In conjunction with Stanford University, TiaLinx, a fabless start-up in Irvine, Calif., is in the process of developing a wafer-scale integration (WSI) of an active antenna array and associated RF circuit for beamforming applications. The technology provides future opportunities to deploy highly integrated radar systems-on-wafer suitable for small footprint, fully electronically controlled and low-cost military and commercial applications, stated Fred Mohamadi, president and founder of TiaLinx. Several challenges in the development of this WSI chip are being addressed. These include design of Si-based substrate RF blocks, RF signal distribution to each element, impact of signal attenuation and cross-talk, accuracy required in phase resolution and beam width management, noise cancellation, DC signal distribution, and control capability of beamforming for tracking as well as beam steering.

The proposed wafer scale array module (WSAM) consists of a 16 by 16 element (a total of 256 elements) antenna and associated electronics circuitry fabricated on a six-inch (150 mm) silicon germanium (SiGe) wafer, respectively. The module provides transmit and receive functions at 35 GHz with a bandwidth of 5%. The WSAM is designed to scan over ±30° at 3 dB drop off from maximum ERP. A possible consideration is to provide a 20 W to 50 W ERP at broadside from the WSAM operating at 3.3 V.

In this scheme, the WSAM utilizes a “tile” array architecture in which the radiating element layer, SiGe RF device layer, and signal distribution layer are parallel to each other. The layers are interconnected using a special manufacturing process. A six-inch wafer is being used for 256-element array with 6 mm pitch (separation plus antenna size). Alternatively, the 256-element array can also be built on an 8-inch wafer substrate with an 8 mm pitch to provide maximum antenna array gain, according to TiaLinx.

The RF portion of the module consists of a 256 network line RF signal divider. Included in each cell are a low noise amplifier (LNA), a programmable power amplifier (PA), a controller that addresses element selection, row or column selection, analog switches to select Rx or Tx modes, an attenuator, and a phase shifter unit.

In essence, this module consists of three main layers, radiating element layer, device layer and signal distribution layer. The radiating element layer can consist of 256 microstrip patch or dipole elements. To provide a low resistance thermal paths for heat dissipation of WSAM, a layer of heat conductive material is used for coating the active devices side, noted Mohamadi. However, he added, an important consideration is the maximum allowable current density of 10⁵ A/ Cm² to address the electromigration design rules. This will set an upper limit of 2 mA/micron length for a 2 micron thick Al-Cu line, explained Mohamadi.

Consequently, in the proposed WSAM implementation, as shown in the figure above, a heavily doped deep conductive junction is formed as a contact junction for the antenna feed prior to any standard process step. This process is similar to a deep diffusion junction process used for manufacturing of double diffused CMOS (DMOS) devices. The deep junction is used for achieving good isolation of the antenna feed lines from active devices (LNA, Phase Shifter and PA). It also provides a region of low resistive signal path that needs to minimize insertion loss to the antenna plates (patch or dipole). The deep junctions will be accessed through the backside of the wafer where antenna plates will be fabricated. Active devices are then fabricated using a standard SiGe process, and later passivated by applying a low temperature deposited porous silica SiOx and a thin layer of Nitridized oxide (SixOyNz) as a final layer of passivation for the device.

Attenuation of RF signal, and jitter deterioration as a result of crosstalk and ISI are the key areas of focus for the robust design of a WSAM, noted Mohamadi. In addition, the developer is investigating the impact of distributed amplifier and spatial phase balancing and crosstalk cancellation on the signal-to-noise ratio (SNR) of the delivered RF signal to active antenna elements. Based on the efforts in process, TiaLinx expects to complete this design with a prototype solution within 18 months.

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