[Technical Master Test Note Series]Seventh: 2601B-PULSE makes VCSEL designers more confident

“LIDAR is a 3D sensing technology that uses pulsed laser light of various wavelengths to measure variable distances and detect objects in front of, behind or to the side of a vehicle. At the heart of a LIDAR system is a vertical cavity surface emitting laser (VCSEL), a semiconductor device that emits laser light vertically from the top surface. The VCSEL emits light vertically from the surface, making it a perfect device for LIDAR systems.

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Author: Tektronix technology big coffee

It’s late at night and you’re driving home. You are tired and want to go to bed quickly. Suddenly, a behemoth appeared in the middle of the road. The car brakes hard to prevent hitting the animal while preventing it from getting off the road. With light detection and ranging technology, also known as LIDAR, your car successfully prevents a car accident.

LIDAR is a 3D sensing technology that uses pulsed laser light of various wavelengths to measure variable distances and detect objects in front of, behind or to the side of a vehicle. At the heart of a LIDAR system is a vertical cavity surface emitting laser (VCSEL), a semiconductor device that emits laser light vertically from the top surface. The VCSEL emits light vertically from the surface, making it a perfect device for LIDAR systems.

One of the advantages of VCSELs is that they can be fabricated on semiconductor wafers for easy testing of electrical and optical properties. Additionally, because LIDAR systems require thousands of individual VCSELs in an array to perform transaction-critical applications, efficient testing must be performed at various stages of the production workflow. Through various stages of testing, defective VCSELs can be eliminated at an early stage, resulting in more predictable results, more controllable yields, and lower casting costs.

So here comes the problem: VCSEL testing presents a long list of challenges that will only increase as technology advances. This blog post is intended to take a closer look at these challenges and how the latest 2601B-Pulse pulser/SMU instrument from Keithley, a Tektronix company, can help designers overcome them.

Challenges of VCSEL Testing Using Traditional SMUs

Designers typically test VCSELs using traditional instrumentation, such as source measure units (SMUs), but as technology continues to advance, the number of test challenges is increasing. Below are five of the biggest challenges.

First, the VCSELs used by LIDAR require higher power and therefore higher test currents. Introducing higher test currents increases the risk of device damage due to self-heating, which can be a problem when testing on wafers as it can severely reduce yield.

Second, pulse current testing on the wafer is required to minimize device heat, generally requiring pulse widths of tens of microseconds and rise times of single-digit microseconds or less.

Third, the cable length and associated inductance used in the test system affects the ability of the instrument to provide a clean, fast rise time pulse with no overshoot or undershoot.

Fourth, for LIDAR applications, VCSELs are typically driven up to 10 A, included in the array. But the big challenge is generating short pulses, as short as 10 microseconds, with very short rise times, typically less than 2 microseconds. This is critical because more than 90% of VCSEL customers require pulsed current waveform stabilization to guarantee the accuracy of optical peak power, near-field and far-field measurements.

Fifth, unfortunately, not all pulser/SMU instruments will output high quality pulses. Some instruments are prone to overshoot and have long rise times and long fall times. Many instruments require manual tuning to improve pulse shape, especially when supplying current pulses to VCSELs, whose inductance may vary from device to device and due to test cable inductance.

Some SMUs are prone to overshoot and long fall times when the current is pulsed with very small pulse widths.

First and foremost, even when the pulsed output is tuned at a specific current level, there is no guarantee that manual tuning will result in a consistent pulse shape, especially when testing pulsed IV amplitude-swept VCSELs. Certain pulser/SMU solutions require numerous tuning parameters such as bandwidth, compensation frequency, pole-zero, load impedance, and rise time. But in production testing, tuning the pulse output on the shop floor is either inconvenient or inefficient. As a result, the industry has been asking companies like Tektronix/Keithley to develop faster, higher current source and measure equipment to deliver the performance they need.

Clients share many concerns with us on these issues, telling us:

• We need shorter current pulses, as low as 10 μs, but as high as 10 A.
• We must minimize the pulse width to provide higher currents for testing VCSELs and arrays.
• We cannot tolerate the device getting hot, possibly even burning the probe tip.
• Any overshoot of the current pulse may cause damage to the device.
• I don’t have time to constantly tune to get the best pulse output.
• If the waveform is not high fidelity, it can lead to incorrect characterization, which can lead to poor yield and field failures.

Keithley Pulser/SMU Solutions

In response to the challenge of customer feedback, Keithley recently developed the 2601B-PULSE System SourceMeter 10 μs Pulser/SMU Instrument. The latest 2601B-PULSE with pulse meter technology is an industry leading high current/high speed pulser that provides the measurements and full functionality of a traditional SMU. This latest pulser provides a class-leading 10 A current pulse output at 10 V and supports pulse widths down to 10 μs, making it ideal for testing VCSELs used in LIDARs, LEDs in lighting and displays, as well as semiconductor device characterization, surge protection test etc.

Keithley 2601B-PULSE System SourceMeter 10µs Pulser/SMU Instrument

The pulser has a built-in 1 Msamples/second (MS/s) 18-bit analog-to-digital converter that can simultaneously acquire pulsed current and voltage waveforms without the need for separate instruments. The 2601B-PULSE is a powerful solution that significantly improves productivity in a variety of applications such as benchtop characterization and highly automated pulsed IV production testing.

The most innovative feature of this new instrument is that it does not require manual tuning, regardless of current magnitude and inductive loads up to 3 μH. Cables and inductance can be a problem when outputting current pulses. Inductance can have a limiting effect or even damage. Inductance often varies from device to device, even when testing laser diodes on wafers. The effect of an inductance on a current source is that the inductance prevents the current from changing. This causes the current source to boost the output voltage, causing overshoot and ringing as the pulse settles, which was unacceptable in testing.

Inductance in the DUT and test system cables can affect the quality of the output current pulses.

As mentioned, some solutions require tuning to compensate for these behaviors, which can take a long time. The control loop system of the 2601B-PULSE does not require manual tuning for any load changes below 10 A and 3 μH, so there is no overshoot and ringing when outputting pulses from 10 μs to 500 μs. This guarantees a fast rise time that can provide current pulses to the device and properly characterize the device or circuit. This is especially important when passing multiple amplitude sweep pulses, such as VCSEL pulsed LIV test sweeps. The figure below shows the high-fidelity current pulse output at various current amplitudes and the inductive load of the 2601B-PULSE.

10 A pulse output on 0 μH, 1 μH, 3 μH loads when using the 2601B-PULSE.

1 A pulse output on 0 μH, 1 μH, 3 μH loads when using the 2601B-PULSE.

0.1 A pulse output on 0 μH, 1 μH, 3 μH loads when using the 2601B-PULSE.

Clearly, the 2601B-PULSE provides industry-leading current pulse output performance, eliminating manual tuning that is inefficient when testing VCSELs on wafers in high-speed automated probing systems. The graph above clearly shows that you can test with confidence to achieve repeatable performance and rise times regardless of current levels and varying inductive loads. This new instrument offers many benefits:

• Use one instrument for DC/pulse current and voltage measurements
• Characterize VCSELs with confidence to help you develop next-generation materials, components and modules
• No need to manually tune the pulse output, ensuring high pulse fidelity, shortening test time and saving production costs
• Minimizes device self-heating and reduces the risk of probe tip burnout, thus protecting VCSELs, VSCEL arrays and LEDs
• Measures sampling rates down to single digit μs while calculating 10 μs, 10 A current pulses at 10V