Single-sided Process Research Articles

TCAD Analysis of Leakage Current and Breakdown Voltage in Small Pitch 3D Pixel Sensors.

Small-pitch 3D pixel sensors have been developed to equip the innermost layers of the ATLAS and CMS tracker upgrades at the High Luminosity LHC. They feature 50 × 50 and 25 × 100 μm2 geometries and are fabricated on p-type Si-Si Direct Wafer Bonded substrates of 150 μm active thickness with a single-sided process. Due to the short inter-electrode distance, charge trapping effects are strongly mitigated, making these sensors extremely radiation hard. Results from beam test measurements of 3D pixel modules irradiated at large fluences (1016neq/cm2) indeed demonstrated high efficiency at maximum bias voltages of the order of 150 V. However, the downscaled sensor structure also lends itself to high electric fields as the bias voltage is increased, meaning that premature electrical breakdown due to impact ionization is a concern. In this study, TCAD simulations incorporating advanced surface and bulk damage models are used to investigate the leakage current and breakdown behavior of these sensors. Simulations are compared with measured characteristics of 3D diodes irradiated with neutrons at fluences up to 1.5 × 1016neq/cm2. The dependence of the breakdown voltage on geometrical parameters (e.g., the n+ column radius and the gap between the n+ column tip and the highly doped p++ handle wafer) is also discussed for optimization purposes.

Ultra-Deep Annular Cu Through-Silicon-Vias Fabricated Using Single-Sided Process

Ultra-deep through-silicon-vias (TSVs) are of great demand for 3D heterogeneous integration. However, most reported deep TSVs adopt double-sided silicon etching and electroplating, which dramatically increases the complexity and cost of the fabrication process. This letter focuses on how to achieve a high-quality Cu seed layer in ultra-deep vias, which is the key technology for the fabrication of single-sided ultra-deep TSVs. By proposing a novel pulsed ultrasound-assisted electroless plating method, continuous and dense Cu seed layers are successfully deposited in silicon vias with a depth as large as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$580 ~\mu \text{m}$ </tex-math></inline-formula> . Combined with a conformal Cu electroplating and a vacuum-assisted Benzocyclobutene (BCB) refilling, BCB-core annular Cu TSVs with depths up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$580 ~\mu \text{m}$ </tex-math></inline-formula> and aspect ratios up to 8 are successfully fabricated. Measured results show that the fabricated TSVs exhibit a depletion capacitance of 1.75 pF, a low leakage current of 0.44 pA at 20 V, indicating good electrical properties.

A Front-Side Microfabricated Thermoresistive Gas Flow Sensor for High-Performance, Low-Cost and High-Yield Volume Production.

In this paper, we present a novel thermoresistive gas flow sensor with a high-yield and low-cost volume production by using front-side microfabricated technology. To best improve the thermal resistance, a micro-air-trench between the heater and the thermistors was opened to minimize the heat loss from the heater to the silicon substrate. Two types of gas flow sensors were designed with the optimal thermal-insulation configuration and fabricated by a single-wafer-based single-side process in (111) wafers, where the type A sensor has two thermistors while the type B sensor has four. Chip dimensions of both sensors are as small as 0.7 mm × 0.7 mm and the sensors achieve a short response time of 1.5 ms. Furthermore, without using any amplification, the normalized sensitivity of type A and type B sensors is 1.9 mV/(SLM)/mW and 3.9 mV/(SLM)/mW for nitrogen gas flow and the minimum detectable flow rate is estimated at about 0.53 and 0.26 standard cubic centimeter per minute (sccm), respectively.

Comparison of processing-induced deformations of InP bonded to Si determined by e-beam metrology: Direct vs. adhesive bonding

In this paper, we employ an electron beam writer as metrology tool to investigate distortion of an exposed pattern of alignment marks in heterogeneously bonded InP on silicon. After experimental study of three different bonding and processing configurations which represent typical on-chip photonic device fabrication conditions, the smallest degree of linearly-corrected distortion errors is obtained for the directly bonded wafer, with the alignment marks both formed and measured on the same InP layer side after bonding (equivalent to single-sided processing of the bonded layer). Under these conditions, multilayer exposure alignment accuracy is limited by the InP layer deformation after the initial pattern exposure mainly due to the mechanical wafer clamping in the e-beam cassette. Bonding-induced InP layer deformations dominate in cases of direct and BCB bonding when the alignment marks are formed on one InP wafer side, and measured after bonding and substrate removal from another (equivalent to double-sided processing of the bonded layer). The findings of this paper provide valuable insight into the origin of the multilayer exposure misalignment errors for the bonded III-V on Si wafers, and identify important measures that need to be taken to optimize the fabrication procedures for demonstration of efficient and high-performance on-chip photonic integrated devices.

Design, Fabrication, and Testing of a Monolithically Integrated Tri-Axis High-Shock Accelerometer in Single (111)-Silicon Wafer.

In this paper, a monolithic tri-axis piezoresistive high-shock accelerometer has been proposed that has been single-sided fabricated in a single (111)-silicon wafer. A single-cantilever structure and two dual-cantilever structures are designed and micromachined in one (111)-silicon chip to detect Z-axis and X-/Y-axis high-shock accelerations, respectively. Unlike the previous tri-axis sensors where the X-/Y-axis structure was different from the Z-axis one, the herein used similar cantilever sensing structures for tri-axis sensing facilitates design of uniform performance among the three elements for different sensing axes and simplifies micro-fabrication for the multi-axis sensing structure. Attributed to the tri-axis sensors formed by using the single-wafer single-sided fabrication process, the sensor is mechanically robust enough to endure the harsh high-g shocking environment and can be compatibly batch-fabricated in standard semiconductor foundries. After the single-sided process to form the sensor, the untouched chip backside facilitates simple and reliable die-bond packaging. The high-shock testing results of the fabricated sensor show linear sensing outputs along X-/Y-axis and Z-axis, with the sensitivities (under DC 5 V supply) as about 0.80–0.88 μV/g and 1.36 μV/g, respectively. Being advantageous in single-chip compact integration of the tri-axis accelerometers, the proposed monolithic tri-axis sensors are promising to be embedded into detection micro-systems for high-shock measurement applications.

Non-conventional approach in single-sided lapping process: kinematic analysis and parameters optimization

The lapping process is strongly affected by a number of input parameters. One of the fundamental mechanical processes in lapping is the abrasive effect of particles. In order to examine the influence of the kinematic parameters on the lapping uniformity of unconventional single-sided machine, numerical simulations of particles sliding trajectories are performed. Changing the kinematic parameters, e.g., rotational velocities or position of the conditioning rings together with the workpieces can be used for correcting the profile of an active surface of the lap. This paper provides a basic guide to the kinematic parameter settings of single-sided lapping process. Furthermore, an unconventional single-sided lapping system, where a conditioning ring performs additional motion during the machining, is described. Simulation results show that the rotational speed ratio of the conditioning ring to the lapping plate k1 and the period ratio of the reciprocating motion to the lapping plate rotary motion k2 represent the relationships among the three basic motions of unconventional lapping systems and are major factors affecting trajectory distribution. Material removal rate and trajectory density parameter are proposed to optimize the kinematic parameters for better uniformity of lapping plate wear. The preferred kinematic parameter values for single-sided lapping system with reciprocating motions and the assumed size of the machine are k1 = 0, 78 and k2 = 2. The general results were validated on a specially designed test stand.

Analysis of Abrasive Particle Trajectories during Single-Sided Lapping Process with Driven Conditioning Ring

In this paper new kinematics systems were studied for a single lapping process. In order to ensure the constant profile wear of the tool a standard kinematic system can be changed. Lapping is carried out by applying loose abrasive grains between two surfaces and causes a relative motion between them. The result is a finish of multi-directional lay. During process, the mechanism of surface formation are decisively affected by a pressure force, a process time and a motion type of grains. It has been proven, that providing the additional movements of the conditioning ring affects the profile wear of the lapping plate. First of all, the kinematics equations of the lapping process have been developed and the process has been simulated. A type of the abrasive particle trajectories in standard single-disk lapping depends on rotational speed ratio of a tool and a conditioning ring. In addition, two new kinematic systems were presented: with radial and secant movement of the conditioning ring. Finally, comparison and analysis of abrasive trajectories were carried out.

Laboratory and testbeam results for thin and epitaxial planar sensors for HL-LHC

The High-Luminosity LHC (HL-LHC) upgrade of the CMS pixel detector will require the development of novel pixel sensors which can withstand the increase in instantaneous luminosity to L=5×1034 cm−2s−1 and collect ∼ 3000 fb−1 of data. The innermost layer of the pixel detector will be exposed to doses of about 1016 neq/ cm2. Hence, new pixel sensors with improved radiation hardness need to be investigated. A variety of silicon materials (Float-zone, Magnetic Czochralski and Epitaxially grown silicon), with thicknesses from 50 μm to 320 μm in p-type and n-type substrates have been fabricated using single-sided processing. The effect of reducing the sensor active thickness to improve radiation hardness by using various techniques (deep diffusion, wafer thinning, or growing epitaxial silicon on a handle wafer) has been studied. The results for electrical characterization, charge collection efficiency, and position resolution of various n-on-p pixel sensors with different substrates and different pixel geometries (different bias dot gaps and pixel implant sizes) will be presented.

Effect of Surface Damage on SiC Wafer Shape

The flatness of a silicon carbide wafer in terms of bow and warp is the result of the combination of factors both material and process related. Sub-surface damage (SSD) from the wafering process steps can be considered as a thin film under compressive stress on the wafer surface. SSD is generally decreased with each subsequent processing step after the multiwire saw. Single-sided process steps can produce very different levels of SSD on opposing wafer surfaces, leading to high bow and warp values. The present study investigates the effects of SSD on wafer flatness at various process steps as well as methods to minimize shape effects due to SSD during and after processing.

Single-Side Fabrication of Multilevel 3-D Microstructures for Monolithic Dual Sensors

Monolithic integration of micromechanical composite sensors needs to fabricate multiple levels of three-dimensional (3-D) microstructures to satisfy individual requirements from individual on-chip sensing elements. Meanwhile, volume fabrication of the composite sensors is needed in many applications that prefer single-sided process in low-cost non-silicon-on-insulator single wafer. A novel single-sided micromachining technique is herein proposed and developed to form such multilevel 3-D structures, where only integrated-circuit (IC) foundry available processes are used, i.e., neither double-sided process nor wafer-bonding is used. With the IC-foundry compatible micromachining process, a six-level 3-D microstructure has been successfully formed for tire-pressure monitoring system (TPMS) sensors. Benefited from the single-side process and the namely pressure-sensor in accelerometer (PinG) dual-sensor architecture, the single-wafer-based dual-sensor features a tiny chip size of 1.25 mm $\times $ 1.25 mm $\times$ 0.45 mm. Supplied with 3.3 V, 0.1-mV/kPa sensitivity for the 500-kPa-ranged pressure sensor and 0.05-mV/g sensitivity for the 120-g-ranged accelerometer are measured. By freely suspending the pressure-sensor structure from the stress-free mass end, the influence of acceleration to the pressure sensor is well eliminated, which was the main problem of the previous PinG sensors. Besides the achieved high-performance TPMS dual sensor, the IC-foundry manufacturable technique for multilevel 3-D microelectromechanical systems (MEMS) structures can be widely used in various monolithic MEMS devices. [2015-0034]

Design, fabrication and characterization of the first dual-column 3D stripixel detectors

This paper presents design, fabrication and the first characterization of the new 3D stripixel detector. The device has dual-column electrodes (both n+- and p+-type) arranged in a quincunx configuration and processed in a single-sided process on n-type silicon bulk. Double-metal technology allows to connect the electrodes by two sets of perpendicular strips providing a X-Y projective read-out. The design of the sensor has been optimized with the use of TCAD Sentaurus simulations. TCT measurements were performed to study the active area of the detector. Finally, 2D position sensitivity was investigated reconstructing the position of a laser beam within a 80 × 80 μm2 pixel, defined by two neighbouring p+-type strips crossing two neighbouring n+-type strips.

Quad-cantilever microsensors with a low-cost single-sided micro-machining technique for trace chemical vapor detection

This paper presents a quad-cantilever microsensor for on-the-spot detection of ultra-low concentration chemical vapors. Compared with conventional dual-cantilever sensors, the quad-cantilever configuration can form a fully cantilever-formed Wheatstone-bridge that possesses a higher sensitivity of twofold and more balanced condition to compensate for environmental noise like temperature fluctuation or air flow. Besides, the four integrated micro-cantilevers are made of SiO 2, with each having a single crystal silicon piezoresistor fully encapsulated by SiO 2. Thus, the quad-lever sensor achieves a very low signal noise of 0.2 μV. For specific detection, sensitive molecule layer is self-assembled on two sensing cantilevers, with another two as reference. The sensors are micro-fabricated with a single-side process from the wafer front-side. With wet anisotropic etch used to release the piezoresistive cantilevers, the new fabrication technique features low-cost and high-yield. Self-assembled with a novel dual-branch specific mono-layer, about 100 ppt level trinitrotoluene (TNT) vapor has been rapidly and repeatedly detected.

A single-sided process for differentially cooled electrothermal micro-actuators

A simple method for increasing the thermal efficiency of shape bimorph electrothermal micro-actuators is proposed, based on a reduction of gas conduction cooling beneath the hot arms by a deep, localized undercut. A single-sided, single-mask SCREAM-type process for fabricating differentially cooled actuators in a bonded silicon-on-insulator material is demonstrated. The process uses deep reactive ion etching and undercut to form suspended parts and isotropic reactive ion etching and lift-off of sacrificial shields to form localized mesas. The advantage of the method is confirmed using folded electrothermal actuators, and an approximate halving of the drive power is demonstrated by variations in the substrate profile in the vicinity of a series of actuators with the same mechanical design.

Development, simulation and processing of new 3D Si detectors

New 3D Si detector structures have been proposed by BNL at the end of 2005. Different from the traditional planar Si detector technology, 3D detector technology places p + and n + electrodes vertically through the entire detector thickness, thus involves 3D processing. Our new 3D structures have some new features either in configuration and/or in processing: (1) all electrodes are processed on one side of the wafer to ensure a simple, true one-sided processing; and (2) stripixel electrode configuration can be arranged to get 2D position sensitive strip-like detectors with single-sided processing. The processing of the first prototype detectors batch of the new 3D detectors with single-column (n + column on p-type substrate) has begun. All n + columns have been etched and the remaining planar processing are been finishing up. Dual electrode columns (p + and n +) one-sided 3D Si detectors are planned for future prototype batches. Electric field simulations and estimation of CCE at SLHC fluences have been carried out for the new 3D Si detectors.

Performance of P-type micro-strip detectors after irradiation to 7.5/spl times/10/sup 15/ p cm/sup 2/

Exploiting the advantages of reading out segmented silicon from the n-side, we have produced test detectors with LHC pitch but 1-cm-long strips which even after proton irradiation at the CERN PS (24 GeV protons) to 7.5/spl times/10/sup 15/ cm/sup -2/ show signal to noise greater than 8:1 using LHC speed electronics. This dose exceeds by a factor of 2 that required for a replacement of the ATLAS semiconductor tracker to cope with an upgrade of the LHC to a Super-LHC with ten times greater luminosity. These detectors were processed on p-type starting material of resistivity /spl sim/2 k/spl Omega/cm and, unlike n-in-n designs, only required single-sided processing. Such technology should therefore provide a relatively inexpensive route to replacing the central tracking at both ATLAS and CMS for Super-LHC (SLHC). The shorter strip length is required to limit the noise. Even at these extreme doses 30% of the nonirradiated signal is seen. This 7000 e/sup -/ signal (in 280-/spl mu/m-thick sensors) is very competitive with the postirradiation performance of other, more exotic detector options . The hit density expected at a SLHC would anyway require a scaling down of the sense element length (for a given pitch) to retain an occupancy of less than 1% in the layers of the central tracker. We therefore propose such a short-strip design as a possible low cost and easily implemented route to achieving the requirements for very high luminosity tracking at an upgraded LHC.

Electron beam and laser testing on the novel stripixel detectors

The novel Si stripixel detector, developed at BNL (Brookhaven National Laboratory), has been applied in the development of a prototype Si strip detector system for the PHENIX Upgrade at RHIC. The Si stripixel detector can generate X – Y two-dimensional (2D) position sensitivity with single-sided processing and readout. Test stripixel detectors with pitches of 85 and 560 μm have been subjected to the electron beam test in a SEM set-up, and to the laser beam test in a lab test fixture with an X – Y – Z table for laser scanning. Test results have shown that the X and Y strips are well isolated from each other, and 2D position sensitivity has been well demonstrated in the novel stripixel detectors.

Development of a novel silicon stripixel detector for RHIC-PHENIX detector upgrade

A new single-sided silicon strip detector has been developed for the silicon vertex tracker of the PHENIX detector at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory (BNL). The novel "stripixel" detector concept developed by the BNL Instrumentation Division was applied to the first prototype silicon sensor. The novel feature of the stripixel concept is two-dimensional (2-D) position sensitivity with single-sided processing achieved by charge sharing between two interleaved electrodes in one pixel. Signals are read out projectively by double layers of aluminum strips. The first prototype stripixel sensors with the size of 34.3 /spl times/ 64.6 mm and thickness of 250 and 400 /spl mu/m were fabricated. The detector with readout electronics was constructed to evaluate the prototype sensor performance by using an analog multiplexer chip with charge sensitive preamplifier-shaper circuits (VA2 chip of IDEAS). The sensor performance on charge sharing, detection efficiency, signal-to-noise ratio, and position resolution has been tested with a radioactive source and a GeV particle beam at KEK-PS. The stripixel detector concept, the specification of the first prototype stripixel sensor, and the test results are presented.

Development of 2nd prototype of novel silicon Stripixel detector for PHENIX upgrade

A novel detector type, named “Stripixel” detector developed at BNL, has been applied in the development of a prototype Si strip detector system for the PHENIX Upgrade at RHIC. This novel detector type can generate X- Y two-dimensional position sensitivity with single-sided processing and readout. The prototype Si Stripixel detector has an active area of about 3 cm × 6 cm, which is divided into two identical halves. Both X and Y pitches are 80 μm, with a stereo angle of 4.6°. There are 384 X strips and 384 Y strips on each half of the detector. The first prototype Si Stripixel detectors have been produced at BNL. The initial tests of detector properties have yielded good results on position resolution in both X and Y coordinates. The design and mask set production of the 2nd prototype Si strip detectors were completed in December 2003. In the 2nd prototype design, the maximum distance between the X and Y interleaving pixels is shortened significantly to solve the asymmetry in charge sharing between X and Y pixels and strips observed in the first prototype Stripixel detectors. Also, a novel pixel inter-connection scheme with overlapping pixels is implemented, which can generate the same position resolution with twice of the pitch size as in the tradition scheme. In this novel scheme, for the same position resolution, one may just use half of the number of strips (in a given detector size), thus saving the readout channels by a factor of two. The first batch of the fabrication run of the 2nd prototype Stripixel detectors have been completed in February 2004. Preliminary electrical tests have revealed good results. Laser, source and beam tests are planned.

Novel silicon stripixel detector for PHENIX Upgrade

Novel detector type, named “Stripixel” detector developed at BNL, has been applied in the development of a prototype Si strip detector for the PHENIX Upgrade at RHIC. This novel detector type can generate X– Y two-dimensional position sensitivity with single-sided processing and readout. The prototype Si stripixel detector has an active area of about 3 cm×6 cm, which is divided into two identical halves. Both X and Y pitches are 80 μm, with a stereo angle of 4.6°. There are 384 X strips and 384 Y strips on each half of the detector. The first batch of prototype Si stripixel detectors has been produced at BNL. The initial tests of detector electrical properties have yielded good results. Charge collection tests on test-structure strip detectors have shown both X and Y position sensitivities to laser light. Beam tests on the Si strip detectors have been done at RIKEN in November 2002.