Andrzej Dziedzic, Leszek Golonka

Proc. 12 th European Microelectronics Conference, Harrogate (United Kingdom), 1999, p.313 -319

PULSE DURABILITY OF POLYMER, CERMET AND LTCC THICK-FILM RESISTORS Jarosław KITA, Andrzej DZIEDZIC, Leszek J. GOLONKA, Grzegorz ŻUK Institute of Microsystem Technology, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

This paper discusses the effect of medium and high voltage pulses on the behaviour of different thick-film resistors fabricated by standard cermet and polymer as well as LTCC technology. The Ferro FX87 LTCC resistors, different cermet ones fabricated on alumina substrate and self-made carbon black/polyesterimide elements with resistance from 1 k to 1 M have been tested. The durability to series of “short” high voltage pulses (Emax = 1000 V/mm) and I(U) and R(U) dependencies under “long” medium voltage pulses (Emax = 100 V/mm) of asfired/as-cured and laser trimmed devices have been measured and analysed. Moreover the long-term stability of fired/cured resistors, laser trimmed ones as well as exposed for high voltage pulses have been tested.

INTRODUCTION The susceptibility to high voltage pulses and electrostatic discharges is one of the very important electrical properties of thick-film resistors. Although this phenomenon has been examined for more than 20 years [1-3] still there is no generally accepted method for measurement either high-voltage discharge or voltage coefficient of resistance as there is with TCR testing or high temperature ageing. We can tell that similarly as for measurement of the electrical properties of varistors the so-called “long” (rectangular shape surges with 1 to 20 ms duration) or “short” (8/20 s with proper pulse waveform) pulses are used during examinations. The second ones usually are generated as a result of capacitive discharge. But also “short” (duration time below 1 ms), rectangular, high voltage pulses of varying width and amplitude are used very often. So far the diagnostics of thick-film resistors affected by single or series of “long” voltage (current) pulses have been carried out at the Wrocław University of Technology [4-5]. This paper presents the behaviour of different thick-film resistive devices, fabricated by standard cermet and polymer as well as LTCC technology, under “short” rectangular pulses. One should note that a pulse voltage trimming method has been developed to realise a noncut trimming of cermet and LTCC thick-film resistors without any damage to the resistor surface [6-8], especially for very small devices. Moreover, because low-ohm thick film resistors are widely used to protect telecommunication network equipment against lighting surge voltages therefore these investigations are applied for overstress characterization of low sheet resistance thick-film systems under standard telecommunication waveforms [9].

EXPERIMENTAL PART The commercial or self-made thick-film resistors presented in Table 1 have been tested. All of them have been fabricated according to recommendations of ink manufacturers; chosen information about their technology and fabrication have been published elsewhere [10-13]. Most of measurements have been performed on 2 2 mm2 resistor test samples (1.65 1.65 mm2 in the case of LTCC devices) but in one of

the applied test the resistors with different geometry (1 1, 2 1, 3 1, 4 1 and 5 1 mm2) have been applied. A part of examined structures has been laser trimmed by about +30 to +40% above as-fired/ascured resistance prior to pulse exposures and long-term thermal ageing. Table 1. List of tested resistors and their sheet resistance Cermet Nominal Type of LTCC Nominal Type of Polymer Nominal Thick-Film R [k /] experi-ment Thick-Film R [k /] experi- Thick-Film R [k /] Resistors Resistors ment Resistors A3 1 I, II, III F3B 1 I, II HS 3 2.5 (buried) A4 10 I, II, III F3S 1 I, II MS 13 2.5 (surface) A5 100 I, II F4B 10 I, II G 10 2.5 A6 1000 I, II, III F4S 10 I, II HS 0.8 50 B4 10 I, II F5B 100 I, II TU8/3 1 B6 1000 I, II F5S 100 I, II TU5/4 10

Type of experiment I, II, III I, II, III I, II I, II, III I, II I, II, III

The self-made, PIC-programmed, IGBT-switched pulse generator with pulse voltage regulated in the range from 400 to 2000 V, pulse width between 10 s and 10 ms, off time between pulses from 10 ms to 10 s, and number of pulses from 1 to 1000 has been used in our research [14] for investigation of durability to high voltage pulses. The influence of number of pulses, N (I, N = 1, 10, 50, 250), pulse time, ton (II, ton = 20, 50, 100, 200, 1000 s) and resistor geometry (III, 1 1, 2 1, 3 1, 4 1 and 5 1 mm2 samples) on the resistance changes of all kind of thick-film resistors have been tested. Moreover, the voltage amplitude has been changed in all tests. The voltage amplitudes applied for LTCC resistors have been multiplied by shrinkage factor (0.825) in comparison with those used for cermet and polymer ones to preserve the same electrical field in tested resistor. The current-voltage (I-V) characteristics were measured by the “pulse” method where “long” medium pulses have been applied [15]. The Keithley Source Measure Unit Model 2400 working in logarithmic stair mode has been applied. The voltage source (sweep) with current measurement has been chosen Because of the resistance of tested samples. The amplitude of 5 ms voltage pulses has been changed from 0.1 to 100 V. 25 points (voltage levels) per decade with 2 s pulse off time between every voltage pulse has been applied and the current has been measured at the end of every pulse. The R-V dependence has been calculated based on these measurements. Moreover, these measurements have been applied for comparative analysis of different thick-film resistors voltage nonlinearity. The long-term stability of electrical properties qualifies resistor usability in commercial applications. Therefore the relative resistive changes in resistance ( R/Ro) versus the storage time at 150oC have been measured and tested. The four groups of every resistor compositions were aged: - as-fired/as-cured, - fired/cured and then laser trimmed to about 30 to 40% above initial resistance, - fired/cured and then subjected to series of 50 high voltage pulses (ton = 20 s, toff = 200ms, U = 660 (F3, F4, F5), 800 (A4, B4, G 10, M 13, HS 3), or 1200 V (A5, B6, HS 0.8 resistors)), - fired/cured and laser trimmed and then subjected to series of high voltage pulses.

HIGH VOLTAGE DURABILITY Chosen results of high voltage pulses are shown in Figures 1 and 2. During this experiment the following have been found:


1. The first voltage pulse causes the strongest resistance change when the moderate electric field (E 800 V/mm) is applied to the resistor. Successive pulses affect the resistance level much weaker. 2. The effect of high-voltage pulses is dependent on resistor system (manufacturer). For example films A show relative resistance changes larger by about 2 orders of magnitude than resistors B with the same sheet resistance. 3. Some of polymer thick-film resistors are characterized by surprisingly good durability to shortest applied pulses (20 s). The changes observed in this case are less than for a part of cermet resistors and much better than for other polymer thick-film resistors [16]. 4. The resistance decreases for cermet and LTCC resistors whereas its increase is observed for some polymer thick-film resistors even for moderate electric field. 5. In most cases the changes in trimmed resistors show opposite tendency in comparison with untrimmed ones. For films with excellent high voltage durability (e.g. B4) the laser trimming causes noticeable deterioration of voltage durability whereas the absolute level of the relative resistance changes of trimmed and untrimmed compositions for general purposes (e.g. A4, A5) is similar. 6. Prolongation of the pulse width and/or increase of its amplitude create destruction of the film even during the first pulse. 7. The observed changes practically do not depend on the resistor geometry; rather not absolute voltage but its gradient (electrical field) causes resistance shift. A4 trimmed

A4 0

U=400 V U=800 V U=1200 V U=1600 V

8

-2

6

-6

R/R [%]

R/R [%]

-4

-8 -10

2

U=400 V U=800 V U=1200 V U=1600 V

-12 -14 -16

1

4

0 10

100

1

N 0,00

B4

1,5

R/R [%]

R/R [%]

-0,10 -0,15

-0,30

U=400 V U=800 V U=1200 V U=1600 V U=2000 V 1

100

U=400 V U=800 V U=1200 V U=1600 V

-0,05

-0,25

N

B4 trimmed 2,0

-0,20

10

1,0

0,5

0,0

10

100

N

1

10 N

100

Fig.1. Resistive changes as a function of the number of applied pulses for different cermet resistors

3

F3B trimmed

F3B

U=330 V U=660 V U=1000 V

50

0 40

-3

30 R/R [%]

R/R [%]

-6 -9 -12

10

U=330 V U=660 V U=1000 V

-15 -18 1

5

20

10

N

0 -10

100

1

10

N

100

HS 3 trimmed

HS 3 U=400 V U=800 V U=1200 V U=1600 V

4

0

R/R [%]

R/R [%]

-5

3

2

-10

-15

1

U=400 V U=800 V U=1200 V

-20

0 1

10 N

100

1

10

100 N

Fig.2. Resistance changes in LTCC and polymer resistors as a function of the number of applied pulses

VOLTAGE NONLINEARITY The resistance-voltage characteristics of different thick-film resistors are given in Fig. 3. One should note that the level of voltage susceptibility of high performance resistors (B4, B6) after laser trimmed is much larger than before. The R-V curves for general purpose system (represented by A4 and A5 compositions) and LTCC resistors are not affected by trimming. It is interested that nonlinearity of polymer resistors is comparable with the others. The empirical relationship I = G·V , where G is a constant with conductance dimension and is a parameter related to nonlinearity of samples, has been used for fitting of measured I-V characteristics. For all cases is insignificantly greater than 1 (for example 1.0006 for B4, 1.003 for A4, 1.004 for trimmed and 1.008 for untrimmed F4B, 1.007 for HS 0.8 and 1.012 for MS 13 resistors). Such values of confirm generally very weak nonlinearity of all tested kinds of thick-film resistors. Except of the above relationship it is possible to approximate I-V data by third order polynomial V = AI + BI3 And very recently such an equation has been used for description of samples with somewhat noticeable nonlinearity i.e. polymer thick-film resistor [15] or integral resistors in LTCC [17].

0,005

0,001

0,000

0,000

R/R1V

R/R1V


-0,005

-0,001

-0,002 -0,010

-0,020

-0,003

A4 untrimmed A4 trimmed A5 untrimmed A5 trimmed

-0,015

1

B4 untrimmed B4 trimmed B6 untrimmed B6 trimmed

-0,004

10 U[V]

100

1

0,005

10 U[V]

100

0,01 0,00

0,000

R/R1V

R/R1V

-0,01

-0,005

-0,010

-0,03

F3S untrimmed F3S trimmed F3B untrimmed F3B trimmed

-0,015

-0,020

-0,02

1

10 U[V]

HS0.8 untrimmed HS0.8 trimmed HS3 untrimmed HS3 trimmed

-0,04 -0,05

100

1

10 U[V]

100

Fig. 3. Resistance – voltage characteristics

LONG-TERM STABILITY The relative resistance changes of various types of thick-film resistors as-fired/cured and subjected to different laser beam/high voltages exposures are compared in Fig. 4. Almost all tested compositions exhibit very small resistance changes, less than 0.2 % after 500 h exposure at 150 oC. Additional exposures before thermal ageing affect relative resistance changes strongly. For example, the voltage pulses cause such destruction of LTCC resistors, that during ageing their resistance is decreased all the time and R/Ro is in the range –1 up to –4% after 500 h at 150oC. On the other hand in the case of polymer resistors laser trimming creates stronger changes than “short” voltage pulses. In the case of cermet resistors their long-tem stability is somewhat worse if prior to stability test devices have been laser trimmed and/or exposed to voltage pulses. In general after additional exposures the stability is decreased but there is no simple correlation between such additional exposures and long-term behaviour of thick-film resistors, it is dependent very strong on kind (manufacturer) of resistors.

B6

A5 1,8

1,6

1,6

1,4

1,4

1,2 1,0

1,0

R/R [ % ]

R/R [ % ]

1,2

FIRED ( F ) F+VOLTAGE PULSES ( VP ) F+LASER TRIMMED ( LT ) F+LT+VP

0,8 FIRED ( F ) F+VOLTAGE PULSES ( VP ) F+LASER TRIMMED ( LT ) F+LT+VP

0,6 0,4 0,2

0,8 0,6 0,4 0,2 0,0

0,0 -0,2

-0,2 1

10

100

1000

1

TIME [ h ]

100

1000

TIME [ h ]

F4B

F3S 0,0

0,0 FIRED ( F ) F+VOLTAGE PULSES ( VP ) F+LASER TRIMMED ( LT ) F+LT+VP

-0,2

FIRED ( F ) F+VOLTAGE PULSES ( VP ) F+LASER TRIMMED ( LT ) F+LT+VP

-0,5 -1,0 R/R [ % ]

-0,4 R/R [ % ]

10

-0,6

-1,5 -2,0

-0,8

-2,5 -1,0

-3,0 -1,2 1

10

100

1000

1

TIME [ h ]

1000

HS 3 CURED ( C ) C+VOLTAGE PULSES ( VP ) C+LASER TRIMMED ( LT ) C+LT+VP

6

R/R [ % ]

R/R [ % ]

8

CURED ( C ) C+VOLTAGE PULSES ( VP ) C+LASER TRIMMED ( LT ) C+LT+VP

2

100

TIME [ h ]

HS 0.8 3

10

1

4

2

0

0 -1

-2 1

10 TIME [ h ]

100

1000

1

10

100

TIME [ h ]

Fig.4. Influence of high voltage pulses and laser trimming on long-term stability

1000


CONCLUSIONS Three different types of thick film resistors – cermet, LTCC, and polymer ones – have been subjected to the same electrical tests for the first time. Their sensitivity to the “short” and “long” high voltage pulses have been evaluated for as-fired/cured as well as laser trimmed samples. Based on “short” pulses it is possible to determine the voltage durability whereas “long” medium ones permit to calculate voltage nonlinearity. Moreover, the long-term stability of all analysed resistor types has been measured and analysed. It has been shown that standard ageing of modern resistors do not permit to differentiate their behaviour. However prior exposition of tested devices to laser beam and especially to high voltage pulses makes possible much more selective analysis of their quality. ACKNOWLEDGEMENTS This work was supported by the Polish State Committee for Scientific Research, Grant No 8T11B 029 13 REFERENCES [1] [2] [3] [4] [5]

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