Fig. 3c), the circuit shows a constant band width of 600 MHz for SBR3 and ESL thick film circuit irrespective of the thickness of the overlay and 400 MHz for all thicknesses of overlay for SBR4 thick film circuit, which is higher than for no overlay. For the thin film circuit when the pellet overlay is kept at the output side, the band width decreases drastically due to 1000 lm thick overlay. But as the thickness increases, the band width increases becoming almost 700 MHz. 3.2.2. Effect on midband transmittance Fig. 4a shows that with the pellet overlay at the input side, the thin film circuit shows higher transmittance compared to thick film circuits of all compositions for all thicknesses of overlay. In all cases, the value is lower
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S. Rane, V. Puri / Microelectronics Reliability 42 (2002) 1953–1958
Fig. 4. (a–c) Midband transmittance of microstrip band pass filter as a function of thickness of overlay at different positions.
than that of the no overlay situation. When the Al2 O3 pellet is kept at the center of the circuit as overlay (Fig. 4b), the midband transmittance decreased from no overlay situation for all the cases. When the overlay is kept on the Cu thin film circuit, initially up to 1500 lm thickness the transmittance decreases and then an increasing trend is observed. For the thick film circuits, there is almost no change of transmittance due to thickness of overlay. When the overlay is placed at the output side (Fig. 4c), the thin film circuit and SBR3 thick film circuit show an initial decrease in midband transmittance as compared to the no overlay case. For the other two thick film circuits there is almost no change in transmittance. 3.2.3. Effect on midband frequency From Fig. 5a, it is seen that when the pellet overlay is kept at the input side, all the circuits show approximately constant midband frequency except the thick film circuit of SBR3 composition.
Fig. 5. (a–c) Midband frequency of microstrip band pass filter as a function of thickness of overlay at different positions.
When the overlay is placed at the center position (Fig. 5b), there is a decrease in the midband frequency as compared to no overlay condition. The thin film circuit shows almost thickness independent characteristics except at a thickness of 1500 lm where the midband frequency decreases. For the circuit of SBR3 composition, the midband frequency decreases drastically from 10.05 to 9.85 GHz and then increases in the thickness of overlay. The circuit of SBR4 composition shows 9.85 GHz midband frequency for all thicknesses of overlay and that of ESL composition shows 10.0 GHz midband frequency for all thickness of overlay. For both these circuits there is no change in frequency due to increases in thickness of overlay. The thin film circuits show a gradual increase in midband frequency as the thickness of overlay increases though initially there is a large decrease in the midband frequency at 1000 lm compared to the no overlay situation due to the overlay at output position (Fig. 5c). The circuit with SBR3 and ESL composition shows a constant 10.1 GHz midband frequency for all the overlay
S. Rane, V. Puri / Microelectronics Reliability 42 (2002) 1953–1958
thickness whereas for SBR4 composition the midband frequency is 10.0 GHz for all the overlay thickness. There is almost no change from no overlay condition. From the above results, it can be stated that the characteristics of the circuits vary when the overlay position is changed and if there is partial covering of the circuit. Thin film circuits seem to be affected more than the thick film circuits.
4. Discussion The band pass filter consists of a series of half wavelength resonators with the parallel side coupling along a distance of quarter wavelength. The characteristic impedance (Z0 ) affects the relative phase velocity of even and odd modes of the parallel-coupled microstriplines. Since the energy transfer has to take place seven times as the wave travels from the input to the output end, there is a complex field distribution involving the coupling regions spread by the field and radiative components. The geometrical sizes of the resonators (especially first and last two) of the SBR3 paste circuit are closer to the design values in comparison to SBR4 paste circuit. Also, the edge definition of the SBR3 circuit is slightly less than the SBR4 paste circuit. The geometrical sizes of the ESL paste circuit are slightly different than the design value. The thick film circuits show higher losses compared to the thin film circuits due to inferior edge definition, spreading of metallization and also due to binder content [11]. Since microstrip discontinuities have associated reactive elements, the losses are increased [12]. The leaky modes from each resonator length will also contribute in a complex way to the properties of the sevensection band pass filter. The circuit properties depends on the material used for the fabrication. In the present study different binder percentage was used for SBR series of pastes. The binder percentage of ESL paste was unknown. The variation in the binder content may be the reason for the different results with different materials. The reflectance data is not exactly complementary to the transmittance data for the thick film circuits, though the reflectance in band pass region of all the samples are less due to impedance matching. The thick film metallization does not create additional impedance mismatches though there are very negligible changes in the reflectance due to metallization [13]. The microstrip circuits overlayed with dielectric material improve the circuit performance [4–6]. Free et al. [14] reported the suitability of the basic thick film process for fabricating the microstrip circuits and also the effect of glaze on the relative permittivity by performing the simulation of a 30 GHz filter. They observed that the performance of the filter is not significantly affected over the range of dimensional error. The overlay is in pellet form and simply kept over the microstripline. The presence of air gap and consequent
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incomplete confining of the fringing field results in changes in the characteristic impedance of lines. Changes in the characteristic impedance of the order of 1–3 X for a 50 X microstripline due to air gap has been reported by Das and Pozar [15]. At high frequency existence of surface wave and transverse mode coupling have been reported [16]. Due to overlay in pellet form, where the thickness is greater than substrate thickness these surface wave phenomenon and dispersive effects may become predominant. Joshi et al. [17] have reported that for thick covers, a change in mode also occurs. Rane and Puri [7] reported that losses are dependent on thickness of overlay. Also, the dielectric constant of overlay material does not affect the properties of microstriplines. The coupling action of the coupled circuits relies on the proximity effect of two lines. In most of the circuits, the design is in such a way that the odd mode is used for coupling [18]. Similar to microstriplines, in the coupled circuits the fringing fields in air are present. All the coupled circuits have modal equivalent fringing field even mode (Dle ) and odd mode (Dlo ) lengths [19]. The changes in the equivalent fringing field lengths affect the odd mode and even mode propagation in the presence of overlay over the sample. The values of Dle are one order of magnitude larger than the values of Dlo . The fringing fields in the odd mode, which are mainly concentrated in the substrate, are spread less than the fringing fields in the even mode, which are mainly placed in the air. If the dielectric in the form of either thin film or thick film is deposited on the coupling section,the effective widths of the conducting strip increases both in the main line and the resonator section. Also the dielectric overlay is deposited in between the coupling gap. As the coupling gap decreases, the value of parallel capacitance (Cp ) slightly increases, the value series capacitance (Cs ) decreases and also the value of Dlo increases and that of Dle remains practically not affected [20]. The edge effect in the even mode is much more pronounced than the edge effects in the odd mode. This phenomenon might be influencing more the properties of thick film circuits than those of the thin film circuits. Whether it is fringing fields or edge effects, both the even and odd modes are affected due to the presence of a dielectric over the microstripline filter. Depending upon the type of filter and type of overlay, the effects are different. Metallization as well as thick film composition dependent behavior were observed for the band pass filter overlayed by thick film Al2 O3 [21]. When the pellet overlay is kept at different positions of the thin film circuit, due to a number of edges present, the overlay might be partially covering the edges and the coupled sections. There is a charge density singularity at the strip edges, which gets modified due to partial overlay. On the underside of the overlay, the fields are fairly strong. At the overlay edges, the field amplitudes are comparatively weak. When the wave is traveling
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either from an overlaid section to a non-overlaid section or vice versa, it meets a complex situation enroute to the output. The fact that the characteristics of the filter has not changed much due to thickness of the pellet or position of the pellet, indicates that since the thickness of overlay is very large, the fringing fields are absorbed by the overlay and results only in attenuation of the transmission curve. When the overlay is kept in the input side there is some thickness dependent changes. The situation due to pellet overlay on thick film circuit is different from thin film circuit. This might be due to the specific properties associated with thick film circuits such as edge definition, irregular surface morphology and comparatively large edge discontinuities, the separation of coupling gap gets affected more than the thin film circuits.
[2]
[3]
[4]
[5]
[6]
5. Conclusion It is seen that out of two indigenous pastes (SBR3 and SBR4), SBR3 paste is comparable to imported (ESL) paste. Also, it is seen that for circuits with more coupling structures, thick film technology is comparable to the conventional thin film technology even though there are slightly higher losses. For multicoupled circuits, just by shifting the position of overlay, tuning of the filter can be achieved. Since the pellet is physically placed over the circuit it does not react with the metallization or the substrate and can be removed very easily. It is felt that given the nature of overlay, proper choice of dielectric material for use as overlay over the microstrip circuits, will enable one to achieve highly accurate properties without costly and time consuming design processes. Dielectric overlay on microstrip circuits are of great significance to high-density large-scale integrated circuits. Use of bulk overlay can be reliable for modifying parameters such as coupling and cross talk, as and when required, since these are directly related to changes in the field distribution in the vicinity of the conductor.
[7]
[8]
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[10] [11]
[12] [13]
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Acknowledgements The work was carried out at Thick & Thin Film Device Lab, Department of Physics, Shivaji University, Kolhapur, India. The author S. Rane gratefully acknowledges for the same. Similarly, V. Puri acknowledges University Grants Commission, India for the award of ÔResearch Scientist BÕ. References [1] Barnwell P, Wood J, Reynolds Q. A microwave circuit fabrication technology using an advanced thick film
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