Three-dimensional wafer stacking via Cu-Cu bonding integrated with 65-nm strained-Si/low-k CMOS technology

The authors report the first demonstration of integrating wafer stacking via Cu bonding with strained-Si/low-k 65-nm CMOS technology. Sets of 330 mm wafers with active devices such as 65-nm MOSFETs and 4-MB SRAMs were bonded face-to-face using copper pads with size ranging between 5 /spl mu/m/spl times/5 /spl mu/m and 6 /spl mu/m/spl times/40 /spl mu/m. The top wafers were thinned to different thicknesses in the range 5 to 28 /spl mu/m. Through-silicon-vias (TSVs) and backside metallization were used to enable electrical testing of both wafers in the Cu-stacked configuration. We tested individual transistors in the thinned silicon of bonded wafer pairs where the thinned silicon thickness ranged from 14 to 19 /spl mu/m. All results showed that both n- and p-channel transistors preserved their electrical characteristics after Cu bonding, thinning, and TSV integration. We also demonstrated the functionality of stacked 65-nm 4-MB SRAMs by independently testing the cells in both the thinned wafer and the bottom wafer. For the SRAM, we tested a wider thinned wafer thickness range from 5 to 28 /spl mu/m. On all tested samples, we did not find any impact to the electrical performance of the arrays resulting from the three-dimensional (3-D) integration process. The stacked SRAM is an experimental demonstration of the use of 3-D integration to effectively double transistor packing density for the same planar footprint. The results presented in this letter enable further exploratory work in high-performance 3-D logic, which takes advantage of the improved interconnect delays offered by this Cu-bonding stacking scheme integrated with modern CMOS processes.     

Extrusion spin coating: an efficient and deterministic photoresist coating method in microlithography

Extrusion spin coating was developed to reduce photoresist waste and to improve coating uniformity in microlithography. This new method uses an efficient extrusion coating technique to apply a thin film of resist to a wafer prior to spinning. This initial layer of photoresist eliminates the spreading phase, the most inefficient step in conventional spin coating. The initial layer also provides existing spin coating models with preset initial conditions, allowing the prediction of coating thickness and uniformity a priori. This paper compares the experimental results with Emslie et al.'s predictive models of spin coating. A solvent concentration of 80% or higher in the coating chamber environment was found to be necessary to attain a predictable coating thickness with 5-/spl Aring/ uniformity. With optimized process variables, the mean coating thickness matched theoretical predictions within a variation of 0.01 /spl mu/m. Defect-free coating results were achieved with coating efficiencies as high as 40%.     

Mismatch in diffusion resistors caused by photolithography

During the qualification of a 0.35-/spl mu/m CMOS process, it was observed that diffusion resistors showed a systematic mismatch, depending on the position on the wafer. The mismatch increased from the center of the wafer to the outer regions. Various experiments showed that the mismatch was caused by spinning the wafer during the resist development process. Changing this process eliminated the systematic diffusion resistor mismatch.     

Generation and characterization of polymeric tridimensional microstructures for micromachine application

In this work, we use the thick layer of polymethylmethacrylate polymer, for micromachining development. In the development of the structures, a three layer process is used. In a silicon wafer is deposited the thick layer spin coating. Over this layer is deposited a thin layer of silicon. The third layer is 1.5 μm of e-beam resist deposited by spin coating. After the deposition of the layers, we perform the e-beam lithography in the top layer resist. This pattern is transferred by plasma etching for the silicon layer. The resolution limits of this process is the resolution of the electron resist and is increased to 0.25 μm (nanometric resolution), using an electron beam spot size of 50 nm and dry development.     

Silicon micromachined EBG resonator and two-pole filter with improved performance characteristics

A novel micromachined resonator at 45 GHz based on a defect in a periodic electromagnetic bandgap structure (EBG) and a two-pole Tchebyshev filter with 1.4% 0.15 dB equiripple bandwidth and 2.3 dB loss employing this resonator are presented in this letter. The periodic bandgap structure is realized on a 400 /spl mu/m thick high-resistivity silicon wafer using deep reactive ion etching techniques. The resonator and filter can be accessed via coplanar waveguide feeds.     

Pattern transfer of microstructures between deeply etched surface for MEMS applications

Photolithography plays a vital role in micromachining process however; coating a thin and uniform resist layer on a non-planar surface is a challenging task for micro-electro-mechanical system (MEMS). Conventional spin coating of photoresist (PR) over an un-even surface would deliver streaks all over the wafer surface. Spray coating of PR is a promising technique when compared to other candidates. This paper presents an efficient pattern transfer of microstructures between the bulk micromachined cavities over silicon and glass wafers using an uncommon photoresist mixture being spray coated. The method is simple and highly cost effective. Finally we implemented this technique for a MEMS application to prove the feasibility of spray coating for microstructure fabrication.     

Low-noise metamorphic HEMTs with reflowed 0.1-/spl mu/m T-gate

A 0.1-/spl mu/m T-gate fabricated using e-beam lithography and thermally reflow process was developed and applied to the manufacture of the low-noise metamorphic high electron-mobility transistors (MHEMTs). The T-gate developed using the thermally reflowed e-beam resist technique had a gate length of 0.1 /spl mu/m and compatible with the MHEMT fabrication process. The MHEMT manufactured demonstrates a cutoff frequency f/sub T/ of 154 GHz and a maximum frequency f/sub max/ of 300 GHz. The noise figure for the 160 /spl mu/m gate-width device is less than 1 dB and the associated gain is up to 14 dB at 18 GHz. This is the first report of a 0.1 /spl mu/m MHEMT device manufactured using the reflowed e-beam resist process for T-gate formation.     

Full-field wafer level thin film stress measurement by phase-stepping shadow Moire/spl acute/

A wafer topography measurement system has been designed and demonstrated based on shadow Moire/spl acute/. Three-step phase-stepping and phase unwarping techniques are also incorporated to enhance the system resolution. Wafer curvatures or bows can be achieved by analyzing the Moire/spl acute/ fringe patterns and film stress can be obtained subsequently by transforming this wafer curvature using a conversion equation such as Stoney's formula. Wafer bow of plasma enhanced chemical vapor deposition nitride and oxide coated wafers are measured by this shadow Moire/spl acute/ system and are subsequently verified by the KLA-Tencor FLX 2320 system. The discrepancy between both bow measurements is within 2/spl mu/m, regardless of the magnitude of the measurement. Therefore, this system is especially suitable for stress characterization of thicker, stiffer, or highly stressed films. In comparison with the traditional laser scanning method, wafer curvature obtained by shadow Moire/spl acute/ is based on full-field information and it would have a better accuracy. By integrating this system with a more accurate wafer curvature to film stress conversion formula, this system should also provide a better film stress characterization.     

New micromachined membrane switches in silicon technology

Presents a new generation of micromachined membrane switches containing a glass substrate and a silicon membrane. Upon external actuation, two planar substrate contacts are closed by a metal layer deposited on the switching membrane. The switching position is constant to better than 1 /spl mu/m and has been observed to move about 0.7 /spl mu/m over the achieved lifetime of 20 million electromechanical cycles under electrical loads of 12 V and 10 mA.     

Room-temperature CW operation of InGaAsP lasers on Si fabricated by wafer bonding

1.3-/spl mu/m InGaAsP-InP lasers have been successfully fabricated on Si substrates by wafer bonding with heat treatment at 400/spl deg/C. A pressure of 4 kg/cm/sup 2/ has been applied on the wafers before the heat treatment and this pressure application has enabled us to achieve bonding strength required for the device fabrication even when the bonding temperature is as low as 400/spl deg/C. Room-temperature continuous-wave operation with threshold current of 49 mA has been achieved for 7-/spl mu/m-wide mesa lasers.     

Effect of crystallinity and thickness on the diffusion barrier behavior of electroless nickel deposit between Cu and solder

The crystallinity of electroless nickel deposit was manipulated by applying bath stabilizer including lead acetate and thiourea. A crystalline deposit and a higher deposition rate of electroless nickel were achieved with thiourea than with lead acetate. The effect of crystallinity on the diffusion barrier performance of the electroless nickel deposit was studied between solder and Cu deposit. The thickness of the electroless nickel deposit investigated includes 1, 3, and 5 /spl mu/m. It was found that both crystalline and amorphous deposits perform similarly in barrier performance except when the thickness of the deposit is as thin as 1 /spl mu/m. Cross sectional elemental analysis results indicate that a 3 /spl mu/m thickness deposit can withstand ten times of reflow without counter diffusion between Sn and Cu, although Ni-Sn intermetallic compounds were formed. The 3 /spl mu/m thickness is also adequate for barrier function after 1000 hours of aging at 150/spl deg/C.     

A surface micromachined optical scanner array using photoresist lenses fabricated by a thermal reflow process

This paper presents the design, fabrication, and operation of a newly developed micromechanical optical scanner array using a translating microlens. We have used photoresist reflow technique to form a microlens on a surface micromachined XY-stage of the scratch-drive actuation mechanism. The lens scanner is placed at the focal length from an incident optical fiber to collimate the transmitting light. The collimated beam is steered two-dimensionally by the XY-motion of the microlens with respect to the incident fiber. We also have developed a theoretical model to predict appropriate initial resist thickness and diameter for the scanning lens. An optical scanning angle of /spl plusmn/7/spl deg/ has been demonstrated by sliding a microlens of 670-/spl mu/m focal length at a physical stroke of /spl plusmn/67 /spl mu/m. Typical angular positioning resolution has been estimated to be 0.018/spl deg/.     

Spray coating of PMMA for pattern transfer via electron beam lithography on surfaces with high topography

Spray coating of polymethylmethacrylate (PMMA) as electron beam resist on non planar surfaces is presented as a reliable technique for deposition of uniform resist layers with adjustable thickness at wafer scale. In the experiments a commercial spray coating system with an ultrasonic spray nozzle was used. Parameters which influence the quality of the resist layer with respect to uniformity across a 4 in Si wafer, such as ultrasonic power and dispensed volume, were evaluated. The suitability of spray coated PMMA for the pattern transfer on surfaces with high topography was proven by PMMA spray coating of 8 μm deep trenches etched into Si wafers. The PMMA was then electron beam exposed and chromium line patterns were transferred on the Si surface via a lift-off process.     

A millimeter-wave bandpass waveguide filter using a width-stacked silicon bulk micromachining approach

A 30-GHz bandpass filter is realized in a novel waveguide topology, through the use of bulk micromachining of standard (low-resistivity) silicon wafers. In this new design, the width of the rectangular waveguide structure is created through the stacking of etched silicon wafer pieces. This width-stacking approach eliminates the presence of convex corners in the design, resulting in more controllable etching. Also, this design enables the simple implementation of the split-block technique, which alleviates Ohmic contact resistance issues. This latter aspect, combined with a double-sided etching strategy that enables deep cavities to be formed, leads to very high-Q silicon micromachined resonators (Q/sub 0//spl ap/4500). A three-cavity bandpass filter was fabricated and tested leading to a deembedded insertion loss of 1dB at a center frequency of 29.7GHz, with a 3-dB bandwidth of 0.654GHz (2.2%). These results validate this new micromachined waveguide approach, and demonstrates a significant improvement over other millimeter-wave micromachined waveguide filters.     

Single-chip multiple-frequency VHF low-impedance micro piezoelectric resonators

This letter reports on the design, fabrication, and testing a new class of low-impedance multiple-frequency micromechanical resonator using piezoelectric transduction mechanism. The resonator is fabricated out of a 3-/spl mu/m-thick silicon layer on a silicon-on-insulator wafer with a low-temperature post-CMOS compatible process. Two types of low-impedance resonators using the same design principle are presented here. The center frequency of the novel resonator is set by the lithographically defined in-plane dimension of the device (width of bar or radius of disk). The 200-/spl mu/m-diameter disk-shaped resonator is measured to have an impedance of 1.6 k/spl Omega/ at the resonant frequency of 45.4 MHz, with a quality factor of 1100. The bar-type resonator is measured to have an impedance of 920 /spl Omega/ at 60 MHz, exhibiting a Q factor of 1400.     

Chalcogenide glass thin films through inverted deposition and high velocity spinning

The fabrication of optical chalcogenide glass thin films through inverted deposition and high velocity spinning is presented. Good quality films up to 275 /spl mu/m thick of this infrared transmitting material, with propagation loss of 0.3 dB cm/sup -1/ at 1.064 /spl mu/m, were engineered using this technique.     

High transmission gain inverted-F antenna on low-resistivity Si for wireless interconnect

Inverted-F antennas of 2-mm axial length are designed and fabricated on a low-resistivity silicon substrate (10 /spl Omega//spl middot/cm) using a post back-end-of-line process. For the first time, their performances are measured up to 110 GHz for wireless interconnects. Results show that a sharp resonance can be seen at 61 GHz for the antenna, and a high transmission gain of -46.3 dB at 61 GHz is achieved from the pair of inverted-F antennas at a separation of 10 mm on a standard 10 /spl Omega//spl middot/cm silicon wafer of 750-/spl mu/m thickness.     

A novel trench capacitor enhancement approach by selective liquid-phase deposition

For the first time, a novel and simple trench bottle integrated process is demonstrated on dynamic random access memory (DRAM) manufacturing by selective liquid phase deposition (S-LPD) oxide. After photoresist (PR) filled into a deep trench (DT) and was recess etched at around 1.3 /spl mu/m depth, LPD oxide can be selected as a deposit onto the DT sidewall but not as a deposit on the PR surface. This S-LPD oxide is formed by using hexa-fluosilic acid (H/sub 2/SiF/sub 6/) and water without H/sub 3/BO/sub 3/. After the PR is removed, the LPD oxide becomes a protective layer on DT upper portion. Thus, the DT bottom area can be enlarged to form a trench bottle by NH/sub 4/OH wet etching. Compared to conventional DT trench, 20% of capacitance was enhanced by this S-LPD process. This novel and low-cost method is for the first time demonstrated on 200-mm wafer 110-nm trench DRAM technology.     

Development of low loss organic-micromachined interconnects on silicon at microwave frequencies

We present the design and development of an ultra-violet (UV) LIGA (a German acronym for electroplating, lithography and molding) micromachining process on silicon substrates at microwave/millimeter wave frequencies. The process employs an ultra-thick negative photoresist SU-8 that can be spin-coated and processed using conventional lithography techniques. Using this process, we have developed micromachined coplanar waveguide (CPW) interconnects on Si substrates. The conductor-backed micromachined CPW on Si (7.2 /spl Omega/-cm) achieves a measured attenuation of 0.18 dB/cm at 20 GHz.     

Large-signal performance of deep submicrometer AlGaN/AlN/GaNHEMTs with a field-modulating plate

This is the first time that the microwave performance of a 0.1-/spl mu/m gate in a silicon nitride window opening, with a field-modulating plate on an AlGaN/AlN/GaN heterojunction structure, is reported. The material structure was grown by organometallic vapor phase epitaxy on SiC substrates with an averaged channel sheet resistance of 313.5 ohms/square. Approximately 80-nm-thick plasma-enhanced chemical vapor deposition silicon nitride is used as the dielectric between gate metal extension and semiconductor surface. Transistors of a total gate width of 250 /spl mu/m and a 0.1 /spl mu/m gate footprint, with a 0.36 /spl mu/m long overhang on top of the silicon nitride, can be operated at a drain bias of 40-V high. Output power density of 9.5 W/mm, with 36% power-added efficiency in class AB regime, was demonstrated at 10 GHz in a continuous wave power measurement.