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Thin film systems for low TCR resistors
Vacuum/volume 38/numbers Printed in Great Britain
8-l O/pages
689 to 692/l
0042-207X/88$3.00+.00 Pergamon Press plc
988
Thin film systems for low TCR resistors ARazborsek and F Schwager, Mettler lnstrumente AG, 8606 Greifensee, Switzerland Resistors with TCR = 0 are important as reference resistors in many measurement instruments. We developed thin film resistors with resistance R between 7 and 5 kOhms and adjustable TCR’s between - 150 ppm “C-’ and +500 ppm “C-‘. The resistors consist of TaN thin films which are covered with overlying rectangular Nipads. R and TCR could be adjusted by varying process and geometry parameters. The thin films were sputter deposited and structured by photolithography. TCRs were found to be reproducible within f3?3 from batch to batch. We were able to design thin film systems with TCR = 0 for temperatures between 20 and 80°C.
1. Introduction Many electronic instruments, like electronic balances, use a constant current source which is obtained by applying a constant voltage on a reference resistor. This resistor should be very stable and independent of environmental conditions like temperature. Resistors with low temperature coefficients of resistivity (TCR) can be obtained with certain metal alloys like NiCr’,* with some oxygen content. These resistors are shaped into wires. For certain applications it might be advantageous to have thin film resistors, e.g. in combination with an integrated circuit. Thin film resistors of NiCr with very low TCR can be made and are even commercially available, but they are not easy to reproduce because the TCR depends heavily on the amount of oxygen in the films. We used a different approach to produce low TCR resistors than using a homogeneous material. It consisted of combining a material of negative TCR with a material with positive TCR. This shifts the problem of producing a very accurate alloy composition with TCR = 0 to the problem of accurate geometric and thickness control of the thin films of the two materials. GoreckaDrzazga et al’ have made thin film resistors by this method. They chose among other combinations tantalum-nitride as the material with the negative TCR, and nickel as the one with the positive TCR. The resistors were produced by first depositing Ta-nitride which possesses a higher specific resistivity than Ni which was deposited on top of it in rectangular pads. The resistors obtained by this method were accurate to within 5% of the calculated values. The authors claimed that errors were mainly due to misalignment of the photomask during photolithography. We used a similar method to produce resistors but improved the photolithographic shaping of the films to prevent alignment errors and also reduced the error due to undercutting of the thin films. 2. Experiments 2.1. Materials. Thin films of TaN and Ni were sputter deposited from a hot pressed TaN- and a mechanically machined Ni-target in argon onto a ceramic substrate. The 33 x 33 mm* Al,O,-substrate (Rubalit 710) was chosen because it offers excellent adhesion properties for the thin films, conducts heat very well, has a high dielectric constant of 9.9 and is temperature resistant. Its surface roughness is 0.1 pm.
2.2, Equipment. The thin films were deposited by sputtering in a Leybold-Heraeus Z 400 sputtering machine with load-lock. The machine can be equipped with 3 sputtering targets at the same time. For the thin-film resistors we used TaN-, Ni- and Autargets of 100 mm diameter. Argon was used as sputtering gas but we also investigated the TCR-dependence of the resistors by sputtering TaN in an argon/nitrogen atmosphere. Typical sputter conditions were pressure 1.5 x lOA2 mbar; gas flow 100 seem; power 5 W cmm2; electrode-distance 35 mm. 2.3. Measurements. The resistors were investigaed with a fourpoint probe and the values corrected by the method of Maloney4. For accurate temperature control, the resistors were immersed in a thermostatically controlled bath of an electrically inert liquid (Fluorinert FC43). The temperature could be varied between 0 and 80°C with an accuracy of +O.Ol”C. The TCRs were determined by equation (1) :
TCR = (R, -&)/R,(T,
-To)
[K-l],
(1)
where R, is the resistance at room temperature T,, and R, the resistance at some other temperature T,. The TCR is usually expressed in ppm. 2.4. Resistors. The layout of the resistors was produced in a way so that their value was of the order of l-5 kOhms. They consisted basically of meandering lines of 40 mm length and 0.25 mm width. The thin films were fabricated by photolithographically shaping the 3 layers of TaN (cu. 100-200 nm thick), Ni (100-200 nm) and Au (500 nm) on top. The Au-layer not only protected the underlying layers but it also acted as a masking layer for the successive etching of the Ni-layer, and reduced undercutting compared to using only photoresist as a masking layer. With the first mask continuous meandering lines were copied into the photoresist, and Au, Ni and TaN were etched away successively. Then photoresist was applied again and the second mask for the Ni-pads was copied. Figure 1 shows that the masks for the Nipads are wider than the TaN-lines. This way it was made sure that even with a misaligned Ni-mask, the TaN-lines were covered with Ni across the whole line width. The Au- and the Ni-layers between the pads were then etched away, and after stripping off the photoresist and the remaining Au on the Ni-pads the resistors 689
A Razborsek
Thin
andF.Schwager:
film
systems
for low TCR resistors
Ni 500 /Jm
rl
500 pm
TaN
0
350 pm
2-
Figure 2. TCR
250 pm 1lOOym
i 2050 pm
Figure 1. Layout
1
N
of thin
I
film systems
I
1000pm
t 2000m
for low TCR
P
resistors
were
cleaned in acetone and isopropanol. The resistors were then measured with a four-point probe. On a 33 x 33 mm’ alumina-substrate 12 resistors with different ratios of Ni-coverage were deposited. The mask for the meandering TaN-lines was the same for all resistors. The TCR was varied by changing the length of the Ni-pads, the number of the Nipads being kept constant. Widths of TaN-lines and Ni-lines were 0.25 and 0.35 mm, respectively. We used 2 masks to investigate the behaviour of the TCR over a wide range of different Nicoverages. The coverages ranged between 10 and 55%, and 45 and 90%, respectively, in steps of 5%. In addition, 2 resistors of pure TaN and Ni, respectively, were deposited for reference measurements on each substrate. The measurements have shown that the point of intersect of the TCR curve with the O-line lies in the region of 55 and 70% coverage depending on the TCR of TaN. Another set of masks with coverages between 50 and 80% in 1% steps was used for a more accurate determination of the point of intersect.
concentration in the sputtergas rbl
of TaN with different
obtain reproducible the plateau region here however, has argon. These films age value of TCR
N,-doping
films. For good reproducibility the films of should be used. Most of the work presented been carried out with TaN sputtered in pure could be reproduced within i_ 5% of an aver= - 102.9 ppm.
3.2. Resistor combinations. Figure 3 shows a plot of TCR-values for resistors with nitrogen doped TaN and Ni-coverages between 45 and 90%. TCR = 0 is reached at a coverage of 72%. The slope of the curve at TCR = 0 is 12 ppm per 1% of coverage change. The temperature dependence of the TCR was determined for a Ni-coverage of 70% between 25 and 75’C. For this purpose the resistances were measured in 10” intervals between 20 and 80°C and the TCRs calculated by means of equation (1). Figure 4 shows that the curve is approximately linear between 20 and
Figure 3. TCR vs Ni coverage
or Nz-doped
TaN
3. Results 3.1. Nickel and tantalum nitride. We first determined the TCRs of films of pure Ni and TaN. For Ni a TCR of 4700 ppm “C-’ was obtained between 20 and 50°C which compares with the values of bulk Ni with 6900 ppm “C _ ’ ‘. This difference is due to different structures of bulk Ni and sputtered Ni and also depends on the thermal expansion coefficient of the substrate material. Figure 2 shows TCR-values of TaN for varying nitrogen concentrations in the sputter gas. A distinct plateau which has been described beforeG8 exists between cu. 0.2 and 0.8% N2. The plateau expresses stoichiometric composition of TaN. Whereas TaN has a nitrogen deficiency at low N,-concentrations in the sputter gas it has a surplus at N,-concentrations above 1%. It would be advantageous to produce TaN-films with more negative TCRs but it is difficult to adjust the sputtering parameters to 690
20
40
Figure 4. TCR of resistor
so Temperature
for different
SO
[C]
temperatures.
A Razborsek and F Schwager:
Thin film systems for low TCR resistors
Figure 5. Dependence of TCR on doping with nitrogen (1%).
Figure 7. Long term stability of undoped resistors after tempering.
50°C. For most of the work we measured resistances at 20 and 5O”C, only. Theoretical considerations show that optimal results for TCRs should be obtained when the sheet resistances of both the TaNand the Ni-films are the same. This could not be confirmed with Ni-layers of too high sheet resistances (e.g. 50 Ohms square-‘). We found that after heat treatment of such films for 60 min at 300°C the value of the TCR reaches the values for TaN. As the Ni-film for a sheet resistance of 50 Ohms square-’ is only cu. 15 nm thick compared to cu. 100 nm of the TaN-film, this result can be explained by the oxidation of Ni during heat treatment. Further investigations have shown that a minimal Ni-film thickness of ca. 60 nm is required to obtain stable resistors. The influence of N,-doping of TaN on the TCR of the resistors can be seen in Figure 5. The curve is shifted by a constant value of about 100 ppm in the negative direction. This means that for TCR = 0 the Ni-coverage has to be roughly 20% higher on doped TaN (1% N,/99’% Ar) than on undoped TaN. A similar influence can be observed during heat treatment of the resistors. Microelectronic components are usually exposed to heat treatment for aging purposes. We heated the resistors for 6 h at 300°C in air. Comparison of the TCR curves of heat treated and untreated resistors show a shift in the negative direction during heat treatment (Figure 6). For undoped resistors the shift amounts to 4 ppm, for resistors doped with 1% N2 in Ar to 15 ppm in the negative direction. It is not surprising that undoped resistors are more stable to heat treatment than those with a nitrogen surplus in the TaN.
coverage were slightly rotated around the point with TCR = 0. This behaviour can be derived from theory. We found that the TCR at the coverage of the original O-point had deviated by less than 1 ppm after one year. At lower Ni-coverages the TCR was shifted in the negative direction and at higher TCRs in the positive direction with time. In Figure 7 the TCR-behaviour near the O-point is shown. It can be seen that heat treatment of the sample at 300°C for 6 h shifted the TCR by about 4 ppm in the negative direction. Aging of the resistor for one year did not change the TCR-plot significantly.
3.3. Long tern stability. Long term stabilities of heat treated nitrogen-doped and undoped resistors were investigated over a period of 1 year. We found that the curves in a plot of TCR vs Ni-
.__ 0.4
05
06
07
08
0.9
Ni - coverage
Figure 6. Resistor before and after N,-concentration in sputter gas.
tempering-TaN
films with 0.5%
3.4. Reproducibility. The advantage of the production technique described here is that reproducibility of the resistors is not dependent on reproducibility of the photolithographic process for the shaping of the resistors. However, reproducibility is influenced by varying sputter conditions from one batch to another, and inhomogeneous distribution of the thin film properties (thickness, structure) over the substrate. It was found that the TCR of the TaN-films is more negative when the target was exposed to air. Stable sputter conditions after exposure were obtained after sputtering off a layer of approximately 1 pm thickness. For reproducibility tests 35 equivalent samples were produced. They were measured and only 20 of them heat treated at 300°C for 6 h. For TaN we obtained a TCR of 102.9 f 5 ppm and for Ni 4700 + 100 ppm. Heat treatment not only improved long term stability but also reduced the deviation of the TCRs from an average value.
4. Conclusions Resistors with TCR = 0 could be made with a combination of sputtered TaN- and Ni-thin films. The TCR-values of TaN are dependent on the amount of nitrogen in the sputter gas. Values of TCR = -200 ppm have been obtained with 1% N2 present in the gas compared with TCR = - 100 ppm for sputtering in pure argon. To obtain resistors with TCR = 0, 70% of the nitrogen-doped TaN film (1% N, in Ar) had to be covered with a Ni-layer whereas the undoped TaN-films had to be 50% covered. The thickness of the Ni-layer is not very important but should not be less than about 60 nm for stable resistors. Heat treatment is of great importance for stabilizing the resistors. TCRs of resistors with nitrogen-doped TaN are shifted by about 15 ppm in the negative direction, whereas the influence of heat treatment on undoped resistors is only about 4 ppm. All heat treated resistors have excellent long term stabilities. Resistors with TCR = 0 change less than 1 ppm after one year of shelf life. 691
A Razborsek andF
Schwager:
Thin film systems for low TCR resistors
Acknowledgement The authors comments.
thank
C Steinbrtichel
for theoretical
support
and
References
’ D M Buczek, J Vat Sci Technol, E(2), 370 (1978). ’ S Schiller, U Heisig, K Goedicke, H Bilz, J Henneherger, W Brode and
692
W Dietrich, Thin Solid Films, 119,21 I (1984). D Muszynska and E Prociow, Thin Solid Films, 92, 111 (1982). 4Timothy J Maloney, Microelec Manufc Tear. 6(10), 43 (1983). ’ Landolt-BBmstein, Band II, Teil 6, s7 (1959). 6Toshiaki Koikeda and Shun Kumagai, Trans Just Comm Jpn Section E, 53(12), 28 (1970). ‘Ralph Trampsoch, Thin Film for Hybrids, MRC Sputtering School, Materials Research Corporation. ‘M Nakamura, M Fujimori and Y Nishimura, Fujilsu Sci-Tech J, 7(l), 131 (1971).
’ A Gorecka-Drzazga,
8-l O/pages
689 to 692/l
0042-207X/88$3.00+.00 Pergamon Press plc
988
Thin film systems for low TCR resistors ARazborsek and F Schwager, Mettler lnstrumente AG, 8606 Greifensee, Switzerland Resistors with TCR = 0 are important as reference resistors in many measurement instruments. We developed thin film resistors with resistance R between 7 and 5 kOhms and adjustable TCR’s between - 150 ppm “C-’ and +500 ppm “C-‘. The resistors consist of TaN thin films which are covered with overlying rectangular Nipads. R and TCR could be adjusted by varying process and geometry parameters. The thin films were sputter deposited and structured by photolithography. TCRs were found to be reproducible within f3?3 from batch to batch. We were able to design thin film systems with TCR = 0 for temperatures between 20 and 80°C.
1. Introduction Many electronic instruments, like electronic balances, use a constant current source which is obtained by applying a constant voltage on a reference resistor. This resistor should be very stable and independent of environmental conditions like temperature. Resistors with low temperature coefficients of resistivity (TCR) can be obtained with certain metal alloys like NiCr’,* with some oxygen content. These resistors are shaped into wires. For certain applications it might be advantageous to have thin film resistors, e.g. in combination with an integrated circuit. Thin film resistors of NiCr with very low TCR can be made and are even commercially available, but they are not easy to reproduce because the TCR depends heavily on the amount of oxygen in the films. We used a different approach to produce low TCR resistors than using a homogeneous material. It consisted of combining a material of negative TCR with a material with positive TCR. This shifts the problem of producing a very accurate alloy composition with TCR = 0 to the problem of accurate geometric and thickness control of the thin films of the two materials. GoreckaDrzazga et al’ have made thin film resistors by this method. They chose among other combinations tantalum-nitride as the material with the negative TCR, and nickel as the one with the positive TCR. The resistors were produced by first depositing Ta-nitride which possesses a higher specific resistivity than Ni which was deposited on top of it in rectangular pads. The resistors obtained by this method were accurate to within 5% of the calculated values. The authors claimed that errors were mainly due to misalignment of the photomask during photolithography. We used a similar method to produce resistors but improved the photolithographic shaping of the films to prevent alignment errors and also reduced the error due to undercutting of the thin films. 2. Experiments 2.1. Materials. Thin films of TaN and Ni were sputter deposited from a hot pressed TaN- and a mechanically machined Ni-target in argon onto a ceramic substrate. The 33 x 33 mm* Al,O,-substrate (Rubalit 710) was chosen because it offers excellent adhesion properties for the thin films, conducts heat very well, has a high dielectric constant of 9.9 and is temperature resistant. Its surface roughness is 0.1 pm.
2.2, Equipment. The thin films were deposited by sputtering in a Leybold-Heraeus Z 400 sputtering machine with load-lock. The machine can be equipped with 3 sputtering targets at the same time. For the thin-film resistors we used TaN-, Ni- and Autargets of 100 mm diameter. Argon was used as sputtering gas but we also investigated the TCR-dependence of the resistors by sputtering TaN in an argon/nitrogen atmosphere. Typical sputter conditions were pressure 1.5 x lOA2 mbar; gas flow 100 seem; power 5 W cmm2; electrode-distance 35 mm. 2.3. Measurements. The resistors were investigaed with a fourpoint probe and the values corrected by the method of Maloney4. For accurate temperature control, the resistors were immersed in a thermostatically controlled bath of an electrically inert liquid (Fluorinert FC43). The temperature could be varied between 0 and 80°C with an accuracy of +O.Ol”C. The TCRs were determined by equation (1) :
TCR = (R, -&)/R,(T,
-To)
[K-l],
(1)
where R, is the resistance at room temperature T,, and R, the resistance at some other temperature T,. The TCR is usually expressed in ppm. 2.4. Resistors. The layout of the resistors was produced in a way so that their value was of the order of l-5 kOhms. They consisted basically of meandering lines of 40 mm length and 0.25 mm width. The thin films were fabricated by photolithographically shaping the 3 layers of TaN (cu. 100-200 nm thick), Ni (100-200 nm) and Au (500 nm) on top. The Au-layer not only protected the underlying layers but it also acted as a masking layer for the successive etching of the Ni-layer, and reduced undercutting compared to using only photoresist as a masking layer. With the first mask continuous meandering lines were copied into the photoresist, and Au, Ni and TaN were etched away successively. Then photoresist was applied again and the second mask for the Ni-pads was copied. Figure 1 shows that the masks for the Nipads are wider than the TaN-lines. This way it was made sure that even with a misaligned Ni-mask, the TaN-lines were covered with Ni across the whole line width. The Au- and the Ni-layers between the pads were then etched away, and after stripping off the photoresist and the remaining Au on the Ni-pads the resistors 689
A Razborsek
Thin
andF.Schwager:
film
systems
for low TCR resistors
Ni 500 /Jm
rl
500 pm
TaN
0
350 pm
2-
Figure 2. TCR
250 pm 1lOOym
i 2050 pm
Figure 1. Layout
1
N
of thin
I
film systems
I
1000pm
t 2000m
for low TCR
P
resistors
were
cleaned in acetone and isopropanol. The resistors were then measured with a four-point probe. On a 33 x 33 mm’ alumina-substrate 12 resistors with different ratios of Ni-coverage were deposited. The mask for the meandering TaN-lines was the same for all resistors. The TCR was varied by changing the length of the Ni-pads, the number of the Nipads being kept constant. Widths of TaN-lines and Ni-lines were 0.25 and 0.35 mm, respectively. We used 2 masks to investigate the behaviour of the TCR over a wide range of different Nicoverages. The coverages ranged between 10 and 55%, and 45 and 90%, respectively, in steps of 5%. In addition, 2 resistors of pure TaN and Ni, respectively, were deposited for reference measurements on each substrate. The measurements have shown that the point of intersect of the TCR curve with the O-line lies in the region of 55 and 70% coverage depending on the TCR of TaN. Another set of masks with coverages between 50 and 80% in 1% steps was used for a more accurate determination of the point of intersect.
concentration in the sputtergas rbl
of TaN with different
obtain reproducible the plateau region here however, has argon. These films age value of TCR
N,-doping
films. For good reproducibility the films of should be used. Most of the work presented been carried out with TaN sputtered in pure could be reproduced within i_ 5% of an aver= - 102.9 ppm.
3.2. Resistor combinations. Figure 3 shows a plot of TCR-values for resistors with nitrogen doped TaN and Ni-coverages between 45 and 90%. TCR = 0 is reached at a coverage of 72%. The slope of the curve at TCR = 0 is 12 ppm per 1% of coverage change. The temperature dependence of the TCR was determined for a Ni-coverage of 70% between 25 and 75’C. For this purpose the resistances were measured in 10” intervals between 20 and 80°C and the TCRs calculated by means of equation (1). Figure 4 shows that the curve is approximately linear between 20 and
Figure 3. TCR vs Ni coverage
or Nz-doped
TaN
3. Results 3.1. Nickel and tantalum nitride. We first determined the TCRs of films of pure Ni and TaN. For Ni a TCR of 4700 ppm “C-’ was obtained between 20 and 50°C which compares with the values of bulk Ni with 6900 ppm “C _ ’ ‘. This difference is due to different structures of bulk Ni and sputtered Ni and also depends on the thermal expansion coefficient of the substrate material. Figure 2 shows TCR-values of TaN for varying nitrogen concentrations in the sputter gas. A distinct plateau which has been described beforeG8 exists between cu. 0.2 and 0.8% N2. The plateau expresses stoichiometric composition of TaN. Whereas TaN has a nitrogen deficiency at low N,-concentrations in the sputter gas it has a surplus at N,-concentrations above 1%. It would be advantageous to produce TaN-films with more negative TCRs but it is difficult to adjust the sputtering parameters to 690
20
40
Figure 4. TCR of resistor
so Temperature
for different
SO
[C]
temperatures.
A Razborsek and F Schwager:
Thin film systems for low TCR resistors
Figure 5. Dependence of TCR on doping with nitrogen (1%).
Figure 7. Long term stability of undoped resistors after tempering.
50°C. For most of the work we measured resistances at 20 and 5O”C, only. Theoretical considerations show that optimal results for TCRs should be obtained when the sheet resistances of both the TaNand the Ni-films are the same. This could not be confirmed with Ni-layers of too high sheet resistances (e.g. 50 Ohms square-‘). We found that after heat treatment of such films for 60 min at 300°C the value of the TCR reaches the values for TaN. As the Ni-film for a sheet resistance of 50 Ohms square-’ is only cu. 15 nm thick compared to cu. 100 nm of the TaN-film, this result can be explained by the oxidation of Ni during heat treatment. Further investigations have shown that a minimal Ni-film thickness of ca. 60 nm is required to obtain stable resistors. The influence of N,-doping of TaN on the TCR of the resistors can be seen in Figure 5. The curve is shifted by a constant value of about 100 ppm in the negative direction. This means that for TCR = 0 the Ni-coverage has to be roughly 20% higher on doped TaN (1% N,/99’% Ar) than on undoped TaN. A similar influence can be observed during heat treatment of the resistors. Microelectronic components are usually exposed to heat treatment for aging purposes. We heated the resistors for 6 h at 300°C in air. Comparison of the TCR curves of heat treated and untreated resistors show a shift in the negative direction during heat treatment (Figure 6). For undoped resistors the shift amounts to 4 ppm, for resistors doped with 1% N2 in Ar to 15 ppm in the negative direction. It is not surprising that undoped resistors are more stable to heat treatment than those with a nitrogen surplus in the TaN.
coverage were slightly rotated around the point with TCR = 0. This behaviour can be derived from theory. We found that the TCR at the coverage of the original O-point had deviated by less than 1 ppm after one year. At lower Ni-coverages the TCR was shifted in the negative direction and at higher TCRs in the positive direction with time. In Figure 7 the TCR-behaviour near the O-point is shown. It can be seen that heat treatment of the sample at 300°C for 6 h shifted the TCR by about 4 ppm in the negative direction. Aging of the resistor for one year did not change the TCR-plot significantly.
3.3. Long tern stability. Long term stabilities of heat treated nitrogen-doped and undoped resistors were investigated over a period of 1 year. We found that the curves in a plot of TCR vs Ni-
.__ 0.4
05
06
07
08
0.9
Ni - coverage
Figure 6. Resistor before and after N,-concentration in sputter gas.
tempering-TaN
films with 0.5%
3.4. Reproducibility. The advantage of the production technique described here is that reproducibility of the resistors is not dependent on reproducibility of the photolithographic process for the shaping of the resistors. However, reproducibility is influenced by varying sputter conditions from one batch to another, and inhomogeneous distribution of the thin film properties (thickness, structure) over the substrate. It was found that the TCR of the TaN-films is more negative when the target was exposed to air. Stable sputter conditions after exposure were obtained after sputtering off a layer of approximately 1 pm thickness. For reproducibility tests 35 equivalent samples were produced. They were measured and only 20 of them heat treated at 300°C for 6 h. For TaN we obtained a TCR of 102.9 f 5 ppm and for Ni 4700 + 100 ppm. Heat treatment not only improved long term stability but also reduced the deviation of the TCRs from an average value.
4. Conclusions Resistors with TCR = 0 could be made with a combination of sputtered TaN- and Ni-thin films. The TCR-values of TaN are dependent on the amount of nitrogen in the sputter gas. Values of TCR = -200 ppm have been obtained with 1% N2 present in the gas compared with TCR = - 100 ppm for sputtering in pure argon. To obtain resistors with TCR = 0, 70% of the nitrogen-doped TaN film (1% N, in Ar) had to be covered with a Ni-layer whereas the undoped TaN-films had to be 50% covered. The thickness of the Ni-layer is not very important but should not be less than about 60 nm for stable resistors. Heat treatment is of great importance for stabilizing the resistors. TCRs of resistors with nitrogen-doped TaN are shifted by about 15 ppm in the negative direction, whereas the influence of heat treatment on undoped resistors is only about 4 ppm. All heat treated resistors have excellent long term stabilities. Resistors with TCR = 0 change less than 1 ppm after one year of shelf life. 691
A Razborsek andF
Schwager:
Thin film systems for low TCR resistors
Acknowledgement The authors comments.
thank
C Steinbrtichel
for theoretical
support
and
References
’ D M Buczek, J Vat Sci Technol, E(2), 370 (1978). ’ S Schiller, U Heisig, K Goedicke, H Bilz, J Henneherger, W Brode and
692
W Dietrich, Thin Solid Films, 119,21 I (1984). D Muszynska and E Prociow, Thin Solid Films, 92, 111 (1982). 4Timothy J Maloney, Microelec Manufc Tear. 6(10), 43 (1983). ’ Landolt-BBmstein, Band II, Teil 6, s7 (1959). 6Toshiaki Koikeda and Shun Kumagai, Trans Just Comm Jpn Section E, 53(12), 28 (1970). ‘Ralph Trampsoch, Thin Film for Hybrids, MRC Sputtering School, Materials Research Corporation. ‘M Nakamura, M Fujimori and Y Nishimura, Fujilsu Sci-Tech J, 7(l), 131 (1971).
’ A Gorecka-Drzazga,