WO2005086183A1

WO2005086183A1 - Electrical trimming of resistors

Info

Publication number: WO2005086183A1
Application number: PCT/US2004/003148
Authority: WO
Inventors: Fen Chen; Cathryn J. Christiansen; Roger A. Dufresne; Tom C. Lee; Baozhen Li; Alvin W. Strong; Kimball M. Watson; Jean E. Wynne
Original assignee: International Business Machines Corporation
Priority date: 2004-02-03
Filing date: 2004-02-03
Publication date: 2005-09-15

Abstract

A method (300) for trimming resistors (422), and an apparatus (400) for trimming resistors are provided for resistors made of a material whose resistance changes in response to applied voltage. The method comprises measuring (310) the resistance of the resistor. The measured resistance is compared (320) with a target resistance. Based. on the difference (330) between the measured resistance and the target resistance, trimming (340) control parameters (maximum sweep voltage, sweep rate, pulse voltage level, pulse duration, pulse shape) of the applied voltage are calculated. An electric power corresponding to the calculated trimming control parameters is applied across the resistor so as to cause the resistor's resistance to approach the target resistance. The resistor's resistance is measured again and compared with the target resistance. If the difference is not sufficiently small, process described above is repeated until the difference between the measured resistance and the target resistance is less than a pre-specified value.

Description

ELECTRICAL TRIMMING OF RESISTORS Background of the Invention

1. Technical Field

The present invention relates to resistors, and more particularly, to resistor trimming.

2. Related Art

TaN (tantalum nitride) resistors have been widely used on semiconductor chips as compact and reliable electronic devices. However, because of the small size resistors, less than 0.01mm², and the difficulty of controlling the nitrogen content during the process, it is very difficult to obtain stable and precise resistance values with less than 6% variation. Air abrasive trimming and laser trimming are two conventional post-process methods which are widely used to improve the resistance deviation from the desired value after fabrication. The air abrasive trimming method is not workable under current process environments because it adds more process steps and introduces more impurities, and it is not compatible with current Si IC

(Integrated Circuit) technology. The laser trimming method is also not workable due to the smaller feature size of the current resistor, due to a decrease in the power handling capability of the laser trimmed resistor, and due to the possible surface damages/micro-cracks of TaN resistors after laser trimming. Another indirect way to trim resistance is by fuse selection, which currently is also used in the industry. In this approach, multiple resistors are fabricated with slightly different geometry. Wafer final test is performed on kerf structures to determine the TaN sheet resistance, and then the resistor which is likely to have resistance closest to the desired value is selected by blowing an e-fuse. However, this method has space and cost disadvantages. In addition, this method does not trim the resistor directly.

Therefore, there is a need for a method for directly trimming the resistors. Also, there is a need for an apparatus for directly trimming the resistors.

Summary of the Invention

The present invention provides a method for trirnming a resistor, the method comprising the steps of (a) measuring a first resistance of the resistor; (b) comparing the first resistance and a target resistance; (c) if the first resistance and the target resistance are not sufficiently close together, determining first trimming control parameters for a first electric power needed to permanently change the resistance of the resistor from the first resistance to a second resistance, the second resistance being different from the first resistance and being closer to the target resistance than the first resistance; and (d) applying the first electric power across the resistor according to the first trimming control parameters.

The present invention also provides a structure, comprising (a) a resistor; (b) a pulse generator, electrically coupled to the resistor; (c) a processor electrically coupled to the resistor and the pulse generator; and (d) an analyzer electrically coupled to the resistor and the processor, wherein the analyzer is configured to measure a first resistance of the resistor, wherein the processor is configured to compare the first resistance and a target resistance, wherein if the first resistance and the target resistance are not sufficiently close together, the processor is further configured to determine first mming control parameters for a first electric power needed to permanently change the resistance of the resistor from the first resistance to a second resistance, the second resistance being different from the first resistance and being closer to the target resistance than the first resistance, wherein if the voltage pulse method is used, the pulse generator is configured to apply the first electric power as an electric pulse across the resistor according to the first trimming control parameters, and wherein if the voltage sweep method is used, the analyzer is configured to apply the first electric power as a voltage sweep across the resistor according to the first trimming control parameters. The present invention also provides a method for trimming a plurality of resistors, the method comprising the steps of (a) simultaneously applying an initial electric power across each of the plurality of resistors according to initial trimming control parameters; (b) trirnming a first resistor of the plurality of resistors; and (c) trirnming a second resistor of the plurality of resistors. The present invention also provides a method for directly trimming the resistors and also provides an apparatus for directly trimming the resistors.

Brief Description of the Drawings

FIG. 1 illustrates a TaN (tantalum nitride) resistor under an applied voltage.

FIG. 2A illustrates the relationship between maximum sweep voltage applied across the TaN resistor of FIG. 1 and the resulting resistance of the resistor, in accordance with embodiments of the present invention. FIG. 2B illustrates the relationship between the peak voltage of a square-pulse having a pulse width of 1ms applied across the TaN resistor of FIG. 1 and the resulting resistance of the resistor, in accordance with embodiments of the present invention. Fig. 3 illustrates a flow chart of a method for trimming the resistor of FIG. 1, in accordance with embodiments of the present invention.

FIG. 4 illustrates an apparatus for trirnming resistors, in accordance with embodiments of the present invention.

Detailed Description of the Invention The present invention is a non-cut, electrical method of mming TaN

(tantalum nitride) resistor with no potential surface damage or micro-cracks. The change of resistance after electrical friniming is thought to be intrinsic to the TaN material and is permanent. This changed TaN material is very stable and still follows the same small TCR (temperature coefficient of resistance) characteristics for precision resistance purpose. Compared to laser trirnming, fuse bank trimming, and air abrasive trimming, the present invention is simple, easy to handle, fast, accurate, cost effective, and Si IC technology compatible. Another advantage of this invention is the direct electrical feedback during trimming to make trimming process very controllable. FIG. 1 illustrates a TaN (tantalum nitride) resistor 100 with a voltage V applied across the resistor 100. The resistance of the resistor 100 may be permanently changed by one or more electric pulses or one or more voltage sweep on the applied voltage N. An electric pulse lasts a very short time (e.g., milliseconds) whereas a voltage sweep lasts much longer (e.g., seconds). The term "permanently" means that under the effect of the electric pulse on the applied voltage N, the resistance of the resistor 100 changes to a new value different from the original value and stays at the new value after the electric pulse or voltage sweep, or in general, any applied electric power of any duration and shape has been applied and removed. Here, the term "electric power" is used to emphasize that the resistor 100 carries a current (constant or changing) when a voltage (constant or changing) is applied across it.

As-deposited TaΝ is a mixture of amorphous and fine crystalline islands. Initial excessive field/heating of TaΝ is believed to cause rupture of the amorphous part and growth of the conductive islands in the material. As a result, the resistance of TaΝ increases due to enhanced scattering. However, after further critical field/heating of TaΝ (i.e., field/heating that exceeds some critical temperature point), a more conductive or percolation path is believed to form due to the fusing of the islands. As a result, the resistance of TaΝ decreases. This characteristic of TaΝ is illustrated below with reference to FIGs. 2A and 2B. The charts clearly illustrate the resistive changes as indicated, the theory is given as the most likely explanation, but the exact physics of the changes is not part of this patent and therefore should not be construed to be limiting. FIG. 2A illustrates the relationship between the maximum sweep voltage (unit

Nolt) of a voltage sweep of the voltage N applied across the TaΝ resistor 100 of FIG. 1 with a sweep rate of lN/s and the resulting resistance (unit Ohm) of the resistor 100, in accordance with embodiments of the present invention. The plot in FIG. 2A shows that for a specific resistor structure, there is a well defined maximum sweep voltage (MSN) window (3N - 4.2N) in which TaΝ resistance linearly increases with increasing maximum sweep voltage. There is also another well-defined maximum sweep voltage window (4.2N - 5N) in which TaΝ resistance linearly decreases with increasing voltage.

From the plot in FIG. 2A, for an increase of the maximum sweep voltage, the resistance could be trimmed by about 30 - 40% upward and about 50% downward. Therefore, by using these two critical voltage windows, the resistance of TaΝ could be easily trimmed or tuned bi-directionally (i.e., up and down) with highly precise resistance control. This observation gives the basis for a N-sweep (voltage sweep) method for triπ-ming resistance in which an increasing voltage is applied across the resistor until the applied voltage reaches a desired maximum sweep voltage.

More specifically, from the plot of FIG. 2A, originally, the resistor 100' s resistance is about 180Ω. If the voltage applied across the resistor increases at lN/s from ON to a maximum sweep voltage of 3N and then instantly drops to ON, the resistor's resistance stays at 180Ω. However, if the voltage applied across the resistor increases at lN/s from ON to a maximum sweep voltage of 4N and then instantly

drops to ON, the resistor's resistance becomes 250Ω (which is higher than the original

resistance of 180Ω). If the voltage applied across the resistor increases at lN/s from

ON to a maximum sweep voltage of 4.5N and then instantly drops to 0V, the resistor's

resistance becomes 100Ω (which is lower than the original resistance of 180Ω). In addition to N-sweep method, a fast N-pulse (voltage pulse) method (NPM) could also be used to achieve the same results as shown in FIG. 2B. FIG. 2B illustrates the relationship between the peak voltage (unit Nolt) of a square-pulse on the applied voltage N having a pulse width of 1ms applied across the TaΝ resistor 100 of FIG. 1 and the resulting resistance (unit Ohm) of the resistor 100, in accordance with embodiments of the present invention.

More specifically, from the plot of FIG. 2B, originally, the resistor 100's

resistance is about 180Ω. If a square-pulse having a pulse width of 1ms and a peak voltage of less than 5N is applied across the resistor, the resistor's resistance stays at 180Ω. If a square-pulse having a pulse width of 1ms and a peak voltage of 6N is

applied across the resistor, the resistor's resistance becomes 220Ω (which is higher

than the original resistance of 180Ω). If a square-pulse having a pulse width of 1ms and a peak voltage of 7V is applied across the resistor, the resistor's resistance

becomes 150Ω (which is lower than the original resistance of 180Ω). This N-pulse trimming process could be very fast (< 1ms) with a single voltage pulse and is very flexible. The trimming process is well modulated by pulse parameters such as pulse width, the number of pulses per group, and the duty factor of the group of pulses. The inherent advantage of using either N-sweep or N-pulse method is that the trimming process has direct adjustment feedback, therefore it has real time, in-situ control.

In one embodiment, the trirnming procedure could be carried out as follows. Step #1: the TaΝ resistor 100 is deposited with initial resistance R0. The value R0 is then measured and compared with a target value Rf. Step #2: A first MSN VO or a first NPM pulse NO is applied on the resistor to coarsely trim the resistance RO up or down to approach the target value Rf. The resulting resistance Rl = RO + dRl. Step #3: If the changed resistance Rl is still higher or lower than Rf, the MSN/VPM with NO + dNl is applied to fine-tune the resistor to achieve the new resistance value R2 = RO + dRl + dR2. Step #4: The resistance value R2 is measured again, and if R2 is still not at the target value Rf , step #3 is repeated to reach a new resistance value R3 = RO + dRl + dR2 + dR3. Step #5: Step #4 is repeated until the measured resistance value reaches the target value Rf = RO + dRl + dR2 + dR3 +...+ dRf. In an alternative embodiment, in step #5, step #4 is repeated until the measured resistance value is sufficiently close to the target value Rf . "Sufficiently close" can be defined as when the error percentage I (Measured resistance - Rf)/Rfl is not greater than a pre- specified acceptable error percentage. For instance, assume that the acceptable error percentage is pre-specified at 5% and that RO = 180Ω and Rf = 200Ω. Then, if the

measured resistance is 190Ω, the error percentage is l(190-200)/200l = 5%, which is not greater than the pre-specified acceptable error percentage of 5%. As a result, the measured resistance value is considered sufficiently close to the target value Rf . A fully computer-controlled trimming/probing method could be easily implemented for large-scale, fast trimming at room temperature. Another feature of this invention is that this acceptable error percentage can be extremely small depending on the amount of fine tuning that is acceptable.

FIG. 3 illustrates a flow chart of a method 300 that summarizes the method described above. As an example of how the method 300 works, assume the TaΝ resistor 100 of FIG. 1 is trimmed using the method 300 of FIG. 3. In Step 310 of the method 300, the resistance of the TaN resistor 100 is measured. Assume that the

measured value is Rml = 180Ω. Then, in step 320, the measured resistance value

180Ω is compared with a target resistance value. Assume that the target resistance

value is Rf = 200Ω. As a result, the error percentage is 10% (l(Rml-Rf)/Rfl). In step

330, a determination is made as to whether the measured resistance value is sufficiently close to the target resistance value. In other words, the determination is made as to whether the error percentage is sufficiently small. Assume that an error percentage of 5% or less is considered sufficiently small (i.e., acceptable). As a result, the error percentage of 10% is not sufficiently small and therefore step 340 is performed. In step 340, the resistance of the TaN resistor 100 is permanently adjusted/changed using the MSN method or NPM method as described above so that the adjusted resistance is closer to the target resistance value Rf.

In one embodiment, the trimming control parameters of the MSN method or NPM method are calculated such that the resistance of the TaΝ resistor 100 is not adjusted past the target resistance value Rf. In the example above, the trirnming control parameters of the MSN method or VPM method are determined such that the resistance of the TaΝ resistor 100 increases but does not exceed the target resistance

value of Rf = 200Ω. Alternatively, the trimming control parameters of the MSV

method or NPM method can be determined such that the resistance of the TaΝ resistor is adjusted to the vicinity (higher or lower) of the target resistance value Rf.

After the trimming using the MSV method or NPM method in step 340, the method 300 loops back to step 310 where the resistance of the TaΝ resistor is again measured. Assume, continuing the example above, that the measured value is Rm2 =

186Ω. Then, in step 320, the measured resistance value 186Ω is compared with the

target resistance value 200Ω. As a result, the error percentage is 7%, which is still not

sufficiently small as determined in Step 330. Therefore, in step 340, the resistance of the TaN resistor is adjusted using the MSV method or VPM method as described above so that the adjusted resistance even closer the target resistance value Rf.

After the trimming using the MSV method or VPM method in step 340, the method 300 loops back to step 310 where the resistance of the TaN resistor is again measured. Assume, continuing the example above, that the measured value is Rm3 =

196Ω. Then, in step 320, the measured resistance value of 196Ω is compared with the

target resistance value of 200Ω. As a result, the error percentage is 2%, which is

sufficiently small (i.e., < 5%) as determined in Step 330. As a result, the method 300 stops.

FIG. 4 illustrates a system 400 for performing the method 300 of FIG. 3, in accordance with embodiments of the present invention. Illustratively, the system 400 comprises a probe chuck 410, a wafer 420, a scanner/switch 430, a pulse generator 450, an analyzer 460, and a processor 470. The wafer 420 includes a TaN resistor 422 to be trimmed. In one embodiment, an oscilloscope 440 can be coupled to the wafer 420 and the scanner/switch 430. The oscilloscope 440 can be used by the system operator to measure the actual pulse voltage levels applied to the wafer 420. The pulse generator 450 is capable of generating pulses, whereas the analyzer 460 is capable of generating voltage sweeps and monitoring the currents and voltages of these voltage sweeps. With these capability, the analyzer 460 is also capable of measuring the resistance of the resistor 422 by generating a voltage level across the resistor 422 and measuring the associated current flowing through the resistor 422. The resistance of the resistor 422 is the ratio of voltage across the resistor 422 over the current flowing through the resistor 422. With reference to both FIGs. 3 and 4, in step 310 of method 300, the resistance of the resistor 422 in the wafer 420 is measured by the analyzer 460, which is electrically coupled to the wafer 420 via the scanner/switch 430. In step 320, the processor 470 obtains the measured resistance value Rm of the resistor 422 from the analyzer 460 and compares the measured resistance value Rm with a target resistance value Rf. If the processor 470 determines that the two values are not sufficiently close together, then in step 340, the processor 470 calculates/determines trimming control parameters for the trimming method used (MSV or VPM). If the VPM method is used, the processor 470 causes the pulse generator 450 to apply an appropriate electric pulse according to the calculated trirnming control parameters (voltage level, duration, pulse shape, etc.) to the resistor 422 via the scanner/switch 430. If the MSV method is used, the processor 470 causes the analyzer 460 to apply an appropriate voltage sweep according to the calculated mming control parameters (maximum sweep voltage, sweep rate, etc.) to the resistor 422 via the scanner/switch 430. In one embodiment, the system 400 may further comprise a sensing circuit

480 electrically coupled to the processor 470; and the entire system 400 is built in a packaged chip (not shown) such that the sensing circuit 480 is located substantially close to the resistor 422. In one embodiment, when a change in the temperature (or any other conditions) of the space immediately surrounding the resistor 422 occurs, in response, the sensing circuit 480 is configured to cause the processor 470 to start the method 300 of FIG. 3 to permanently adjust the resistance of the resistor 422 to a desired resistance value so as to offset the effect of the change in the temperature of the space immediately surrounding the resistor 422.

In the embodiment described above, the chip is field tunable and self- programmable. "Field tunable" means that the resistors even in the packaged chip can be tuned to desired resistance values. "Self programming" means that the chip by itself can adjust the resistances of its own resistors in response to changes in the environment surrounding the resistors.

In one embodiment, all the resistors (not shown) to be trimmed on the wafer 420 are trimmed by a universal trimming pulse. After that, each individual resistor can be precisely trimmed again using the method 300 of FIG. 3 to achieve the target resistance value for the individual resistor. In summary, the present invention provides a post-process non-cut trimming method by electrical ways for TaN resistors. The method involves bi-directional trimming (increasing and/or decreasing resistance to approach target value) of TaN resistors. The method also provides direct resistance feedback during trimming and very controllable trimming. The method is fast (less than 1ms single pulse) and cost effective (no extra layer/process). The method is precise with coarse and fine adjustments. The method can provide multi-device parallel trimming. Also, trimming can be controlled by multiple trirnming control parameters such as maximum sweep voltage, sweep rate, pulse voltage, pulse duration, pulse shape, and pulse duty cycle. Another advantage of the method of the present invention is no surface damage after trimming. In one embodiment, field tunable resistors for self- programming can be used.

In the embodiments described above, the resistor comprises TaN. In general, any material whose resistance permanently changes under electrical power (electric pulse, voltage sweep, etc.) can be used.

In the embodiments described above, the specific trimming control parameters used for the applied pulse are for illustration only. In general, any values for the trimming control parameters for the applied electric power can be used. For instance, pulse widths other than 1ms for electric pulses can be used. Also, sweep duration of any time length can be used.

In the embodiments described above, the physics behind the resistance change of TaN in response to electric pulse(s) do not necessarily apply to other electrically tunable materials. In other words, the physics behind the resistance change of the other electrically tunable materials in response to electric pulse(s) may be different from what hypothesized for TaN described above.

While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.

Claims

ClaimsWe claim:

1. A method (300) for trimming a resistor, the method comprising the steps of: (a) measuring (310) a first resistance of the resistor; (b) comparing (320) the first resistance and a target resistance; (c) if (300) the first resistance and the target resistance are not sufficiently close together, determining first trirnming control parameters for a first electric power needed to permanently change the resistance of the resistor from the first resistance to a second resistance, the second resistance being different from the first resistance and being closer to the target resistance than the first resistance; and (d) applying (340) the first electric power across the resistor according to the first trimming control parameters.

2. The method of claim 1, wherein the first trirnming control parameters comprises a parameter selected from the group consisting of maximum sweep voltage, sweep rate, pulse voltage level, pulse duration, pulse shape, and combination thereof.

3. The method of claim 1, further comprising the steps of: (e) measuring the second resistance of the resistor after step (d) is performed; (f) comparing the second resistance and the target resistance; (g) if the second resistance and the target resistance are not sufficiently close together, determining second trimming control parameters for a second electric power needed to permanently change the resistance of the resistor from the second resistance to a third resistance, the third resistance being different from the second resistance and being closer to the target resistance than the second resistance; and (h) applying the second electric power across the resistor according to the second trimming control parameters.

4. The method of claim 3, wherein the second trimming control parameters comprises a parameter selected from the group consisting of maximum sweep voltage, sweep rate, pulse voltage level, pulse duration, pulse shape, and combination thereof.

5. The method of claim 3, wherein the condition that the second resistance and the target resistance are not sufficiently close together comprises a condition that the percentage difference between the second resistance and the target resistance is greater than a pre-specified acceptable error percentage value.

6. The method of claim 1 , wherein the condition that the first resistance and the target resistance are not sufficiently close together comprises a condition that the percentage difference between the first resistance and the target resistance is greater than a pre- specified acceptable error percentage value.

7. The method of claim 1, wherein the resistor comprises a material whose resistance permanently changes in response to an electric power.

8. The method of claim 1, wherein the second resistance is higher than the first resistance.

9. The method of claim 1 , wherein the second resistance is lower than the first resistance.

10. The method of claim 1, wherein the first electric power is an electric pulse which has a pulse width of less than one millisecond.

11. A structure (400), comprising: a resistor (422); a pulse generator (450), electrically coupled to the resistor (422); a processor (470) electrically coupled to the resistor and the pulse generator (450); and an analyzer (460) electrically coupled to the resistor (422) and the processor (470), wherein the analyzer (460) is configured to measure a first resistance of the resistor (422), wherein the processor (470) is configured to compare the first resistance and a target resistance, wherein if the first resistance and the target resistance are not sufficiently close together, the processor (470) is further configured to determine first trimming control parameters for a first electric power needed to permanently change the resistance of the resistor from the first resistance to a second resistance, the second resistance being different from the first resistance and being closer to the target resistance than the first resistance, wherein if the voltage pulse method is used, the pulse generator (450) is configured to apply the first electric power as an electric pulse across the resistor according to the first trimming control parameters, and wherein if the voltage sweep method is used, the analyzer (460) is configured to apply the first electric power as a voltage sweep across the resistor (422) according to the first trimming control parameters.

12. The structure of claim 11, further comprising a sensing circuit (480) electrically coupled to the processor (470) and positioned substantially close to the resistor (422), wherein the sensing circuit (480) is configured to sense a change in temperature of the space immediately surrounding the resistor (422) and, in response, to trigger the processor (470) to cause the analyzer (460) to measure the first resistance of the resistor (422).

13. The structure of claim 11, wherein the first trimming control parameters comprises a parameter selected from the group consisting of maximum sweep voltage, sweep rate, pulse voltage level, pulse duration, pulse shape, and combination thereof.

14. The structure of claim 11, wherein the analyzer (460) is further configured to measure the second resistance of the resistor (422) in response to and after the first electric power being applied across the resistor (422) according to the first trimming control parameters, wherein the processor (470) is further configured to compare the second resistance and the target resistance, wherein if the second resistance and the target resistance are not sufficiently close together, the processor is further configured to determine second trimming control parameters for a second electric power needed to permanently change the resistance of the resistor (422) from the second resistance to a third resistance, the third resistance being different from the second resistance and being closer to the target resistance than the second resistance, wherein if the voltage pulse method is used, the pulse generator (450) is further configured to apply the second electric power as an electric pulse across the resistor (422) according to the second trirnming control parameters, and wherein if the voltage sweep method is used, the analyzer (460) is configured to apply the second electric power as a voltage sweep across the resistor according to the second trirnming control parameters.

15. The structure of claim 14, wherein the second trimming control parameters comprises a parameter selected from the group consisting of maximum sweep voltage, sweep rate, pulse voltage level, pulse duration, pulse shape, and combination thereof.

16. The structure of claim 14, wherein the condition that the second resistance and the target resistance are not sufficiently close together comprises a condition that the percentage difference between the second resistance and the target resistance is greater than a pre-specified acceptable error percentage value.

17. The structure of claim 11, wherein the condition that the first resistance and the target resistance are not sufficiently close together comprises a condition that the percentage difference between the first resistance and the target resistance is greater than a pre-specified acceptable error percentage value.

18. The structure of claim 11, wherein the resistor (422) comprises TaN.

19. A method for trimming a plurality of resistors, the method comprising the steps of: simultaneously applying an initial electric power across each of the plurality of resistors according to initial trimming control parameters; trimming a first resistor of the plurality of resistors; and trimming a second resistor of the plurality of resistors.

20. The method of claim 19, wherein the step of trimming the first resistor comprises the steps of: measuring a first resistance of the first resistor; comparing the first resistance and a first target resistance; if the first resistance and the first target resistance are not sufficiently close together, determining first trimming control parameters for a first electric power needed to permanently change the resistance of the first resistor from the first resistance to a resistance value being different from the first resistance and being closer to the first target resistance than the first resistance; applying the first electric power across the first resistor according to the first mming control parameters; measuring a second resistance of the second resistor; comparing the second resistance and a second target resistance; and if the second resistance and the second target resistance are not sufficiently close together, determining second trimming control parameters for a second electric power needed to permanently change the resistance of the second resistor from the second resistance to a resistance value being different from the second resistance and being closer to the second target resistance than the second resistance; and applying the second electric power across the second resistor according to the second trimming control parameters.