ULM refines VCSEL manufacturing technique


ULM Photonics, based in Ulm, Germany, manufactures vertical-cavity surface-emitting lasers (VCSELs) for the data communications, sensing, printing, and medical markets. It focuses on high-speed VCSELs that perform up to 10 Gbits/sec. The VCSELs are made in the semiconductor indium aluminum gallium arsenide (InAlGaAs) that enables VCSEL transmission in the 750-1000-nm range.

In combination with flip-chip manufacturing techniques, developers at ULM have achieved high-density arrays of VCSELs—much higher than with conventional wire-bonding techniques. With the flip-chip approach it is possible to achieve multiple VCSEL arrays from 1¥12, 3¥12, 4¥12, up to 8¥8 and 8¥12. ULM's manufacturing approach enables VCSELs to be positioned on the driver chip, which is followed by "one step" soldering.

ULM co-founder and manager, device production, Martin Grabherr says that while it is possible to purchase commercially available VCSEL manufacturing systems, the typical performances are not optimised for the most efficient manufacture, which ULM is pursuing. The other key players in this development work were ULM scientists Christian Wimmer, Lin Robert Borowski, and Tobias Pohl.

The VCSEL consists of a several hundred layers, usually multiples of one-quarter wavelength in height. These layers all need to be precisely controlled during epitaxial growth.

Recent research and development work at ULM has refined the VCSEL manufacturing process by changing certain parameters. "The important feature of our process is to match the optical wavelength of the illuminating source to the 'optical length' of the layers that we want to remove," explains Grabherr. "The optical length is equivalent to the mechanical length of the layer multiplied by the refractive index of the given layer."

VCSEL mesa etching is the process by which the microstructures in the semiconductor material are carved or converted to dielectrics. Mesa etching is a crucial step in the production of highly efficient oxide-confined VCSELs. Depending on the application, such as production of high-speed or singlemode VCSELs, there is a need to perform dry etching of the mesa (pillar) using reactive ion etching or comparable etching technologies like inductive coupling plasma, chemically assisted ion beam, and reactive ion beam.

To control the etch depth in the multilayer structure of the VCSEL cavity, typically laser interferometry or laser reflectometry is used. The simplest setup for optical in situ etch control is schematically drawn in Figure 1, where a laser illuminates the surface of the wafer and a photodiode detects the reflected intensity.

Figure 2 shows the reflected signal of a 633-nm HeNe laser during the mesa etch process of a standard 850-nm VCSEL wafer. When the etching approaches the inner cavity, the signal oscillation is non-periodic. This behaviour can be explained by the interference of the remaining top mirror with the bottom mirror and the sandwiched inner cavity, which is not phase-matched for the 633-nm laser.

Grabherr asks the key question upon which ULM's refinement technique hangs: "What kind of laser is more promising to exactly measure the changing optical characteristics of a 850-nm VCSEL cavity? It is the 850-nm VCSEL itself! This is what became obvious when we thought about a more convenient laser source."

The signal plotted in Figure 3 is obtained using an ULM-built standard 850-nm multimode VCSEL. The graph shows the perfect optical response from the etched 850-nm VCSEL cavity. Since illumination wavelength, distributed Bragg reflector (DBR) stop-bands, and inner cavity length are matched, the reflected signal is not affected by multimirror resonator effects. Closer to the inner cavity, the laser beam increasingly propagates through the remaining few layer pairs of the top DBR and the high reflectivity of the bottom DBR comes into account. Following the nice observation of the one long inner cavity, the signal of a perfect DBR mirror is observed in return.

Now the absolute reflectivity again oscillates with the phase mismatch of the surface layer, but that is steadily reduced down to a certain level, typical of bulk material. The use of 850-nm VCSEL devices to in situ monitor dry etching processes of 850-nm VCSEL wafers is reported for the first time. The signal obtained allows most simple real time interpretation of the ongoing etch process.

Even for non-specialist manufacturing operatives, it is relatively easy to identify the major parts of the VCSEL structure: the top DBR, inner cavity, and bottom DBR. It's possible to count the number of etched layer pairs and measure the optical length of etched bulk material.

Unstable, non-reproducible, or unpredictable etch rates can be compensated by ULM Photonics' advanced in situ monitoring technique, and the desired etch depth of a few microns can be achieved with an accuracy of better than 50 nm. Figure 4 shows the circular mesa of an etched VCSEL structure where etching is controlled and stopped just below the inner cavity by using the advanced in-situ monitoring VCSEL etch technique.

So what will ULM Photonics be doing with its refined manufacturing technique beyond its own manufacturing business function? "We discussed commercialising the technique with some of the etching technology system manufacturers," notes Grabherr, "but the market for such equipment is relatively small—maybe 10 units per year worldwide. So we then decided to publish our findings. If companies need 850-nm VCSELs, then we are the most precise and reliable source."

In mid-2003, ULM is focusing more on the medical and sensing markets while the lull in VCSEL-based datacoms persists. However, Grabherr is confident the telecoms market for his products will pick up later in 2003 and that the free-space optics sector looks particularly promising.

ULM Photonics GmbH, Albert-Einstein-Allee 45, 89081 Ulm, Germany, www.ulm-photonics.de 2 ILM, Germany, www.uni-ulm.de/ilm.

One of the first companies to develop 10-Gbit/sec vertical-cavity surface-emitting lasers (VCSELs), Avalon Photonics (Zurich) has recently completed its second round of financing, raising a total of USD5.5 million in venture capital funding. Investors include Vision Capital, Viventures, Venture Incubator, innotech, and the Swiss Center for Electronics and Microelectronics (CSEM).

Vision Capital led the investment. The round included existing investors, Viventures and CSEM, and two new investors: Venture Incubator from Switzerland and innotech from Germany. The funding will allow the company to reinforce its position in the supply of high-speed and singlemode VCSELs.

In September 2000, Avalon spun off from CSEM. The Avalon team has been developing and producing singlemode and multimode VCSELs for the data communications and sensing markets for more than nine years. Avalon says it can "volume deliver" product today "at the lowest costs" due to its efficiency in manufacturing high-bandwidth VCSEL arrays. The company recently announced a partnership with Alvesta and continues to gain traction in the market.

"We welcome Venture Incubator and innotech as new shareholders," says Dr. Heinz Meier, who joined Avalon last year as chief executive. "These new investors bring complementary strengths that can enhance our business and complement our existing shareholders, Vision Capital, Viventures, Intel Capital, and CSEM. With more than nine years of experience in VCSEL design, development, and manufacturing, [we] will be in a solid position to meet present and future requirements in the short range optical data-communication segment as well as in the sensing market."

"With this second round of financing," notes Sven Lingjaerde, general partner with Vision Capital, the transatlantic venture capital firm, "Avalon is well positioned to be a long-term player in the high-end VCSEL industry." Adds innotech partner Dr. Hazel Arnot, "What especially positions Avalon for success is its strength not only in datacom, but also in the sensing market, where Avalon is one of the leading VCSEL providers for sensing applications."

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