M. Popadic

Ultra Linear Low-Loss Varactor Diode Configurations for Adaptive RF Systems

Two linear low-loss varactor configurations for tunable RF applications are compared. The wide tone-spacing varactor stack provides the best linearity for signals with relative large tone spacing like receiver jammer situations. The narrow tone-spacing varactor stack offers the highest linearity for in-band-modulated signals, and is better suited to adaptive transmitters. Both structures make use of a varactor with an exponential C(VR) relation, and so the different requirements of transmit and receive chains can be addressed in one technology. Both configurations have been realized in a silicon-on-glass technology. The measured Q at 1.95 GHz is from ~ 40 to 200 over a capacitance tuning range of 3.5 with the maximum control voltage of 12 V. The measured OIP3 of both structures are roughly 60 dBm.

Improved RF Devices for Future Adaptive Wireless Systems Using Two-Sided Contacting and AlN Cooling

This paper reviews special RF/microwave silicon device implementations in a process that allows two-sided contacting of the devices: the back-wafer contacted silicon-on-glass (SOG) substrate-transfer technology (STT) developed at DIMES. In this technology, metal transmission lines can be placed on the low-loss glass substrate, while the resistive/capacitive parasitics of the silicon devices can be minimized by a direct two-sided contacting. Focus is placed here on the improved device performance that can be achieved. In particular, high-quality SOG varactors have been developed and an overview is given of a number of innovative highly-linear circuit configurations that have successfully made use of the special device properties. A high flexibility in device design is achieved by two-sided contacting because it eliminates the need for buried layers. This aspect has enabled the implementation of varactors with special Ndx -2 doping profiles and a straightforward integration of complementary bipolar devices. For the latter, the integration of AlN heatspreaders has been essential for achieving effective circuit cooling. Moreover, the use of Schottky collector contacts is highlighted also with respect to the potential benefits for the speed of SiGe heterojunction bipolar transistors (HBTs).

Special RF/microwave devices in Silicon-on-Glass Technology

This paper reviews special RF/microwave silicon device implementations in the back-wafer contacted Silicon-On-Glass (SOG) Substrate-Transfer Technology (STT) developed at DIMES. In this technology, metal transmission lines can be placed on the low-loss glass substrate, while the resistive/capacitive parasitics of the silicon devices can be minimized by a direct two-sided contacting. Focus is placed here on the device level aspects of the SOG process. In particular, complementary bipolar device integration and high-quality varactors for high-linearity adaptive circuits are treated in relationship to developments in back-wafer contacting and the integration of AlN heatspreaders.

Silicon-on-glass technology for RF and microwave device fabrication

This paper reviews the applications and potentials of back-wafer contacted silicon-on-glass (SOG) substrate-transfer technology (STT) particularly for RF and microwave silicon-device-design enhancement. This type of SOG process gives direct access to the part of the device that is usually connected via the bulk Si, by allowing advanced patterning and contacting of the backside of the wafer (back-wafer) with respect to the front of the wafer (front-wafer). In this manner the resistive and capacitive parasitics of the device itself, which in silicon often inhibit high-frequency (HF) performance, can be reduced to a minimum. At the same time new device concepts are made possible. Examples of fabricated devices (varactor diodes, vertical double-diffused MOSFETs (VDMOSFETs) and complementary bipolar transistors) are given and described in relationship to issues such as the very limited thermal budget permitted in the back-wafer processing and the inherently high thermal resistance of the SOG devices

Analytical carrier transport model for arbitrarily shallow p-n junctions

This paper presents for the first time an analytical model of arbitrarily shallow p-n junctions. Depending on the junction depth, electrical characteristics of ultra-shallow p-n junctions can vary from the characteristics of standard Schottky diodes to standard deep p-n junctions. Therefore, this model successfully unifies the standard Schottky and p-n diode expressions. In the crossover region, where the shallow doping can be totally depleted, electrical characteristics phenomenologically substantially different from typical diode characteristics are predicted. These predictions and the accuracy of the presented model are evaluated by comparison with the MEDICI simulations. Furthermore, ultra-shallow n+-p diodes were fabricated, and the anomalous behavior in the crossover regime was experimentally observed.

Ultra-Shallow Dopant Diffusion from Pre-Deposited RPCVD Monolayers of Arsenic and Phosphorus

Reduced-pressure chemical-vapor-depositions (RPCVD) of arsenic and phosphorus monolayers on silicon are investigated as a damage-free source of ultra-shallow dopant diffusion when encapsulated under a deposited oxide. The encapsulation enhances the diffusion into the Si as compared to doping from the gaseous phase, which is confirmed by sheet-resistance measurements and current-voltage characterization of contacts and diodes fabricated with these layers. The latter show the transition from a p-Schottky diode to an n+p diode with increasing annealing temperature on p-doped samples. This effectively represents the means for SBH modulation by ultra-shallow doping. Process simulations have been found inadequate to describe the diffusion process, which, instead, was found to be a mixture of at least two physical mechanisms.

Deep p + junctions formed by drive-in from pure boron depositions

This paper presents a new method of supplying the high doses of boron needed for creating several micron deep p+n junctions. Chemical vapor deposition (CVD), in a Si/SiGe epitaxial reactor, of nanometer-thick pure boron layers is used to fabricate 5 μm deep p+n junctions. The 10 min B deposition is combined with a 195 min drive-in at 1100°C to give a resulting sheet resistance of 3.1 Ω/sq. For as-deposited B-layers in windows through an silicon dioxide isolation to the Si substrate, reactions of the Si with oxide at the perimeter of the deposited windows will be enhanced by the presence of the B-layer during the high-temperature drive-in. Detrimental effects such as lateral contact window widening, small surface defects and/or large spikes formation, are avoided by capping the surface of the windows with either thermal oxide in a selective process or a low-pressure CVD (LPCVD) oxide during the drive-in. A good electrical quality of the oxide capping layer was achieved. The surface morphology was investigated by atomic force and scanning electron microscopy (AFM/SEM) analysis and found to depend on the overall method of fabrication.

Ultra-low-temperature process modules for back-wafer-contacted silicon-on-glass RF/microwave technology

This paper reviews several novel process modules developed for the processing of the backside of the wafer of our substrate-transfer technology called back-wafer-contacted silicon-on-glass (SOG), which is in use for fabricating RF/microwave devices such as high-quality varactors and bipolar transistors. In this technology the silicon wafer is transferred to glass by gluing. The integrity of the acrylic adhesive limits the subsequent processing temperatures to less than 300°C. Ultra-low-temperature process modules have therefore been developed to nevertheless allow the creation of low-ohmic contacts and high-quality ultrashallow junctions. Moreover, a physical-vapor deposition of AlN provides an effective means of integrating a thin-film dielectric heatspreader.

Ultrashallow doping by excimer laser drive-in of RPCVD surface deposited arsenic monolayers

Reduced pressure CVD of arsenic has been investigated as a source of dopants in combination with excimer laser annealing (LA). Energy densities used for LA are above the Si melt limit and abrupt, highly doped, nearly defect-free, ultrashallow junctions have been formed. The junction depth is determined by the melt depth and is independent of the doping level, which is determined by the As deposition. Multiple LA of the surface deposited As layer was performed to yield improved uniformity while multiple cycles of As deposition plus LA have been performed to yield a higher dose and consequently lower sheet resistance, which in the case of three depositions drops to around 80 Ω/sq for layers of an estimated depth of less than 20 nm. Near-ideal diode characteristics have been measured.

RF/microwave device fabrication in silicon-on-glass technology

This paper reviews recent developments in circuit and device implementations based on back-wafer contacted silicon-on-glass (SOG) substrate-transfer technology (STT). This technology has been specifically developed for the enhancement of silicon RF and microwave device and circuit performance. While metal transmission lines can be placed on the low-loss glass substrate, the resistive and capacitive parasitics of the silicon devices can also be minimized by a direct contacting of the parts of the devices that are usually connected via the bulk Si. Focus is placed here on the device level aspects of the SOG process, in particular high-quality varactors for high-linearity adaptive circuits and complementary bipolar device integration are treated in relationship to new developments in back-wafer contacting and the integration of AlN heatspreaders.