Coplanar waveguides and microwave inductors on silicon substrates

In recent years, a high resistivity silicon (HR-Si) substrate is of interest as a substrate upon which integrated radio-frequency (RF) and millimeter wave circuits can be realized. Because the material have an inherent defect density, surface effect and resistivity degradation near the interface between an insulating oxide and a HR-Si substrate tend to overshadow the potentially low RF loss levels in HR-Si substrate. Fixed positive charges within the oxide attract free carriers near the substrate surface, leading to an accumulation or inversion layer on the silicon surface. Consequently, these free carriers act as the thin surface-channel at the Si/oxide interface, thus reducing the resistivity of the silicon surface and increasing the substrate loss. In fact, several surface passivation approaches have been used to overcome these shortcomings. For example, local resistivity can be enhanced by high-dose implantation (e.g, argon) to convert LR-Si substrate to HR-Si substrate [1], or a trap-rich passivation layer (e.g., polysilicon or amorphous silicon) between the oxide layer and the HR-Si substrate to prevent the carrier accumulation [2]. Polysilicon films deposited by LPCVD are the most widely used as a surface passivation layer on on HR-Si substrate and a thickness between 300nm and 400 nm were generally used. For HR-Si surface passivation application, the properties of LPCVD-deposited polysilicon (or amorphous silicon) are known to be significantly dependent on deposition conditions and annealing conditions [3].

Radio-Frequency (RF) losses on High-Resistivity Silicon (HRS) substrates were studied for several different surface passivation layers comprising thin-films of SiC, SiN and SiO 2 In many combinations, losses from conductive surface channels were reduced and increasing the number of interfaces between thin-films was found to be beneficial. In some cases the surface losses were completely eliminated. For example, with plasma-enhanced chemical-vapor deposition (PECVD) α-SiC layers up to a few tens of nm thick and exposed to nitridation or SiN growth at 850°C to form a SiC:N interface layer, values for the total losses of 1.6 dB/cm were achieved. Analysis of these layers was performed by using temperature dependent measurements of the RF losses on Coplanar Waveguides (CPWs), the capacitance-voltage characteristics and the sheet resistance along the Si surface. The overall results can be explained by assuming that the thin-films are so defected that they allow vertical current paths to highly-resistive interface layers where both fixed and mobile charge can be stored.

AlGaN/GaN high-electron-mobility-transistors (HEMTs) with 0.8-/spl mu/m- and 0.3 /spl mu/m-gate-length were successfully fabricated on high-resistivity (HR) silicon substrate. The cutoff frequency f/sub T/ values of 7 GHz, 22 GHz and maximum oscillation frequency f/sub max/ values of 23 GHz, 40 GHz were achieved for 0.8-/spl mu/m- and 0.3-/spl mu/m-gate-length device, respectively. A minimum noise figure (NF/sub min/) of 2.0 dB and an associate gain (G/sub ass/) of 10.3 dB were achieved at 10 GHz in 0.3-/spl mu/m-gate-length device. This shows the promising potential of AlGaN/GaN HEMTs on high-resistivity Si for cost effective low noise amplifiers.

In this paper, spiral inductors on high-resistivity silicon (HR-Si) substrates, which surfaces were passivated by different process methods, were fabricated and measured. The comparison of inductances and quality factors between these inductors shows that the inductances and quality factors of spiral inductors significantly depend on process method and thickness of surface passivation layer. The differently generated passivation layer results in different effective substrate resistivity and hence gives rise to different substrate loss. The experimental results show that using thicker nanocrystalline silicon (nc-Si) as a passivation layer will result in a smaller substrate loss and hence spiral inductors on the substrate have larger inductances and larger quality factors. In this study, the metal thickness of inductors is 1.2 µm and is about half that of the inductors provided by foundries. At higher frequencies, the thinner spiral inductors on the SiO2/nc-Si/HR-Si substrate have larger inductances and larger quality factors as compared with the inductors provided by foundries. With the same metal thickness as that adopted in standard processes of foundries, the identical-geometry inductors on the SiO2/nc-Si/HR-Si substrate probably exhibit superior quality factors.

RF MEMS capacitive switch on semi-suspended CPW using low-loss high-resistivity silicon substrate

SummaryA new analog tunable Ka‐band band‐stop filter is proposed on high resistivity silicon (HRS) substrate using radio frequency microelectromechanical systems (RF MEMS) varactors for satellite and radar applications. The designed filter uses a λ/4 comb‐line resonator on the center line of a coplanar waveguide (CPW) with a new uncommon defected ground structure (DGS). Using the DGS in the proposed structure not only decreases the lateral size of the filter but also increases the quality factor. The tuning range of the proposed filter is 29.8–35.6 GHz with a maximum actuation voltage of 44.5 V and a band rejection between 37 and 45 dB. The quality factors of the designed filter in the up and down states of the MEMS varactors are 162 and 292, respectively. The equivalent circuit model of the filter is exactly extracted, and DGS dimensions are optimized. Based on the frequency simulation using ANSOFT HFSS 14, the insertion loss of the designed filter is about 0.51 dB in the 0–20 GHz range and 2 dB in the 0–15 GHz range in, respectively, up and down state of the bridge. The return losses are 0.51 and 0.33 dB at the center frequency for the up‐ and down‐state configuration of the bridge. A step‐by‐step fabrication process is also proposed for designing the band‐stop filter. High quality factor, small size, and improved structure are the advantages of the proposed design.

This paper explores the usefulness of RF MEMS and reconfigurable antennas in the context of modern communication systems and other applications. Simulation studies on a fractal antennas with switches and phase shifters provide beam steering and beam scanning characteristics. A brief description of the design aspects of two MEMS based coplanar waveguide (CPW) phase shifters for microwave and millimeter wave antenna applications are also provided here. One phase shifter design consists of a unique bilateral interdigital structure on a silicon substrate deposited with a thin film of electrically tunable barium strontium titanate. The dielectric constant of the thin film, and hence the effective permittivity of the transmission line changes by the application of a bias voltage. The second device is a distributed microelectromechanical system (MEMS) phase shifter fabricated on a high resistivity silicon substrate using microstereolithography technique is presented here. The distributed MEMS phase shifter consists of a high-impedance CPW transmission line and several MEMS bridges of uniform height. By applying a voltage the height of these bridges can be adjusted. This results in a variation in the distributed capacitance of the segment of the transmission line, which manifests in the phase shift at the output terminal. The fabrication procedure described here shows significant promise for the use of microstereolithography techniques for RF MEMS devices.

Terahertz wire-grid polarizer with 500 nm grating period on high resistivity silicon (100) is fabricated using nanoimprinting lithography (NIL). Preliminary results show a good polarization characteristic ranging from 0.5 to 5 THz. The method is high throughput and low cost.

Ultra-thick and ultra-thin Silicon PIN detectors are specially applied in high particles detections. The corresponding leakage current is investigated. The ultra-thick and ultra-thin gated diodes structures based on high resistivity silicon substrates are fabricated and tested to analyses the reverse leakage current for silicon PIN detectors application. It is concluded that the contribution of the generation current in the main junction depletion region is more significant in the ultra-thick structures, while the contribution of the generation current generated at the silicon-oxide interface is more evident in the ultra-thin structures.

We investigate the long-term performance of passive millimeterwave microstrip circuits on high resistivity silicon substrates. Three types of test structures were exposed to harsh environmental conditions such as thermal stress, humidity and organic vapors for 400 hours. Measured results at 38 GHz showed excellent long-term stability of the circuits. No significant difference between chips with and without SiO/sub 2/ passivation layer has been found though chips with passivation showed larger photosensitivity and higher loss.

This paper presents design, fabrication, and optimization of high quality factor (Q) inductors on low dielectric polymers implemented on a high resistivity silicon substrate. Both Benzocyclobutene (BCB) and SU-8/spl trade/ dielectrics were used as low dielectric material. A maximum Q > 20 at 8GHz was measured for a 2.5nH inductor on SU8/spl trade/ deposited on high resistivity Si.

High performance undoped AlGaN/AlN/GaN high electron mobility transistors (HEMTs) on high resistivity Si were achieved by implementing an ohmic-recess contact by inductively coupled plasma etching using four layers of Ti/Al/Ni/Au metal. About a 17.5% increase in drain current density and a 12.5% increase in extrinsic transconductance were observed on ohmic-recess-etched HEMTs. This is due to the reduction in the parasitic resistance such as contact resistance, drain resistance, and source resistance of the device. The ohmic-recess-etched AlGaN/AlN/GaN HEMT with a gate–drain spacing exhibited the lowest specific on-resistance of with an off-state breakdown voltage of 194 V. The highest ever reported figure-of-merit has been achieved in the ohmic-recess-etched AlGaN/AlN/GaN HEMTs on silicon. Ohmic recess is useful for undoped AlGaN/AlN/GaN HEMTs to achieve low with high figure-of-merit.

A novel frequency agile floating-patch MEMS (micro-electro-mechanical system) antenna is proposed for 42 GHz applications. The floating-patch MEMS antenna is fabricated on a high resistivity silicon (HRS) substrate using surface micromachining technology. Simulation and experimental results for reflection coefficients and radiation patterns are presented.

In this paper, a wafer-level System-in-Packaging structure using through silicon via (TSV) for integration on both sides of the silicon wafer is presented. It is composed of BCB/ metal multilayers, high-resistivity silicon substrate with TSV. To reduce the transmission loss in microwave frequency, not only the high-resistivity silicon is used, but also a special TSV structure with 6 grounded shielding vias around the core via are adopted. Microstrip line (MSL) is used to transmit high-frequency signal on package plane together with the low permittivity intermediate dielectric polymer, BCB. Descriptions on the interconnection structure and the fabrication process are included. The microwave measurement result of the MSL connected by TSVs is measured up to 35GHz. The results of both the simulation and the measurement are presented.

In this article, design and development of band pass filter at X-band with the FBW of ~ 4% on silicon substrate has been presented using the dual-behavior resonator (DBR) approach. The proposed topology uses single section coupled lines to maintain the compactness and can be easily realizable with the standard microwave integrated circuit fabrication techniques. The designed filter shows high selectivity at upper band and wider stops band characteristics. The insertion loss better than 3.5 dB and return loss better than 12 dB are achieved at center frequency of 8 GHz using high resistivity silicon substrate.