VCSEL is a technology of the future and stands for ‘Vertical Cavity Surface Emitting Laser’. The laser products are especially interesting for recent developments for example facial recognition technology, atomic clocks, proximity sensors, infrared lighting and high-resolution video displays.
Facial Recognition - An Everyday Example
"In facial recognition for mobile devices, the VCSEL acts as a light source that illuminates the face evenly with infrared light. A camera takes pictures of significant features of the user. The image stored in the system is then compared with the captured image to unlock it - if the two match, the device is released. [...] The VCSEL chip can be glued into the housing like an LED chip and is used, for example, as an array (consisting of several hundred individual apertures per VCSEL chip)."¹
High-Volume Manufacturing of Laser Products Possible
Generally, the benefits of VCSELs are that they are inexpensive and optically efficient with a small footprint. They also have the benefit of providing wavelength stability over temperature. Additionally, there is the possibility to combine a VCSEL wafer with various optical elements and then separate this bonded wafer rather than mounting the optical elements specifically for each VCSEL.
The cost-effective mass production of laser products is therefore opened up as a possibility. A further important advantage of VCSELs when compared to edge-emitting semiconductor lasers is that they can be measured and characterized before separation of the wafer. This allows for the detection of quality issues at an early stage.
VCSEL - What Is It Actually?
VCSELs are semiconductor lasers where the light emitted emerges perpendicular to the chip surface. A VCSEL is created from a complicated multilayer structure that is applied to the substrate by molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD).
The epitaxial layers have an active layer that generates the photons trapped between two Bragg reflectors (DBRs). Acting as mirrors, the Bragg reflectors make the light reflect back and forth many times through the active surface to increase amplification.
Generally, VCSELs generate an output power in the 0.5 to 5 mW range and the most frequent emission wavelengths are in the range of 750-980 nm, as achieved with a GaAs/AlGaAs material system.
VCSEL arrays can be utilized to generate extensively greater powers. A 2D VCSEL array with thousands of emitters at a distance of a few tens of micrometers can emit several tens of watts of continued radiation and therefore compete with diode bars, and even with diode stacks based on edge-emitting semiconductor lasers. The output power can be simply increased by making the number of emitters greater.
Modern Surface Measurement Technology
For these complex and varied applications, all-in-one solutions are the most beneficial. They must be able to adhere to the measurement requirements for a large range of processes and handle bonded, standard, and thinned wafers, along with film frames.
Moreover, MicroProf® FS provides all-encompassing measurement solutions for monitoring a range of compound semiconductor substrates for example GaAs, InP, SiC, GaN, ZnO along with transparent materials.
The Technical Solution
The MicroProf® FS is a completely automatic wafer metrology tool for a varied range of applications in different process stages. It can be used for:
- Wafer thickness measurement
- Stress and bow in epi process control
- Haze and defect mapping after epitaxy with high sensitivity to important sub-micrometer defects
- The determination of critical dimensions (CD) and etching depth
- Determining the thickness of step height, dielectric layers, and the width of metallizations
- The taper of bond wafers and inspection of defects in adhesive layers
- TTV, roughness, thickness, and Roll-of-Amount (RoA) post-wafer grinding
- The acquisition of reflection spectra of Bragg reflectors (DBRs)
The modular multi-sensor concept allows the MicroProf® FS to offer high universality and flexibility. It is therefore perfectly suited to a varied range of measurement tasks in VCSEL manufacturing. This flexibility is particularly helpful for VCSEL manufacturers who frequently adjust their research projects to new materials and the most recent nano scientific concepts.
Figure 1. 3D mesa and ring contact measurement of a VCSEL array
Figure 2. Detection of voids in the bond interface
Figure 3. Wafer thickness after grinding
Figure 4. Film thickness analysis of a dielectric layer on wafer using fit algorithm
The Base Component
The basic component is the respected MicroProf 300®. This multi-sensor measuring tool provides both the measurement of wafers in various process stages and, through a hybrid measuring concept, increases the precision of measurements on samples where a single sensor or measuring principle is insufficient.
According to the task, these can be measurements with alternative topography and (layer) thickness sensors, which are made fully automatic using a single recipe. Managed by the in-house software produced by FRT, the sensors instantly combine different information and therefore generate updated information that is not directly accessible.
The tool is controlled by the SEMI-compliant FRT Acquire Automation XT software. This software provides measurements that are recipe based along with total data analysis, including the automatic determination of VCSEL parameters.
The user can select the required evaluation and measurement routine for their measurement task from a range of packages. For repeating structures, a layout wizard with a graphical user interface (GUI) can allow the user to teach in the measurement positions. Additionally, a fine alignment of the samples through pattern recognition is possible as an option.
From manual measurement on the tool to completely automatic measurement with one-button operation and integration into production control systems, for example via a SECS/GEM interface, this software delivers complex functions.
The user can simply configure various measurement tasks with different sensors to be consecutively executed in a sequence of measurement. This also contains the execution of analysis, measurement, and processing with advanced algorithms, output and visualization of the results in the form of reports and the export of the results into various data formats.
Moreover, customers are provided with sample statistics and a pass or fail classification at the wafer and die level to bring more benefits.
The Perfect Workhorse
With practically maintenance-free hardware components and a wafer handling system within an Equipment Front End Module (EFEM), the MicroProf® FS provides high throughput and is the ideal workhorse. The MicroProf® FS allows the customer to keep up with the rapid developments of the VCSEL industry and efficiently amortize investment costs.
Further information can be found here.
For additional comments or questions, please contact FRT where the experts will be happy to provide solutions to measuring tasks.
References and Further Reading
This information has been sourced, reviewed and adapted from materials provided by FRT Metrology.
For more information on this source, please visit FRT Metrology.