VCSEL Testing Using Triggered Synchronous Source Meter

Vertical cavity surface emitting lasers (VCSELs) are gradually replacing traditional edge emitting lasers, especially in low bandwidth and short-distance communication systems where cost factors are particularly important. Edge emitting lasers must be cut from the wafer and the edges polished before testing, while VCSEL manufacturers can test their devices at the wafer level.

Light intensity (L)-current (I)-voltage (V) sweeps are a series of measurements performed to characterize the operation of a VCSEL. LIV testing requires a ramp current to be passed through the VCSEL and the resulting light output to be measured using a photodetector (PD).
Figure 7-29 shows a simple wafer-level test system. Two Model 2400 source meters are used. A wafer probe is electrically connected to each device via a probe card. The probe station also positions the photodetector just above the device. When light from the VCSEL strikes the reverse biased PD, leakage current increases. The magnitude of the leakage current is related to the light intensity striking the active region.

Figure 7-29 Wafer-level testing of a VCSEL

Switch Configuration

If the probe card can connect to multiple devices simultaneously, then a system such as that shown in Figure 7‑30 can be used to test all devices simultaneously each time the probe card connects to the wafer.

An individual VCSEL is selected for testing, the corresponding relay for that VCSEL is closed, and the light density is checked using the PD. The source meter first performs the necessary DC tests, such as forward bias, reverse breakdown voltage, and leakage current. Sufficient current is then forced to illuminate the VCSEL, and the second 2400 measures the leakage added by the PD. After completing this test, the switching channel of the next device is selected.

Depending on the current sensitivity requirements of the test specification, the PD can be measured using either the 2400 or the 6517A. The 2400 can be used to measure currents of approximately 10nA, while the 6517A can reliably measure currents less than 10fA.

The offset current specification of the Model 7011 Multiplexer Card is <100pA, which may exceed the tolerance of the test system. Using the Model 7158 Low Current Scanner Card instead can reduce the offset value to <1pA. Note that since the Low Current Switch Card only has 10 channels for scanning, using the Low Current Switch Card will reduce the number of available channels in the system. [page]

Figure 7‑30 Testing multiple VCSELs

The instruments in the test system are connected via Trigger Link cables, a hardware handshaking bus found in newer Keithley instruments. When the SourceMeter and Switch Mainframe are connected via Trigger Link, they can trigger each other for faster testing. This built-in bus eliminates the need for direct PC control of most system functions. Trigger Link has six trigger lines, and up to six instruments can be controlled using this trigger bus.

Trigger Link has two modes of operation: synchronous and semi-synchronous. In synchronous mode, the input and output triggers use separate trigger lines; while in semi-synchronous mode, the input and output triggers use the same trigger line. There are two Trigger Link connectors on the 7001/7002 switch mainframe. There is one Trigger Link connector on the 2400 SourceMeter. The factory default for trigger line #1 is input trigger, and trigger line #2 is output trigger.

As shown in Figure 7-31, the SourceMeter instrument can receive and send triggers at the beginning or end of each phase of a Source-Delay-Measure (SDM) cycle. An SDM cycle turns on the source signal, performs a programmable delay, and then takes a measurement.

Figure 7-31 SourceMeter Input/Output Trigger

For this application, a synchronous trigger is used to control the following time sequence:

A scan is driven by a front panel button or a bus command sent to the 7001/7002.

Close the channels corresponding to VCSEL #1 and PD #1 on the 7011 switch card. After the delayed stabilization time, the output trigger is sent to the two Model 2400 SourceMeter instruments via line #1 (“pre-source” trigger in the SDM cycle).

The SMUs begin their source-delay-measure cycle. The first Model 2400 SMU outputs a positive bias current to VSCEL #1; the second Model 2400 SMU outputs a negative bias voltage to PD #1.

After a programmable delay time, the second Model 2400 SourceMeter instrument measures the leakage current generated by PD # 1. At the end of its measurement phase, the SourceMeter instrument sends an output trigger (the “after measurement” trigger of the SDM cycle) to the switch host via line # 2.

7001/7002 opens all channels, then closes the channels corresponding to VCSEL #2 and PD #2, and triggers the source instrument.

Perform the above steps for all DUTs. Of course, after each measurement, the probe must be connected to another device on the wafer with the detector positioned above the device.