[Technical Big Coffee Test Notes Series]Seven: 2601B-PULSE makes VCSEL design confident

“One of the advantages of VCSELs is that they can be manufactured on semiconductor wafers and can be easily tested for electrical and optical performance. In addition, because the LIDAR system requires thousands of individual VCSELs in the array to perform critical transactional applications, effective testing must be performed at all stages of the production workflow. Through various stages of testing, bad VCSELs can be eliminated at an early stage, making the results more predictable, making the yield rate more controllable, and reducing casting costs.

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Author: Tektronix Technology Expert

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

It’s late at night and you are driving home. You are tired and want to go to bed quickly. Suddenly, a giant appeared in the middle of the road. The car brakes urgently to prevent hitting animals and at the same time prevent it from leaving the road. Through light detection and distance technology, also known as LIDAR, your car successfully prevented a car accident.

LIDAR is a 3D sensing technology that uses pulsed lasers of various wavelengths to measure variable distances and detect objects in front of, behind, or on the side of the vehicle. The core of the LIDAR system is the vertical cavity surface emitting laser (VCSEL), which emits laser light vertically from the top surface. 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 manufactured on semiconductor wafers and can be easily tested for electrical and optical performance. In addition, because the LIDAR system requires thousands of individual VCSELs in the array to perform critical transactional applications, effective testing must be performed at all stages of the production workflow. Through various stages of testing, bad VCSELs can be eliminated at an early stage, making the results more predictable, making the yield rate more controllable, and reducing casting costs.

So here comes the problem: VCSEL testing faces a long list of challenges, and the challenges will only continue to increase with technological progress. This blog post is intended to examine these challenges in more depth and how Keithley’s latest 2601B-Pulse pulser/SMU instrument, a Tektronix company, can help designers overcome these challenges.

Challenges of using traditional SMU for VCSEL testing

Designers usually use traditional instruments to test VCSELs, such as source measurement units (SMUs), but as technology continues to advance, the number of test challenges is increasing. Here are five of the biggest challenges.

First, the VCSEL used by LIDAR requires higher power and therefore higher test current. Introducing a higher test current increases the risk of device damage due to self-heating, which may become a problem when testing on a wafer because it will severely reduce the yield.

Second, pulse current testing is required on the wafer to minimize the heat of the device. Generally, a pulse width of tens of microseconds and a rise time of one-digit microseconds or less are required.

Third, the cable length and related inductance used in the test system affect the instrument’s ability to provide clean, fast rise time, no overshoot, and no undershoot pulses.

Fourth, for LIDAR applications, VCSELs are usually driven up to 10 A and included in the array. But the huge challenge is to generate short pulses, as short as 10 microseconds, and the rise time is very short, usually less than 2 microseconds. This is very critical, because more than 90% of VCSEL customers require stable pulse current waveforms to ensure the accuracy of optical peak power, near-field and far-field measurements.

Fifth, unfortunately, not all pulsers/SMU instruments will output high-quality pulses. Some instruments are prone to overshoot, have a long rise time and a long fall time. Many instruments require manual tuning to improve the pulse waveform shape, especially when supplying current pulses to the VCSEL, the inductance may vary between different devices and due to the inductance of the test cable.

When the current sends pulses with very small pulse widths, some SMUs are prone to overshoot and have a long fall time.

Most importantly, even if the pulse output is tuned at a specific current level, there is still no guarantee that manual tuning can get a consistent pulse waveform, especially when testing pulsed IV amplitude scan VCSELs. Some pulser/SMU solutions require a large number of tuning parameters, such as bandwidth, compensation frequency, pole-zero, load impedance, and rise time. But in production testing, it is either inconvenient or inefficient to tune the pulse output in the workshop. Therefore, the industry has always required companies like Tektronix/Keithley to develop faster and higher current sources and measurement equipment to provide the performance they need.

Customers have shared many concerns with us on these issues and told us:

• We need shorter current pulses, as low as 10 μs, but current as high as 10 A.
• We must minimize the pulse width to provide higher current for testing VCSELs and arrays.
• We cannot tolerate the heating of the device and may even burn the probe tip.
• Any overshoot of the current pulse may cause damage to the device.
• I don’t have time to tune continuously to get the best pulse output.
• If the waveform is not a high-fidelity waveform, it may cause characterization errors, which may lead to poor yields and field failures.

Keithley Pulser/SMU Solution

To meet the challenges of customer feedback, Keithley has newly developed the 2601B-PULSE system source meter 10 μs pulser/SMU instrument. The latest 2601B-PULSE adopts pulse meter technology, which is an industry-leading high-current/high-speed pulser that provides the measurement and all functions of a traditional SMU. This latest pulser provides a leading 10 A current pulse output at 10 V and supports a minimum pulse width of 10 μs. It is particularly suitable for testing VCSELs used in LIDAR, LEDs in lighting and displays, as well as semiconductor device characterization and surge protection Test etc.

Keithley’s latest 2601B-PULSE system source meter 10μs pulser/SMU instrument

The pulser has a built-in 1 M samples/second (MS/s) 18-bit analog-to-digital converter, which can collect pulse current and voltage waveforms at the same time without using a separate instrument. 2601B-PULSE is a powerful solution that significantly improves production efficiency in various applications, such as benchtop characterization and highly automated pulse IV production testing.

The most innovative feature of this new instrument is that it does not require manual tuning, regardless of the current amplitude, and regardless of the inductive load up to 3 μH. When outputting current pulses, cables and inductors may become a problem. Inductance may have a limiting effect and even cause damage. The inductance is usually different from device to device, even when testing laser diodes on a wafer. The effect of the inductance on the current source is that the inductance prevents the current from changing. This will cause the current source to increase the output voltage, causing overshoot and ringing when the pulse is stable, which is unacceptable in the test.

The inductance in the device under test and the test system cable may affect the quality of the output current pulse.

As mentioned earlier, some solutions require tuning to compensate for these behaviors, which can take a long time. The control loop system of 2601B-PULSE does not require manual tuning for any current load changes below 10 A and below 3 μH, so there is no overshoot and ringing when outputting 10 μs ~ 500 μs pulses. This guarantees a fast rise time and can provide current pulses for the device to correctly characterize the device or circuit. This is extremely important when passing multiple amplitude scan pulses, such as VCSEL pulsed LIV test scans. 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, and 3 μH loads when using the 2601B-PULSE.

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

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

Obviously, the 2601B-PULSE provides industry-leading current pulse output performance, eliminating manual tuning, and the manual tuning efficiency is very poor when testing the VCSEL on the wafer in the high-speed automatic detection system. The figure above clearly shows that you can test with confidence, regardless of current levels and different inductive loads, and you can achieve repeatable performance and rise times. This new instrument provides many benefits:

• Use one instrument for DC/pulse current and voltage measurement
• Confidently characterize VCSELs 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
• Minimize the self-heating of the device and reduce the risk of burning the probe tip to protect the VCSEL, VSCEL array and LED
• Measure the sampling rate of the lowest digit μs, and calculate the current pulse of 10 μs and 10 A at 10V at the same time

What testing challenges are you facing?

Are you fully equipped to characterize VCSELs or other optoelectronic devices used by LIDAR? Are you ready to overcome the growing challenges posed by these evolving devices? Welcome to communicate with us about your pulse test application and discuss how the latest 2601B-PULSE can help you solve your challenges. Please contact our application engineers or visit the 2601B-PULSE product page for more information.