VCSEL testing cuts lead times
Today s price demands coupled with intense competition mean that the pressure is on for chip makers to raise wafer yields and make faster chips at lower costs. To this end, manufacturers are now looking towards simple, automated test systems for wafers that combine electronics for laser diode control and measurement with optical measurement and motion control platforms. The new wafer test systems promise to offer just this (figure 1).
Detectors cause testing problems
There are several important parameters to measure when testing a VCSEL, including threshold voltage, radiant flux and spectral characteristics. Previous-generation test systems typically comprise a probe station, operating software and an optical detector, but a drawback of these systems has been that the accuracy depends on the setup parameters.
For example the detector, typically a photodiode, has to be aligned with the VCSEL to ensure it captures all the emitted light. Previous systems have relied on a large-area diode to ease alignment and maximize light capture, but changes in the diode s responsivity as light hits it at different points have hampered results.
Light polarization also poses problems for VCSEL testing systems with photodiode detectors. As the polarized light from a VCSEL hits the photodiode, some is reflected away from, rather than transmitted into, the photodiode. This leads to power losses and, again, inaccurate measurements.
So how can chip makers increase their yields if they can t even accurately test their chips for defects? German probe-station maker Suss Microtec and US optical measurement company Labsphere believe they have the answer.
Last year the two companies joined forces to create an automated laser diode wafer-level test system for VCSELs. Designed for high-speed diode characterization, the system comprises a probe station, control software and, unlike past test systems, an integrating sphere that houses the detector.
Integrating sphere tackles flux
Used to measure optical radiation, the prime function of Labsphere s integrating sphere is to spatially integrate radiant flux from a light source such as a VCSEL (figure 2). Different spheres exist for different applications and, unlike photodiode detectors, an integrating sphere designed for VCSELs can capture the total radiometric flux emitted from the devices.
The sphere consists of a spherical shell which has its interior coated with a propriety high-reflectance thermoplastic called Spectralon. This material diffuses the incident light. The sphere also has an input port and a photodiode detector mounted on its exterior wall. The input port is machined into the sphere s wall, and different sizes can be chosen depending on the application. The photodiode detector is made of either silicon, for wavelengths up to 1100 nm, or InGaAs or Ge for longer wavelengths.
Light enters the sphere via the port, strikes the diffuse interior surface and scatters equally in all directions. This scattered light or radiance is no longer polarized, which solves the reflectance problems that plagued old-style detectors, and also means that light incident on the detector is an accurate measurement of the device s power. Test results are also less sensitive to the alignment of the detector with the VCSEL. In fact, even with alignment offsets as high as 2-3 mm in the x-direction, the correct reading can still be obtained.
The diffuse scattering of light has another advantage. Since the material scatters in all directions, any point on the inside of the sphere is illuminated equally. The detector can therefore be placed at any location on the exterior wall to give the same reading. The setup is designed, however, so that the detector s field-of-view does not overlap with the direct area of illumination from the incoming laser light. This ensures that the light is scattered and uniformly integrated before striking the detector.
