Use different measurement methods and connection methods to effectively measure resistance

1 Introduction

In the semiconductor process, the important parameters and performance of many devices are related to the sheet resistance. In order to improve the production of thick, thin film integrated circuits and chip resistors, it is necessary to use equipment such as probe stations, laser trimmers to test them or Trimming. Generally used measuring instruments or equipment include connection, excitation, measurement and display units, and sometimes post-data processing units. Different measurement methods and different connection methods introduce different measurement errors to obtain different measurements. Usually the relay contact closure resistance in the switch matrix is about 1Ω, the resistance when the FET switch is opened is more than ten ohms, and the lead resistance is several hundred milliohms. How to reduce the measurement error according to the needs is one of the keys to the test technology.

2. Basic principles of resistance testing

In the resistance test, we often use the constant current pressure measurement method, the Wheatstone bridge (single-arm bridge) and the double-arm bridge method.

2.1 Constant current pressure measurement method

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In Figure 1, r is the sum of lead resistance and contact resistance; I is a programmable constant current source; V is a voltmeter with extremely high input impedance, which does not shunt the constant current source. Apply a known constant current I, flow through the measured resistance R t, and then measure the voltage V across the resistance. When R t>>r, the resistance value can be calculated according to the formula Rt=V/I.

2.2 Wheatstone bridge method

In Figure 2, V1 and V2 are programmable constant voltage sources; Rstd is a standard resistance; Rt is the resistance to be measured; I is an ammeter. When the bridge is balanced, that is, when the current flowing through the ammeter I is zero, V1 /V2=Rstd/Rt, from which Rt=Rstd×V2/V1 can be calculated.

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2.3 Double-arm bridge method

The measurement range of a single-arm bridge is 10～106 Ω. When a single bridge measures a few ohms of low resistance, the lead resistance and contact resistance can no longer be ignored. The double-arm bridge is suitable for the measurement of 10-6～102 Ω resistance. It is an improved single-arm bridge, as shown in Figure 3. Change the low and medium resistances Rt and R in the bridge to four-terminal connection, and add two high resistance resistances R3 and R4 in the bridge circuit, which greatly reduces the influence of lead resistance and contact resistance. Detailed introduction can be found in literature [1].

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This article mainly introduces the constant current pressure measurement method. When the resistance of the measured resistance is much greater than the resistance of the test lead and the contact resistance between the test probe and the test point, the basic two-wire test method shown in Figure 1 is feasible, and a fairly high test can also be obtained.

3, Kelvin connection test technology

When the resistance of the measured resistance is less than a few ohms, the resistance of the test lead and the contact resistance between the probe and the test point cannot be ignored compared with the measured resistance. If the two-wire test method is still used, the test error will increase. At this time, the Kelvin connection method (or four-wire test method) can be used for testing, as shown in Figure 4.

There are two requirements for Kelvin connection: for each test point there is an excitation line F and a detection line S, which are strictly separated and form an independent loop; at the same time, the S line must be connected to a test loop with extremely high input impedance On the upper side, the current flowing through the detection line S is extremely small and approximately zero.

In Figure 4, r represents the sum of the lead resistance and the contact resistance between the probe and the test point. Since the current flowing through the test loop is zero, the voltage drop on r3, r4 is also zero, and the voltage drop of the excitation current I on r1, r2 does not affect the voltage drop of I on the measured resistance, so the voltmeter can Accurately measure the voltage value across Rt, thereby accurately measuring the resistance of Rt. The test result has nothing to do with r, which effectively reduces the measurement error.

According to the function and the level of potential, these four lines are respectively called high potential application line (HF), low potential application line (LF), high potential detection line (HS) and low potential detection line (LS).

4, resistance isolation test technology

In the case that the applied constant excitation current can all flow through the resistance under test, it is very simple to use the above method to test, such as testing a single resistance. However, we often encounter the situation where the resistance under test is connected in parallel with a resistance network. This resistance network has a shunting effect on the applied current, which makes it impossible to use the above methods for testing. In this case, we must use resistance isolation testing technology. The principle of the test circuit is shown in Figure 5.

Rt is the resistance under test, R1 and R2 are connected in series and then connected in parallel with Rt; A1, A2 are high input impedance, high operational amplifiers; DVA is a high input impedance, high differential voltage programmable amplifier amplifier, its output and digital The analog conversion ADC is connected; the DAC is a current output digital-to-analog converter, and the DAC and A1 form a programmable constant current source; according to the computer control, the DAC outputs a different constant current If.

A2 constitutes a voltage follower circuit so that Vc = Vb, so I1 = 0. Therefore, the computer sets the If through the 16-bit current output type DAC to control the current I t flowing through the measured resistance Rt, and then through the voltage detection circuit composed of DVA and ADC to test the voltage across Rt, the resistance value of Rt can be calculated. .

This method is equivalent to disconnecting R1 and separately isolating the measured resistance for testing, so it is called resistance isolation testing technology.

5. Separate the Kelvin connection and test the resistance

Resistance isolation test is applicable to most complex resistor networks, but it will cause some problems when it is applied to a very small number of resistor networks with a large resistance value ratio. As shown in Figure 6, R2/R1=6000, R 3/R1=4000, if you connect as shown in Figure 5 to test the resistance R3 (ie Figure 6-(a) connection), the facility plus current It is 200μA, then both ends of R3 A voltage drop of 8V is generated. Since R2 is isolated, the voltage at both ends of R2 is zero, so 8V voltage must be generated at both ends of R1, resulting in the power consumption of R2 being V2/R1=6.4W, which is obviously not allowed. If the positions of HP and LP are interchanged, since A2 is not an ideal device, there is a certain offset voltage Vos. Even as small as 20μV, a current of 2μA will be generated on R1, causing a 1% deviation of the current flowing through R3, causing The test dropped drastically.

In this case, a deformed Kelvin connection method can be used for testing, instead of using the isolation method. It still uses four wires, but one or two pairs of F and S wires are separately connected to different points for testing as needed. This method is called separate Kelvin connection. In this example, three resistors need to be tested four times to calculate their resistance values. The connection methods of the four tests are shown in the table in Figure 6. Points 1, 2, and 3 in the figure are the connection points. Figure 6-(b) The connection method can directly measure the value of R1; Figure 6-(c) can measure the parallel value R p of R2 and R3; Figure 6-(d) and Figure 6-(e) can be measured separately Resistance R2' and R3'. Analysis shows

R2′/R 3′=R2/R3, 1/Rp=1/R2+1/ R3; from this, R2=Rp×(1+R2′/R3′), R3=R 2×R3′/ R2'.

In the test, it is often encountered that there is no test point on both ends of the resistance under test, which is hidden in the resistance network, such as the R-2R network. At this time, it is also necessary to use a separate Kelvin connection for testing.

6. Measurement technology of extremely small resistance

For the resistance measurement in the extremely small resistance range, the circuit shown in Figure 7 can be used to complete the measurement, which can measure the resistance of 10 ~ 80mΩ. Through the differential operational amplifier circuit, the weak voltage signal generated by the measured resistance is amplified by 100 times, so the actual resistance value is the measured value to be divided by 100. The op amp UI in the picture uses a low-noise, high-speed, precision op amp, such as OP-37EJ, AD645 or MAX400. The resistor R1 connected in series with the high potential application line (HF) is used to match the output load of the current application module. R2 to R5 use high and stable resistors to ensure the stability of the differential amplifier circuit gain, which determines the measurement and repeatability . In order to ensure that the power supply voltage of the op amp is very high, the circuit installation position should be as close as possible to the measured resistance, all probes should be as short as possible, and C2 and C3 should be as close as possible to the op amp.

7, concluding remarks

Since the resistance under test is constantly changed in the automatic test, and the test method and connection method should be flexibly selected according to the situation, the actual production is to use a probe card to connect the circuit under test with the system, which is composed of a relay or FET switch. The switching torque is appropriately switched by the software to improve the test speed and production efficiency. At the same time, different connection methods are used for the probes in different measurements, such as the straight four-point probe method and the square four-point probe method, which can overcome the influence of various factors and optimize the measurement results. As mentioned above, as long as we combine the specific conditions of the resistance to be tested and apply the above-mentioned test techniques flexibly and reasonably, satisfactory test results can be obtained. Manufacture high-quality thick, thin film integrated circuits and chip resistors.