“With the introduction of the new infrastructure concept, the construction of 5G and data centers will enter the fast lane in 2020. The massive demand for optical modules leads the upgrading of the industry and puts forward higher requirements for optical devices. In the field of optical communication testing, there will also be many challenges.

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With the introduction of the new infrastructure concept, the construction of 5G and data centers will enter the fast lane in 2020. The massive demand for optical modules leads the upgrading of the industry and puts forward higher requirements for optical devices. In the field of optical communication testing, there will also be many challenges.

To face future challenges together, Tektronix will launch a series of technical articles on optical communication testing. This article is the first lecture to explain the PD dark current testing of passive devices. The author of this article is Duan Gong, an engineer friend from the Tektronix agent “Kotai Test”. He happened to hear a few small partners engaged in optoelectronics talking about PD dark current and how to test it, and just wanted to share some experience in this regard and understanding.

Dark current, Wikipedia’s explanation is: When no photons pass through a photo sensor (such as a photomultiplier tube, photodiode, and photocoupler element), a tiny current is still generated on the element. In non-optical components it is called leakage current at reverse bias and is present in all diodes. The reason for the formation of dark current is the random generation of electrons and holes in the depletion layer of the device.

This explanation has several points: 1) no light environment, reverse bias, leakage current; 2) dark current exists in any diode; 3) dark current belongs to the thermal noise of the component, which is randomly generated and cannot be eliminated. Popular point: First, this current is not generated by photons from the outside, but from the thermal noise inside the component; secondly, any diode has a theoretical characteristic, that is, forward conduction and reverse cut-off, but in reality diode components, reverse It is impossible to achieve a real cut-off (the reverse saturation current is 0), and finally the dark current cannot be completely eliminated, and can only be reduced by TEC or liquid nitrogen cooling.

Generally speaking, the dark current is very small, basically in the order of uA and nA. In the industrial field, the dark current test is a must-test item. This test indicator is mainly used to judge whether the diode element is broken down and whether the wafer process exists. problem. How can we measure such a small dark current accurately and reliably? Some people say that you can use a general multimeter or ammeter, is it really possible?

The sisters who ride the wind and waves are all working hard, picking out details frame by frame. The daily life of engineers is even more challenging, and any detail bug becomes a stumbling block. In fact, the test of small current is not as simple as it is said, and there are still many difficulties to be overcome. Here is a brief list of the most important ones.
Difficulty 1: How to overcome the input voltage drop caused by the ammeter?

When the multimeter measures currents above mA, the internal resistance of the ammeter can basically be ignored, but the magnitude of the small current is basically at the level of uA or even nA. At this time, the internal resistance of the ammeter cannot be ignored, and the internal resistance of the ammeter will bring Voltage drop, this voltage drop is called “input voltage drop (voltage burden)”, the size of this indicator directly affects the measurement accuracy of the current:

For example: Suppose Vs=0.7V, Is=100uA, Ifs=200uA, Rs=10KΩ, and then the input voltage drop is 200mV:
Then calculate IM=(0.7V-0.2V(100uA/200uA))/10KΩ= 60uA
And ideally IM=0.7/10KΩ=70uA, then the test error=14%;
If the input voltage drop is reduced to 200uV, then the whole test IM=69.99uA, then the test error=0.01%;
Conclusion: From the above simple example, it can be shown that the voltage drop at the input terminal of the ammeter will directly affect the measurement accuracy of the ammeter. The error is smaller.
Difficulty 2: How to add a suitable reverse bias when measuring current?
Commonly used multimeters can only solve the problem of measurement, but at present, many dark current tests need to provide a reverse bias voltage (Bias Voltage), why add a bias voltage? On the one hand, the bias voltage can accelerate the migration process of electrons and holes, reduce the recombination rate of electrons and holes, thereby improving the quantum efficiency and response time; but the reverse voltage cannot be increased indefinitely, and excessive bias voltage may Lead to reverse breakdown of diodes, etc.; on the other hand, many diodes belong to avalanche diodes such as APDs, which themselves require a certain bias voltage to achieve working conditions, resulting in an avalanche effect. Therefore, a voltage source must be added to the loop of the ammeter, which will complicate the test system and introduce more interference conditions, resulting in the inability to guarantee the test accuracy of the entire dark current.
What equipment do the related industries (such as the LED/PD industry) use in the dark current (with bias) test? Through research and visits to several industries, it is found that there are two main options for dark current testing:
(1) SMU source test unit. On the one hand, it can use its voltage source function to complete reverse bias scanning, and on the other hand, use its measurement function to complete small current tests. The advantage of this solution is that the voltage scanning range is large, up to several hundred volts. , and the current measurement function can basically meet the test requirements of the nA level. The disadvantage is that the unit price of the SMU is relatively high, and the cost performance is relatively low.

Keithley SMU 2600
(2) High-precision DMM or picoammeter. These two products are both measuring equipment and can be used for dark current testing. The current test accuracy can even reach pA level. The advantages of the products are moderate price and high measurement accuracy; but the disadvantages of these two products are: 1) Unable to Providing bias voltage can only complete the dark current test in an unbiased environment; 2) The input terminal voltage drop of the high-precision multimeter is relatively high, which will affect the test accuracy of small currents.

Keithley DMM 7510
At present, the 5G infrastructure is in full swing, the optical communication industry is showing explosive growth, and the bandwidth and speed are getting faster and faster. Whether it is passive optical networks such as FTTx, fiber optic cables, or active optical transceiver modules, optical chips, etc. The sensitivity of the terminal has put forward higher and higher requirements, so the increase in sensitivity will inevitably require more and more stringent dark current requirements. After consulting many specifications, a considerable part of the dark current test requirements clearly require dark current ≤ 1nA, Some even require ≤ several hundred pA, while the bias voltage is required to be between 5-15V, and some voltage requirements are ≥ 100V, which is basically powerless for DMM and picoammeter.

Keithley 6485
So is there a meter that can provide both bias scanning and small current testing? The answer is yes, such as Keithley’s 6487 can be achieved. Let’s first look at several important indicators of this biased picoammeter:

・ 10 fA resolution

・ ＜200uV burden voltage

・ Support voltage sweep and Analog output

・ Scanning voltage range 0-505V;

For the three indicators 1, 2 and 4, it fully meets the requirements of the dark current test mentioned earlier in the article, and the voltage source supports synchronous scanning and >100V, and has an analog output function, which can not only describe the IV curve, test high It can also be applied to typical optoelectronic applications such as fiber alignment and PD on-wafer testing.

PD On-Wafer Testing

High Resistance Testing
If you just want to test and evaluate the dark current of diodes or PDs, or you are doing related industry applications such as fiber coupling or PD on-wafer testing, it is recommended to learn about Keithley’s 6487 or 6482/2502 (dual channel) with Biased picoammeter.