Fabrication and electrical properties of laser ... - Semantic Scholar
Microelectronics Reliability 49 (2009) 600–606
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Fabrication and electrical properties of laser-shaped thick-film and LTCC microresistors Damian Nowak a,*, Edward Mis´ a, Andrzej Dziedzic a, Jarosław Kita b a b
Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland University of Bayreuth, Functional Materials Group, Universitätstrasse 30, D-95440 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 12 November 2008 Received in revised form 13 January 2009 Available online 3 April 2009
a b s t r a c t The dimensions of discrete passives, passive integrated components (arrays, networks) and embedded integral ones should be reduced significantly in the nearest future. Therefore the relations between technological accuracy and limitations, minimal geometrical dimensions and electrical as well as stability properties become more and more important. This paper presents systematic studies of thick-film or LTCC microresistors made with the aid of laser shaping. The investigations are concerned with miniaturization of two resistor dimensions, namely length (down to 30 lm) and width (also down to 30 lm). The sheet resistance, hot temperature coefficient of resistance (HTCR) as well as long-term stability and durability of test structures to various short electrical pulses were related to geometrical properties of microresistors. Such investigations proved that combining of current materials and fabrication methods used in modern thick-film and LTCC technologies with laser shaping made possible fabrication of 30 30 lm2 microresistors with satisfactory electrical properties and can serve as interesting alternative for thick-film and LTCC resistors miniaturization. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Modern packaging needs next generation of passives, which should be much smaller, cheaper and much more integrated. There are various attempts that can be used for size reduction of thickfilm and/or LTCC components and circuits. The concept and modern technological approach to the fabrication of discrete, integrated and integral micropassives is presented, e.g. in [1–3]. Photopatterning techniques seem to be the most interesting because they permit to reduce component dimensions down to 20 lm. Unfortunately, only conductive and dielectric photosensitive inks are available as far. There are also experimental photosensitive resistive inks and in the literature one can find information about microresistors made from them [4–6] but these inks are still not commercially available. There are also other attempts to miniaturization of thick-film and LTCC resistors like combination of photopatterned contacts and screen-printed resistive film [7,8], application of microcontact printing [9] or substitution of array with two-terminal resistors by equivalent multicontact resistor [10]. But novel solution of this problem is still needed. Authors propose to use laser shaping for fabrication of thickfilm and LTCC resistors. Although many laser applications, especially related to complete microcircuit size reduction and packaging density increase, are reported in the literature (some results * Corresponding author. E-mail address: [email protected] (D. Nowak). 0026-2714/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2009.02.019
are presented for example in [11–13]), according to authors’ knowledge there is no information about geometrical, electrical and stability properties of thick-film or LTCC microresistors made by the aid of laser shaping. This paper presents systematic studies of such microresistors. The individual resistors were created on alumina or LTCC substrates by laser cutting of conductive and resistive films after screen-printing and firing. Two kinds of microresistors, with variable length or width, have been prepared. The geometrical parameters were correlated with: sheet resistance and its distribution, hot temperature coefficient of resistance, normalized temperature dependence of resistance in a wide temperature range, durability of microcomponents to various short electrical pulses and longterm stability at step-increased ageing temperature. The results obtained for laser-shaped microresistors were compared with previously investigated microresistors made by standard screen-printing or by using full Fodel process and experimental Fodel resistive ink [3,5–8]. 2. Fabrication and geometrical properties of laser-shaped microresistors The test structures were made on alumina (96% Al2O3) or LTCC (DP 951 tape from DuPont). The microresistors with variable length (30–300 lm), were created by laser cutting of fired (300 lm wide) PdAg-, Au- or Ag-based conductive films (Fig. 1a). Such small dimensions were patterned with frequency-tripled
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
601
Fig. 1. (a) Conductive path with laser made gap (b) gap filled with screen-printed resistive ink (c) top view of resistors with regulated width.
(beam wavelength of 355 nm) Microline 350L laser system (LPKF, Germany) equipped with an arc lamp pumped Nd:YAG-laser with Q-switching. Relatively low energy laser beam is needed to avoid injury of the substrate. Next DP 2021 (100 ohm/sq.) or DP 2041 (10 kX/sq.) ink has been screen-printed (overlapping 200 lm) and fired. For comparison of manufacturing methods standard screen-printed resistors with lengths from 300 to 1800 lm were prepared on the same substrate. The microresistors with constant length and variable width down to 30 lm (Fig. 1c) were prepared in two technological versions – first by proper cutting of 1 1 mm2 DP 6620 (100 ohm/ sq.) or DP 6641 (10 kX/sq.) fired resistors (laser equipment presented above was used). In the second way conductors were made in full Fodel process whereas the resistive film was screen-printed. Photosensitive Ag-based DP 6453 Fodel conductor was used for terminations whereas DP 2031 (1 kX/sq.), CF 041 (10 kX/sq.) or R 8951 (100 kX/sq.) were applied as resistive films. KrF excimer laser (LPX 210 Lambda Physik model, wavelength k = 248 nm, 30 lm laserspot diameter on the surface, repetition rate 200 Hz, energy density on the surface 40 J/cm2, shaping by scanning with one pulse per lm) was used for shaping [14]. The test structures consisted of six resistors 170 lm wide and 200, 400, 600 or 800 lm long (see e.g. Fig. 2) or two resistors 1000, 200, 100, 50 or 30 lm wide and 1000 lm long, every separated by 30 lm notch. One should note that it is not necessary to use technological margins between adjacent terminations (the distance between them is determined by laser notch).
Laser profilometer (surface measuring system OME lScan AF2000 from Nanofocus) was used for three-dimensional (3D) characterization of investigated structures. A typical two-dimensional (2D) profile is shown in Fig. 2. It is clearly visible that the thickness of resistive film is not identical at every point. The mean thickness of these films is about 10 lm, both on alumina and LTCC substrates. The depth of laser kerf is dependent on cut material and kind of substrate – the same pulse energy of laser gives much deeper notches in LTCC substrates in comparison with alumina ones (for alumina substrate the notch exists only in resistive or conductive film – not in alumina). Moreover it is much more difficult to make cut in fired conductive film than resistive one.
3. Basic electrical properties Examples of sheet resistance (Rsq) vs. resistor length dependences are shown in Figs. 3 and 4. When the resistor length is increased from 200 to 800 lm (or 1800 lm) then the sheet resistance is increased by 20–50%. The increase level is dependent on kind of resistive film. The ±15% resistance distribution is noted for laser-shaped structures. This distribution is much less in comparison with screen-printed microresistors and comparable with Fodel ones [5,6]. Hot temperature coefficient of resistance (HTCR) was also measured and analyzed. HTCR is defined as:
602
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
Fig. 2. Planar thickness map of 800 170 lm2 resistors (DP2031/LTCC).
where R(125 °C) and R(25 °C) are resistances at 125 °C and 25 °C, respectively. Resistors with regulated length except of Au terminals show TCR decrease with length increase (Fig. 5). Resistors with regulated width exhibit HTCR value practically independent on their width (Fig. 6). The Keithley 2000 Multimeter interfaced to personal computer for data acquisition and presentation was used in measurements of resistance vs. temperature in the range from 170 to 130 °C. The examples of normalized temperature dependence of resistance and differential TCR, where TCRdiff ¼ dR=ðR dTÞ, are shown in Figs. 7 and 8. The presented characteristics are typical for thick-film resistors – they exhibit resistance minimum of at a certain temperature. This minimum shifts towards lower temperature when the resistor aspect ratio is increased. This together with results presented in Figs. 3 and 4 suggests normal dimensional effect. Such Fig. 3. Sheet resistance of DP2041 films vs. resistor length.
Fig. 4. Sheet resistance of R8951/Al2O3 resistors vs. resistor length.
HTCR ¼
½Rð125 CÞ Rð25 CÞ 106 Rð25 CÞ½125 C 25 C
ð1Þ Fig. 5. HTCR as a function of resistor length (DP 2041 resistive layer).
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
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Fig. 6. HTCR as a function of resistor width.
Fig. 8. Normalized temperature dependence of resistance (a) and differential TCR (b) for DP6641 resistors.
Fig. 7. Normalized temperature dependence of resistance (a) and differential TCR (b) for 800 170 lm2 resistors (R8951/Al2O3 structures).
resistors exhibit smaller resistance drift. This means that ageing processes within resistor volume give smaller fractional resistance changes than those appearing at the resistor/conductor interface. Screen-printed resistors exhibit similar stability level under the same ageing conditions [6,15]. This suggests that laser affected zone, appearing during shaping is very small and can be neglected during analysis of electrical and stability properties for structures with resistor width larger than 150 lm. 5. Pulse durability
changes of R(T) characteristics are a result of interactions between conductive and resistive film. 4. Long-term thermal stability We analyzed long-term stability based on measurements of resistance drift induced by long-term thermal ageing at three different temperatures 150, 200 and 250 °C. The samples were kept at every temperature for about 300 h. Some examples of changes observed after long-term exposure at 250 °C are shown in Fig. 9. Rather insignificant resistance changes are observed in general. Resistors with Au-based terminations have better stability as those with Ag- or PdAg-based contact layers. Moreover longer and wider
Authors used voltage pulses for two kind of tests (Fig. 10) – analysis of allowable voltage between adjacent terminals and investigation of resistors pulse durability. The first test indicates the class of electronic circuits, where such resistor ladders can be applied. The results are collected in Table 1. It is very interesting that although the distance between adjacent resistors is very small (approximately only about 30 lm, i.e. much less than for other thick-film and LTCC microresistors design and solution) such structures withstand voltage pulses of several hundred volts. In spite of deeper laser notch for resistors on LTCC substrate the allowable pulse voltages are somewhat larger for structures on
604
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
Fig. 9. Long-term stability of laser-shaped microresistors: (a) resistive ink 2021, (b) resistive ink 2041, (c) resistive inks 6620 and 6641 (s – standard screen-printing, p – laser shaping).
Table 1 Resistance to high voltage shocks between terminals for two-terminal resistors.
Fig. 10. Concept of measuring: (a) resistance to high voltage shocks between terminals and (b) pulse durability for two-terminal resistors.
Substrate
l (lm)
U (V), tp = 10 ls
U (V), tp = 1 ms
Al2O3 Al2O3 Al2O3 Al2O3 LTCC LTCC LTCC LTCC
200 400 600 800 200 400 600 800
>475 >475 >475 >475 >475 >475 410 280
320 >475 460 365 410 450 345 237
alumina. This is probably caused by larger reactivity of resistive film with LTCC substrate. For longer resistors the breakdown voltage becomes somewhat smaller probably because notch width is not constant but there are places where it becomes smaller. Pulse treatment is used e.g. for trimming of thick-film resistors [16] – more information about properties of such trimmed structures one can find in [15]. But investigations of pulse durability can help also to fix the critical electric field and next surface or volume power density for microresistors in dependence of resistor dimensions or pulse duration [6]. Voltage pulse exposures were realized with the aid of self made Programmed Pulse Generator
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
605
Fig. 14. Resistance changes vs. surface power density for 2021 resistors (tp = 20 ls). 2
Fig. 11. Resistance changes vs. electric field for 200 170 lm resistors.
DP2031/Al2O3
is no noticeable differences between standard screen-printed, Fodel-made and laser-shaped microresistors. 6. Conclusions
Fig. 12. Resistance changes vs. pulse surface power density for 200 170 lm2 DP2031/Al2O3 resistors.
This paper presents fabrication and wide spectrum of geometrical, electrical and stability properties of laser-shaped thick-film and LTCC microresistors. The results show that this is very interesting solution for miniaturization of thick-film components. From geometrical point of view the quality of design image is much better than for standard screen-printing and can be even better than obtained by photopatternable layers (it is not necessary to include the technological offset into the design procedure). Till this moment we do not notice differences in obtained geometry and/or electrical properties concerned with kind of laser used for shaping. Therefore we think that after improvement of thickness uniformity the distribution of resistance will be significantly reduced, even for so small planar dimensions. Next the electrical properties are very similar to resistors prepared in standard thick-film technology. This suggest that heat affecting zone, which usually appears during interaction of laser beam with thick-film materials has very limited area and does not affect electrical and stability properties.
Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education, Grant No. R02 017 02 and Wroclaw University of Technology, Grant No. 343 578 W-12. References Fig. 13. Resistance changes vs. electric field for 2021 resistors (tp = 20 ls).
[17]. We chose the rectangular pulses with duration time equal to 20, 200 or 1000 ls. As an example we present relative resistance changes as a function of electrical field E and surface power density Ps (Figs. 11–14) – both above mentioned dependencies were calculated based on real dimensions of microresistors. We assumed that the pulse amplitude and width were called critical if during or after the series of two rectangular pulses (with various amplitude and duration but 2 s break between pulses) the resistance of tested components was changed by more than ±10% or investigated devices were destroyed completely. The received level of critical electrical field and critical surface power density and dependencies of these parameters on pulse duration and resistor dimensions are similar to reported in [6,17]. This suggests that from the point of pulse durability there
[1] Dziedzic A. Electrical and structural investigations in reliability characterisation of modern passives and passive integrated components. Microelectron Reliab 2002;42:709–19. [2] Ulrich RK, Schaper LW, editors. Integrated passive component technology. Wiley Interscience; 2003. [3] Dziedzic A. Modern micropassives: fabrication and electrical properties. Bull Pol Ac Sci: Technical Sci 2006;54:9–18. [4] Minalgiene J, Baltrusaitis V. Photoimageable thick film implementation of very high density ceramics technology products. In: Proc 39th IMAPS Nordic annual conf, Stockholm; September 2002. p. 233–42. [5] Dziedzic A, Rebenklau L, Golonka LJ, Wolter KJ. Fodel microresistors – processing and basic electrical properties. Microelectron Reliab 2003;43:377–83. [6] Dziedzic A, Mis E, Rebenklau L, Wolter KJ. Geometrical and electrical properties of LTCC and thick-film microresistors. Microelectron Int 2005;22(1):26–33. [7] Henke M, Bauer R, Golonka LJ, Rebenklau R, Dziedzic A, Wolter KJ. Investigations on LTCC-multilayer with high density pattern and cofired resistors. In: Proc 22nd int spring seminar on electronics technology, Dresden, Germany; May 1999. p. 105–10. [8] Bauer R, Dziedzic A, Golonka L, Wolter KJ. Investigation for miniaturization of thick film resistors by application of a postpatterning thick film system. In: Proc 1st Eur microelectronics and packaging symp, Prague; 2000. p. 134–9.
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[9] Lakeman C, Fleig P. High resolution integration of passives using micro-contact printing (lCP). Microelectron Int 2003;20(1):52–5. [10] Mis E, Dziedzic A. Multiterminal thick-film and LTCC microresistors – contact effect simulation and trimming procedure consideration. Elektronika 2006;47(12):16–8. [11] Kita J, Dziedzic A, Golonka LJ. Non-conventional application of laser in LTCC and thick-film technology – preliminary results. In: Proc 23rd int spring seminar on electronics technology, Balatonfured, Hungary; May 2000. p. 219–24. [12] Illyefalvi-Viteez Z. Laser processing for microelectronics packaging applications. Microelectron Reliab 2001;41:563–70. [13] Drue KH. Precise drilling and structuring of LTCC materials using a 355 nm YAG-laser. In: Proc EMPC 2005, Brugge; June 2005. p. 410–5.
[14] Mis E, Borucki M, Dziedzic A, Kamin´ski S, Rebenklau L, Wolter KJ, et al. ‘‘Lasershaped thick-film and LTCC microresistors”. In: ESTC 2006, Dresden; 2006. p. 954–60. [15] Dziedzic A, Kolek A, Ehrhardt A, Thust H. Advanced electrical and stability characterization of untrimmed and variously trimmed thick-film and LTCC resistors. Microelectron Reliab 2006;46:352–9. [16] Tobita T, Takasago H. New trimming technique for a thick film resistor by the pulse voltage method. IEEE Trans CHMT 1991;14:613–7. [17] Dziedzic A, Golonka LJ, Kita J, Roguszczak H, Zdanowicz T. Some remarks about ‘‘short” pulse behaviour of LTCC and thick-film microsystems. In: Proc 1st Eur microelectronics and packaging symp, Prague, Czech Republic; June 2000. p. 194–9.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Fabrication and electrical properties of laser-shaped thick-film and LTCC microresistors Damian Nowak a,*, Edward Mis´ a, Andrzej Dziedzic a, Jarosław Kita b a b
Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland University of Bayreuth, Functional Materials Group, Universitätstrasse 30, D-95440 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 12 November 2008 Received in revised form 13 January 2009 Available online 3 April 2009
a b s t r a c t The dimensions of discrete passives, passive integrated components (arrays, networks) and embedded integral ones should be reduced significantly in the nearest future. Therefore the relations between technological accuracy and limitations, minimal geometrical dimensions and electrical as well as stability properties become more and more important. This paper presents systematic studies of thick-film or LTCC microresistors made with the aid of laser shaping. The investigations are concerned with miniaturization of two resistor dimensions, namely length (down to 30 lm) and width (also down to 30 lm). The sheet resistance, hot temperature coefficient of resistance (HTCR) as well as long-term stability and durability of test structures to various short electrical pulses were related to geometrical properties of microresistors. Such investigations proved that combining of current materials and fabrication methods used in modern thick-film and LTCC technologies with laser shaping made possible fabrication of 30 30 lm2 microresistors with satisfactory electrical properties and can serve as interesting alternative for thick-film and LTCC resistors miniaturization. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Modern packaging needs next generation of passives, which should be much smaller, cheaper and much more integrated. There are various attempts that can be used for size reduction of thickfilm and/or LTCC components and circuits. The concept and modern technological approach to the fabrication of discrete, integrated and integral micropassives is presented, e.g. in [1–3]. Photopatterning techniques seem to be the most interesting because they permit to reduce component dimensions down to 20 lm. Unfortunately, only conductive and dielectric photosensitive inks are available as far. There are also experimental photosensitive resistive inks and in the literature one can find information about microresistors made from them [4–6] but these inks are still not commercially available. There are also other attempts to miniaturization of thick-film and LTCC resistors like combination of photopatterned contacts and screen-printed resistive film [7,8], application of microcontact printing [9] or substitution of array with two-terminal resistors by equivalent multicontact resistor [10]. But novel solution of this problem is still needed. Authors propose to use laser shaping for fabrication of thickfilm and LTCC resistors. Although many laser applications, especially related to complete microcircuit size reduction and packaging density increase, are reported in the literature (some results * Corresponding author. E-mail address: [email protected] (D. Nowak). 0026-2714/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2009.02.019
are presented for example in [11–13]), according to authors’ knowledge there is no information about geometrical, electrical and stability properties of thick-film or LTCC microresistors made by the aid of laser shaping. This paper presents systematic studies of such microresistors. The individual resistors were created on alumina or LTCC substrates by laser cutting of conductive and resistive films after screen-printing and firing. Two kinds of microresistors, with variable length or width, have been prepared. The geometrical parameters were correlated with: sheet resistance and its distribution, hot temperature coefficient of resistance, normalized temperature dependence of resistance in a wide temperature range, durability of microcomponents to various short electrical pulses and longterm stability at step-increased ageing temperature. The results obtained for laser-shaped microresistors were compared with previously investigated microresistors made by standard screen-printing or by using full Fodel process and experimental Fodel resistive ink [3,5–8]. 2. Fabrication and geometrical properties of laser-shaped microresistors The test structures were made on alumina (96% Al2O3) or LTCC (DP 951 tape from DuPont). The microresistors with variable length (30–300 lm), were created by laser cutting of fired (300 lm wide) PdAg-, Au- or Ag-based conductive films (Fig. 1a). Such small dimensions were patterned with frequency-tripled
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
601
Fig. 1. (a) Conductive path with laser made gap (b) gap filled with screen-printed resistive ink (c) top view of resistors with regulated width.
(beam wavelength of 355 nm) Microline 350L laser system (LPKF, Germany) equipped with an arc lamp pumped Nd:YAG-laser with Q-switching. Relatively low energy laser beam is needed to avoid injury of the substrate. Next DP 2021 (100 ohm/sq.) or DP 2041 (10 kX/sq.) ink has been screen-printed (overlapping 200 lm) and fired. For comparison of manufacturing methods standard screen-printed resistors with lengths from 300 to 1800 lm were prepared on the same substrate. The microresistors with constant length and variable width down to 30 lm (Fig. 1c) were prepared in two technological versions – first by proper cutting of 1 1 mm2 DP 6620 (100 ohm/ sq.) or DP 6641 (10 kX/sq.) fired resistors (laser equipment presented above was used). In the second way conductors were made in full Fodel process whereas the resistive film was screen-printed. Photosensitive Ag-based DP 6453 Fodel conductor was used for terminations whereas DP 2031 (1 kX/sq.), CF 041 (10 kX/sq.) or R 8951 (100 kX/sq.) were applied as resistive films. KrF excimer laser (LPX 210 Lambda Physik model, wavelength k = 248 nm, 30 lm laserspot diameter on the surface, repetition rate 200 Hz, energy density on the surface 40 J/cm2, shaping by scanning with one pulse per lm) was used for shaping [14]. The test structures consisted of six resistors 170 lm wide and 200, 400, 600 or 800 lm long (see e.g. Fig. 2) or two resistors 1000, 200, 100, 50 or 30 lm wide and 1000 lm long, every separated by 30 lm notch. One should note that it is not necessary to use technological margins between adjacent terminations (the distance between them is determined by laser notch).
Laser profilometer (surface measuring system OME lScan AF2000 from Nanofocus) was used for three-dimensional (3D) characterization of investigated structures. A typical two-dimensional (2D) profile is shown in Fig. 2. It is clearly visible that the thickness of resistive film is not identical at every point. The mean thickness of these films is about 10 lm, both on alumina and LTCC substrates. The depth of laser kerf is dependent on cut material and kind of substrate – the same pulse energy of laser gives much deeper notches in LTCC substrates in comparison with alumina ones (for alumina substrate the notch exists only in resistive or conductive film – not in alumina). Moreover it is much more difficult to make cut in fired conductive film than resistive one.
3. Basic electrical properties Examples of sheet resistance (Rsq) vs. resistor length dependences are shown in Figs. 3 and 4. When the resistor length is increased from 200 to 800 lm (or 1800 lm) then the sheet resistance is increased by 20–50%. The increase level is dependent on kind of resistive film. The ±15% resistance distribution is noted for laser-shaped structures. This distribution is much less in comparison with screen-printed microresistors and comparable with Fodel ones [5,6]. Hot temperature coefficient of resistance (HTCR) was also measured and analyzed. HTCR is defined as:
602
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
Fig. 2. Planar thickness map of 800 170 lm2 resistors (DP2031/LTCC).
where R(125 °C) and R(25 °C) are resistances at 125 °C and 25 °C, respectively. Resistors with regulated length except of Au terminals show TCR decrease with length increase (Fig. 5). Resistors with regulated width exhibit HTCR value practically independent on their width (Fig. 6). The Keithley 2000 Multimeter interfaced to personal computer for data acquisition and presentation was used in measurements of resistance vs. temperature in the range from 170 to 130 °C. The examples of normalized temperature dependence of resistance and differential TCR, where TCRdiff ¼ dR=ðR dTÞ, are shown in Figs. 7 and 8. The presented characteristics are typical for thick-film resistors – they exhibit resistance minimum of at a certain temperature. This minimum shifts towards lower temperature when the resistor aspect ratio is increased. This together with results presented in Figs. 3 and 4 suggests normal dimensional effect. Such Fig. 3. Sheet resistance of DP2041 films vs. resistor length.
Fig. 4. Sheet resistance of R8951/Al2O3 resistors vs. resistor length.
HTCR ¼
½Rð125 CÞ Rð25 CÞ 106 Rð25 CÞ½125 C 25 C
ð1Þ Fig. 5. HTCR as a function of resistor length (DP 2041 resistive layer).
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
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Fig. 6. HTCR as a function of resistor width.
Fig. 8. Normalized temperature dependence of resistance (a) and differential TCR (b) for DP6641 resistors.
Fig. 7. Normalized temperature dependence of resistance (a) and differential TCR (b) for 800 170 lm2 resistors (R8951/Al2O3 structures).
resistors exhibit smaller resistance drift. This means that ageing processes within resistor volume give smaller fractional resistance changes than those appearing at the resistor/conductor interface. Screen-printed resistors exhibit similar stability level under the same ageing conditions [6,15]. This suggests that laser affected zone, appearing during shaping is very small and can be neglected during analysis of electrical and stability properties for structures with resistor width larger than 150 lm. 5. Pulse durability
changes of R(T) characteristics are a result of interactions between conductive and resistive film. 4. Long-term thermal stability We analyzed long-term stability based on measurements of resistance drift induced by long-term thermal ageing at three different temperatures 150, 200 and 250 °C. The samples were kept at every temperature for about 300 h. Some examples of changes observed after long-term exposure at 250 °C are shown in Fig. 9. Rather insignificant resistance changes are observed in general. Resistors with Au-based terminations have better stability as those with Ag- or PdAg-based contact layers. Moreover longer and wider
Authors used voltage pulses for two kind of tests (Fig. 10) – analysis of allowable voltage between adjacent terminals and investigation of resistors pulse durability. The first test indicates the class of electronic circuits, where such resistor ladders can be applied. The results are collected in Table 1. It is very interesting that although the distance between adjacent resistors is very small (approximately only about 30 lm, i.e. much less than for other thick-film and LTCC microresistors design and solution) such structures withstand voltage pulses of several hundred volts. In spite of deeper laser notch for resistors on LTCC substrate the allowable pulse voltages are somewhat larger for structures on
604
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
Fig. 9. Long-term stability of laser-shaped microresistors: (a) resistive ink 2021, (b) resistive ink 2041, (c) resistive inks 6620 and 6641 (s – standard screen-printing, p – laser shaping).
Table 1 Resistance to high voltage shocks between terminals for two-terminal resistors.
Fig. 10. Concept of measuring: (a) resistance to high voltage shocks between terminals and (b) pulse durability for two-terminal resistors.
Substrate
l (lm)
U (V), tp = 10 ls
U (V), tp = 1 ms
Al2O3 Al2O3 Al2O3 Al2O3 LTCC LTCC LTCC LTCC
200 400 600 800 200 400 600 800
>475 >475 >475 >475 >475 >475 410 280
320 >475 460 365 410 450 345 237
alumina. This is probably caused by larger reactivity of resistive film with LTCC substrate. For longer resistors the breakdown voltage becomes somewhat smaller probably because notch width is not constant but there are places where it becomes smaller. Pulse treatment is used e.g. for trimming of thick-film resistors [16] – more information about properties of such trimmed structures one can find in [15]. But investigations of pulse durability can help also to fix the critical electric field and next surface or volume power density for microresistors in dependence of resistor dimensions or pulse duration [6]. Voltage pulse exposures were realized with the aid of self made Programmed Pulse Generator
D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
605
Fig. 14. Resistance changes vs. surface power density for 2021 resistors (tp = 20 ls). 2
Fig. 11. Resistance changes vs. electric field for 200 170 lm resistors.
DP2031/Al2O3
is no noticeable differences between standard screen-printed, Fodel-made and laser-shaped microresistors. 6. Conclusions
Fig. 12. Resistance changes vs. pulse surface power density for 200 170 lm2 DP2031/Al2O3 resistors.
This paper presents fabrication and wide spectrum of geometrical, electrical and stability properties of laser-shaped thick-film and LTCC microresistors. The results show that this is very interesting solution for miniaturization of thick-film components. From geometrical point of view the quality of design image is much better than for standard screen-printing and can be even better than obtained by photopatternable layers (it is not necessary to include the technological offset into the design procedure). Till this moment we do not notice differences in obtained geometry and/or electrical properties concerned with kind of laser used for shaping. Therefore we think that after improvement of thickness uniformity the distribution of resistance will be significantly reduced, even for so small planar dimensions. Next the electrical properties are very similar to resistors prepared in standard thick-film technology. This suggest that heat affecting zone, which usually appears during interaction of laser beam with thick-film materials has very limited area and does not affect electrical and stability properties.
Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education, Grant No. R02 017 02 and Wroclaw University of Technology, Grant No. 343 578 W-12. References Fig. 13. Resistance changes vs. electric field for 2021 resistors (tp = 20 ls).
[17]. We chose the rectangular pulses with duration time equal to 20, 200 or 1000 ls. As an example we present relative resistance changes as a function of electrical field E and surface power density Ps (Figs. 11–14) – both above mentioned dependencies were calculated based on real dimensions of microresistors. We assumed that the pulse amplitude and width were called critical if during or after the series of two rectangular pulses (with various amplitude and duration but 2 s break between pulses) the resistance of tested components was changed by more than ±10% or investigated devices were destroyed completely. The received level of critical electrical field and critical surface power density and dependencies of these parameters on pulse duration and resistor dimensions are similar to reported in [6,17]. This suggests that from the point of pulse durability there
[1] Dziedzic A. Electrical and structural investigations in reliability characterisation of modern passives and passive integrated components. Microelectron Reliab 2002;42:709–19. [2] Ulrich RK, Schaper LW, editors. Integrated passive component technology. Wiley Interscience; 2003. [3] Dziedzic A. Modern micropassives: fabrication and electrical properties. Bull Pol Ac Sci: Technical Sci 2006;54:9–18. [4] Minalgiene J, Baltrusaitis V. Photoimageable thick film implementation of very high density ceramics technology products. In: Proc 39th IMAPS Nordic annual conf, Stockholm; September 2002. p. 233–42. [5] Dziedzic A, Rebenklau L, Golonka LJ, Wolter KJ. Fodel microresistors – processing and basic electrical properties. Microelectron Reliab 2003;43:377–83. [6] Dziedzic A, Mis E, Rebenklau L, Wolter KJ. Geometrical and electrical properties of LTCC and thick-film microresistors. Microelectron Int 2005;22(1):26–33. [7] Henke M, Bauer R, Golonka LJ, Rebenklau R, Dziedzic A, Wolter KJ. Investigations on LTCC-multilayer with high density pattern and cofired resistors. In: Proc 22nd int spring seminar on electronics technology, Dresden, Germany; May 1999. p. 105–10. [8] Bauer R, Dziedzic A, Golonka L, Wolter KJ. Investigation for miniaturization of thick film resistors by application of a postpatterning thick film system. In: Proc 1st Eur microelectronics and packaging symp, Prague; 2000. p. 134–9.
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D. Nowak et al. / Microelectronics Reliability 49 (2009) 600–606
[9] Lakeman C, Fleig P. High resolution integration of passives using micro-contact printing (lCP). Microelectron Int 2003;20(1):52–5. [10] Mis E, Dziedzic A. Multiterminal thick-film and LTCC microresistors – contact effect simulation and trimming procedure consideration. Elektronika 2006;47(12):16–8. [11] Kita J, Dziedzic A, Golonka LJ. Non-conventional application of laser in LTCC and thick-film technology – preliminary results. In: Proc 23rd int spring seminar on electronics technology, Balatonfured, Hungary; May 2000. p. 219–24. [12] Illyefalvi-Viteez Z. Laser processing for microelectronics packaging applications. Microelectron Reliab 2001;41:563–70. [13] Drue KH. Precise drilling and structuring of LTCC materials using a 355 nm YAG-laser. In: Proc EMPC 2005, Brugge; June 2005. p. 410–5.
[14] Mis E, Borucki M, Dziedzic A, Kamin´ski S, Rebenklau L, Wolter KJ, et al. ‘‘Lasershaped thick-film and LTCC microresistors”. In: ESTC 2006, Dresden; 2006. p. 954–60. [15] Dziedzic A, Kolek A, Ehrhardt A, Thust H. Advanced electrical and stability characterization of untrimmed and variously trimmed thick-film and LTCC resistors. Microelectron Reliab 2006;46:352–9. [16] Tobita T, Takasago H. New trimming technique for a thick film resistor by the pulse voltage method. IEEE Trans CHMT 1991;14:613–7. [17] Dziedzic A, Golonka LJ, Kita J, Roguszczak H, Zdanowicz T. Some remarks about ‘‘short” pulse behaviour of LTCC and thick-film microsystems. In: Proc 1st Eur microelectronics and packaging symp, Prague, Czech Republic; June 2000. p. 194–9.
