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Technical Insight

Peregrine sees through sapphire (Cover Story - VCSELs)

Peregrine Semiconductor has leveraged its CMOS-on-sapphire technology to fabricate integrated VCSEL and detector arrays with silicon ICs, writes Tim Whitaker.
As the leading proponent of silicon-on-sapphire technology, Peregrine Semiconductor has developed a process combining the integration levels and low power of CMOS with the high speed and low parasitics that come from using an insulating sapphire substrate. However, sapphire is also transparent, which allows optical signals to pass through the substrate. Because of this, it is possible to fabricate modules in which arrays of vertical cavity lasers (VCSELs) and detectors are flip-chip bonded to the CMOS circuitry, with the fibers attached to the backside of the sapphire substrate as depicted in the figure, right. "Packaging is the major issue in achieving high speed performance in an optoelectronic system," says Ron Reedy, Peregrine s chief technology officer. "Getting the full data rate electrical signals on and off the fiber is a problem that can not be done simply if the electrical packaging parasitics are in the way. Flip-chip packaging offers a good solution, but it must be based on a substrate with high speed, low power electronics and optical access between fibers, lasers and photodetectors." Module assembly The starting point for the module shown in the is a sapphire wafer covered with an ultrathin (100 nm), high quality epitaxial silicon layer called UTSi. Standard CMOS processing is carried out on the UTSi layer to build ICs such as VCSEL drivers, amplifiers, clock and data recovery circuits, and serialize/deserialize elements. Peregrine runs a 0.5 m version of the UTSi process at its 6 inch fab, which provides 3.125 Gbit/s operation at very low power. The company will soon introduce a 0.25 m process for 12 Gbit/s operation, followed by 0.13 m UTSi CMOS for data rates up to 40 Gbit/s. Next, VCSEL and photodetector arrays are flip-chip bonded onto the CMOS circuitry using standard goldgold or goldtin eutectic die attach techniques. For both transmitters and receivers, an open area is left in the silicon to allow light to pass through. On the transmit side, a small silicon monitor photodiode is included to measure the power output of each VCSEL. "We also build in feedback circuitry, which allows active control of each VCSEL in an array," says Reedy. "This is useful for power monitoring, temperature or ageing compensation and eye safety control." Finding a way to couple light from the VCSEL into the fiber is a big challenge, but fortunately sapphire has some advantageous properties. When the VCSEL beam leaves the surface of the device it begins to expand; this expansion is modified by the sapphire, which has a refractive index of 1.76. "The thickness of the sapphire layer is such that by the time the beam emerges from the backside of the sapphire, the beam width is about the same as the core diameter of multi-mode fiber, i.e. about 5060 m," says Reedy. Because of this, there is no need to incorporate microlenses into the structure, although Peregrine has demonstrated such lenses, and they would probably be required for single-mode fiber. "Although our initial products are aimed at short reach multi-mode fiber markets, this approach is directly applicable to receivers in single-mode systems, where parasitics at 10 and 40 Gbit/s are the most severe," Reedy added. Attaching the fibers The module demonstrated by Peregrine at the recent Optical Fiber Communication conference (OFC) was slightly different from the one shown above, since it used fibers with end facets cut at an angle of 45 to change the direction of the light entering and leaving the fiber. The arrays of fibers are held in plastic V-groove holders with the same pitch (250 m) as the VCSEL and detector arrays. Therefore, only a single alignment step is needed to position the plastic V-groove onto the backside of the sapphire. Again, the transparency of the sapphire helps by allowing alignment marks to be placed on the V-groove and on the top side of the substrate. The arrangement shown is fine for 1-D arrays but is not suitable for 2-D arrays."We ve received a lot of inquiries about 2-D arrays," says Reedy. "For these, we ll use a different approach in which the fibers are butt-coupled to the backside of the sapphire." Functionality The module configuration allows a great deal of functionality to be integrated, as well as making it relatively easy to increase data rates by moving to faster CMOS processes. In addition to demonstrating VCSEL drivers, TIAs and limiting amplifiers, Peregrine is developing crosspoint switches, clock and data recovery ICs, MUX/DEMUX functions and integrated EEPROM (non-volatile memory). These can be seen as building blocks to more complex systems, ultimately offering fiber-in and fiber-out connections with high-speed electronics between. "Under this scenario, only relatively low speed control signals are required, thereby reducing power consumption, crosstalk, and size," says Reedy. "For example, you could build a 12 12 crosspoint switch between the detector and VCSEL arrays. The device would have fibers in and fibers out, with multiple functions carried out by the on-chip electronics. Functions such as routing, retiming, pulse shaping and jitter reduction could be done on-chip without handling high speed electronic signals anywhere but on the surface of the sapphire." Starting with incoming optical signals, it would be possible to deserialize the signals electronically on the surface of the chip, and provide relatively low speed digital outputs. "Router companies dislike having to deal with multi-GHz analog signals at the board level," says Reedy. "Crosstalk and power consumption cause real problems; they d much rather get digital input/output signals." Peregrine expects these types of products to start sampling at the end of the year. RF devices While optoelectronic modules represent a new direction for Peregrine, the company continues to manufacture high performance RFICs using its UTSi CMOS-on-sapphire process. For example, Peregrine recently introduced a family of low insertion loss RF switches targeted at applications up to 2.5 GHz; the company claims these exhibit superior performance to GaAs switches. The SPDT devices have >36 dB isolation at 1 GHz and have single-pin control voltage levels that are CMOS compatible and readily interfaced with other CMOS devices. The switches also have extremely low power consumption, and are priced at 65 cents in volumes of 10k units. Peregrine also ships over a million mixer products per month with extremely high linearity (input IP3 of +38 dBm) and has recently announced a new mixer family with integrated high performance baluns (transformers). "This is an example of where our UTSi products are displacing GaAs and Si bipolar in certain high performance markets," says Reedy.
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