978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 82 33rd Int. Spring Seminar on Electronics Technology

Miniaturization of Thick–Film Resistors by Laser–Shaping

Damian Nowak1), Andrzej Dziedzic1), Marko Hrovat2), Jena Cilenšek2)

1) Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

2) Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia damian.nowak@pwr.wroc.pl

Abstract: This paper presents manufacturing process of laser-shaped thick-film resistors as well as their chosen electrical, geometrical, microstructural and stability properties. Test structures were based on commercially available conductive and resistive pastes deposited on Al2O3 substrates. Resistors made in standard screen printing process after firing were patterned using laser beam in order to minimize their planar dimensions to tenths of millimeter and below. Prepared samples were subjected to number of electrical tests and some characteristic parameters like temperature coefficient of resistance (TCR) were calculated based on these measurements. Test samples were also exposed to long-term thermal ageing. Moreover the laser profilometer was used for three-dimensional characterization of components. Scanning electron microscope (SEM) was used for the microstructural analysis and the EDS (Energy Dispersive X-ray Spectroscopy) of samples is given.

1. INTRODUCTION

Miniaturization is a crucial problem in all electronic applications. A continuous reducing of dimensions is caused by number of factors. One is the cost reduction, another is necessity of permanent circuit speed increase due to ever more sophisticated applications operating at higher frequency.

Thick-film technology has been well known since many years. It has many advantages like low cost and durability on its side. However there are some disadvantages like quite big planar dimensions of components. It is impossible to scale thick-film passives down under some few hundred micrometers by using standard screen-printing. That is why new methods of miniaturization in this technology are investigated.

New class of photosensitive inks promise further downscaling of passives, resistors made with use of such materials were studied in [1]. Although usage of these inks may significantly reduces passives dimensions, systems covering full spectrum of necessary pastes (conductive, resistive, dielectric) are commercially not available and probably described

technology will not be considered as standard one for long time.

In this paper the results of experimental attempts towards search of physical limits of thick-film resistors miniaturization made of standard materials are presented. A classical manufacturing process was combined with highly non classical production way involving laser as main tool for structure shaping, so far commonly applied only for trimming of resistors [2]. Similar approach to miniaturization is described in [3,4].

2. TEST STRUCTURES FABRICATION

The microresistors with constant length and regulated width from 200 μm down to 30 μm (Fig. 1) have been prepared by proper cutting of 1×1 mm2 DP 6620 (100 ohm/sq.) or DP 6641 (10 kohm/sq.) fired thick-film resistors. Such small dimensions were patterned with frequency-tripled (beam wavelength of 355 nm) Microline 350L laser system (LPKF, Germany) equipped with an arc lamp pumped Nd:YAG-laser with Q-switching.

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 83 33rd Int. Spring Seminar on Electronics Technology

Fig. 1. Top view of resistors with regulated width

3. BASIC ELECTRICAL PROPERTIES

3.1.Resistance vs geometry

Resistive

paste

Width

[µm]

R

[kΩ]

30 16,5

50 9,2

100 5,1

200 2,6

6620

1000 0,5

30 246,5

50 150,8

100 76,5

200 38,0

6641

1000 8,4

Tab. 1. Resistance of resistors with different width

3.2. Temperature coefficient of resistance

Hot temperature coefficient of resistance (HTCR) was also measured and analyzed. HTCR is defined as:

610]25125)[25(

)]25()125([×

°−°°°−°

=CCCR

CRCRHTCR (1)

where R(125°C) and R(25°C) are resistances at 125°C and 25°C, respectively. Resistors with regulated width exhibit HTCR value practically independent of their width (Fig. 2).

Fig. 2. HTCR as a function of resistor width

3.3. Temperature dependence of resistance

The Keithley 2000 Multimeter interfaced to personal computer for data acquisition and presentation was used for measurements the dependence of resistance as a function of temperature in the range from -170°C to 130°C. The examples of normalized temperature dependence of resistance and differential TCR, where

dTRdRTCRdiff ⋅

= (2)

are shown in Figs. 3 and 4.

Fig. 3. Normalised temperature dependence of resistance for microresistors with regulated width

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 84 33rd Int. Spring Seminar on Electronics Technology

Fig. 4. Differential temperature coefficient of resistance for microresistors with regulated width

The presented characteristics are typical for thick-film resistors - they exhibit resistance minimum at a certain temperature.

4. LONG–TERM STABILITY

We characterized and analyzed long-term stability based on measurements of resistance drift induced by long-term thermal ageing at temperature of 200°C in two steps for 150 hours. The results are shown in Fig. 5.

Resistors exhibit relative resistance changes from 0.5% to 3.5%, dependent on resistors’ width. Thus, influence of laser beam on structures stability was observed. So called heat affected zones caused larger resistance changes for narrower resistors.

-4,00-3,50-3,00-2,50-2,00-1,50-1,00-0,500,000,501,00

30 50 100 200 1000

w [µm]

ΔR/

R [%

]

6620 step I6620 step II6641 step I6641 step II

-4,00-3,50-3,00-2,50-2,00-1,50-1,00-0,500,000,501,00

30 50 100 200 1000

w [µm]

ΔR/

R [%

]

6620 step I6620 step II6641 step I6641 step II

Fig. 5. Long-term stability of laser-shaped microresistors

5. GEOMETRICAL AND MICROSTRUCTURAL PROPERTIES

Optical profilometer (Talysurf CCI Lite from Tylor Hobson) was used for geometrical characterization of investigated structures. A three-dimensional and cross-section profiles are shown in Figs. 6 and 7.

Fig. 6. 3D thickness map of 200x1000 µm2 resistors

Fig. 7. Profile of 200x1000 µm2 resistor through resistive film

It is visible that the thickness of resistive film is not identical at every point. The average thickness of these films is about 10 µm.

A JEOL JSM 5800 scanning electron microscope (SEM) was used for the microstructural analysis [4]. For the investigation prior to analysis in the SEM the polished cross-sections of samples were coated (sputtered) with a carbon to provide electrical conductivity and to avoid charging effects. The microphotographs were taken either by backscattered electrons (BEI – back-scattered electrons image) or secondary electrons (SEI - secondary electrons image) imaging.

SEM images show cross-sections of overlap area (Fig. 8) and resistor (Fig. 9). Fig. 8 shows that resistive (a) and conductive (b) layers exhibit different resistance to cutting with laser beam. The kerf is wider in resistive layer. Moreover they show that laser

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 85 33rd Int. Spring Seminar on Electronics Technology

beam removed also about 10 μm of alumina substrate (c). Cracks in resistors’ cross-sections and in alumina substrates as well as debris in laser cuts are due to the preparation of samples – cutting and polishing.

Fig. 8. The microstructure of the sample 6620 (Rsq = 100

Ω/sq). Cross-section, mag. 2500x, SEI.

Fig. 9. The microstructure of the sample 6641(Rsq =

10 kΩ/sq). Cross-section, mag. 2500x, SEI

The EDS analysis of the resistor film over the area of around 5x5 µm2 is shown in Fig. 10. The ratio between Bi and Ru indicates that bismuth ruthenate is the conductive phase in the resistor. In Fig. 11. the results of the analysis of lighter phase between alumina grains near the interface of the alumina substrate and thick film layers in the substrate is presented. As the analyzed phase is too small for an exact analysis the EDS also include surroundings which explain the high concentration of the alumina. The presence of lead as well as relatively high concentration of the silicone indicates a diffusion of glass phase from thick film layers into the alumina

substrate. Elements not detected are denoted by asterisk in Tables.

Fig. 10. EDS analysis of the thick-film resistor (DP 6620)

Fig. 11. EDS analysis of the interface phase (DP 6620)

6. CONLUSIONS

This paper presents fabrication and wide spectrum of 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. We think that after improvement of manufacturing process distribution of electrical parameters will be significantly reduced, even for so small planar dimensions. Next the electrical properties are 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. However resistors made with the aid of laser beam present somewhat worse stability properties.

REFERENCES

[1] A. Dziedzic, L. Rebenklau, L.J Golonka., K.J. Wolter, "Fodel microresistors – processing and basic electrical properties", Microelectronics Reliability, vol. 43 (2006), pp. 377 – 383.

978–1–4244–7850–7/2010/$26.00 ©2010 IEEE 86 33rd Int. Spring Seminar on Electronics Technology

[2] S. Kamiński, E. Miś, M. Szymendera, A. Dziedzic, "New trim configurations for laser trimmed thick-film resistors – experimental verification", Journal Microelectronics and Electronic Packaging, 2 (2005), no 1, pp. 19-24.

[3] E. Miś, M. Borucki, A. Dziedzic, S. Kamiński, L. Rebenklau, K.-J. Wolter, F. Sonntag, "Laser-Shaped Thick-Film and LTCC Microresistors", Proceedings of Electronics Systemintegration Technology Conference, Dresden, Germany, 2006, pp. 954 – 960.

[4] D. Nowak, E. Miś, A. Dziedzic, J. Kita, "Fabrication and electrical properties of laser-shaped thick-film and LTCC microresistors", Microelectronics Reliability, 49 (2009), pp. 600-606

[5] M. Hrovat, D. Belavic, Z. Samardzija, J. Holc, "A characterisation of thick film resistors for strain gauge applications", Journal of Materials Science, vol. 36 (2001), pp. 2679-2689