Optical Waveguides Integrated with Sea-of-Leads WLP

In the future — some say the very near future — it's likely that clocking signals will be distributed throughout the chip using optics. With this approach, the light signal is generated off the chip by a laser, sent through optical wave-guides built on the chip, eventually reaching some key distribution points (between 4 and 256) where they are decoded into electrical signals (see "Electrical Beats Optical for Short Distances ").

Researchers at the Georgia Institute of Technology (Atlanta) and Rensselaer Polytechnic Institute (Troy, N.Y.) have moved this one step closer to reality by integrating optical waveguides with sea-of-leads (SoL) wafer-level packaging (WLP) technology. SoL is a new high-density packaging technology that extends wafer-level batch fabrication of multilevel interconnect networks to include the chip's I/O leads (see "SoL-Compliant Wafer-Level Package Technologies ," Semiconductor International , April 2002).

As reported at the International Interconnect Technology Conference (IITC) in June, the researchers built optical waveguides constructed from a transparent epoxy on top of silicon dioxide (which typically serves as a passivation layer in high-performance microelectronics). A buried air-gap upper cladding region was formed using a photosensitive polycarbonate composite to provide a refractive index contrast, Δn, between waveguide core and cladding regions of 0.52.

"Polymer waveguides offer lower refractive indices than most semiconductor waveguides," noted Georgia Tech graduate research assistant Anthony Mulé. "Polysilicon, for example, has a higher refractive index, more than twice that of your typical polymer. If you want to do something like intra-chip optical data interconnection, where you're communicating data from one site to another, you want to have a low effective index to make that time-of-flight delay as low as possible. The other advantage, which applies to optical clock distribution, is that polymer waveguides are compatible with the fabrication processes that we've developed for doing the wafer-level packaging and for forming chip-level volume grating couplers."

SoL structure includes detector (a), grating coupler layer (b), CMOS metalization (c), passivation (d), waveguide core (e), and embedded air gap (f). (Source: Georgia Institute of Technology)

"In our approach, we use the region atop the passivation layer to avoid via blockage that would result if we try to route waveguides anywhere between the passivation and the devices, leaving electrical I/O to dominate restrictions on waveguide placement," Mulé explained. "Using volume couplers, we go from the waveguides located above the passivation layer down to device-level detectors."

The process begins by defining bond pad regions atop a 1.8 µm layer of PECVD SiO₂. The SiO₂ serves as both passivation for underlying circuitry and the lower cladding for optical waveguides (although no circuitry is present on samples fabricated to date). Preparation of the SiO₂ surface for waveguide application is performed by applyin g either hexamethyldisilazane (HMDS) or AP3000 primer. A UV curable alkoxy-siloxane epoxy waveguide material obtained from Polyset Inc. is spun onto the sample at a thickness of 1.2 µm. A 5.1 J/cm² blanket exposure and 1.5 hr oven bake at 150°C are performed to cure the waveguide layer.

In experiments performed to date, 50-µm-wide channel waveguide regions are defined via reactive ion etching with an 8:1 O₂/CHF₃ plasma created at 300 W power and 5 mTorr pressure. Approximately 0.8 µm of the 1.2 µm core region is etched to leave a rib waveguide geometry. Next, 4-5 µm of a sacrificial photosensitive polycarbonate composite (Unity 200) is applied to create embedded air-gap regions. The sacrificial layer is patterned with a 1 J/cm ² dose of 240 nm radiation to initiate decomposition within the exposed regions. Complete decomposition occurs during a subsequent 2 min 110°C soft bake. A 3-4 µm layer of Avatrel polymer is then applied as an overcoat. Finally, the sample is cured at 160°C for 4 hr, during which time the remaining polycarbonate regions decompose to leave an embedded air gap.

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