Average Sheet Resistance Research Articles

Growth and characterization of plasma-assisted molecular beam epitaxial-grown AlGaN/GaN heterostructures on free-standing hydride vapor phase epitaxy GaN substrates

We have grown AlGaN/GaN high electron mobility transistor (HEMT) structures by plasma-assisted molecular beam epitaxy on free-standing n-GaN substrates grown by hydride vapor phase epitaxy. Reflection high energy electron diffraction patterns of the as-loaded wafers exhibit narrow streaks which persist throughout the growth. Atomic force microscopy shows smooth AlGaN surfaces with root-mean-square roughness of 10 Å over a 20×20μm2 area. High resolution x-ray diffractometry indicates that the AlGaN peak is ∼20% narrower than for similar structures grown on SiC. Hall mobilities, electron sheet densities, and sheet resistances were measured on ten 60×60μm2 Hall test patterns defined photolithographically across the surface of the 10×10mm2 sample. Buffer leakage measurements demonstrate that a Be:GaN layer effectively isolates the channel from the conductive substrate. Average sheet resistances and sheet densities were 380Ω∕◻ and 0.94×1013cm−2, respectively. These HEMT structures exhibit room-temperature Hall mobilities in excess of 1900cm2∕Vs. In addition, devices on these structures exhibit excellent pinch-off, low gate leakage, and saturated drain current densities of almost 700 mA/mm. Further details regarding the structural and electrical properties will be described along with device testing.

Growth and characterization of 0.8-μm gate length AlGaN/GaN HEMTs on sapphire sub-strates

AlGaN/GaN high electron mobility transistor (HEMT) structures were grown on 2 inch sapphire substrates by MOCVD, and 0.8-μm gate length devices were fabricated and measured. It is shown by resistance mapping that the HEMT structures have an average sheet resistance of approximately 380 Ω/sq with a uniformity of more than 96%. The 1-mm gate width devices using the materials yielded a pulsed drain current of 784 mA/mm at Vgs=0.5 V and Vds=7 V with an extrinsic transconductance of 200 mS/mm. A 20-GHz unity current gain cutoff frequency (ƒT ) and a 28-GHz maximum oscillation frequency (ƒmax) were obtained. The device with a 0.6-mm gate width yielded a total output power of 2.0 W/mm (power density of 3.33 W/mm) with 41% power added efficiency (PAE) at 4 GHz.

Ohmic contact characterization of AlGaN/GaN device layers with spatially localized LEEN spectroscopy

Low energy electron-excited nano-luminescence (LEEN) spectroscopy has been used to correlate higher intensities of deep level emissions with higher ohmic contact values on AlGaN/GaN device layers. Deep level defect emissions in the AlGaN layers have also been identified by LEEN as a signature of uncharacteristically high sheet resistances for a given wafer. The spectral features observed locally at the ohmic contact region are compared with direct electrical measurements of the same device structures. Ohmic contacts formed by annealing Ti/Al/Ni/Au at 800 °C for 30 s in nitrogen range from 1.0×10 −6 to 1.0×10 −4 Ω cm 2 (0.2–3.9 Ω mm), with average sheet resistance values ranging from 650 to 2275 Ω/□ as Al mole fraction decreases. The depth dependent characterization capacity of LEEN allows the density of defect emissions to be assigned to particular interfaces. For these samples, the deep level emissions found at 2.20 and 2.60–3.15 eV found near the surface of the sample while the defect emissions associated with high sheet resistance are found to be in the AlGaN layer itself. Unprocessed material shows that samples with the lowest amounts of midgap luminescence produce the lowest contact resistance. This information can be used to determine the outcome of device fabrication prior to contact formation.

Electrical characterisation of AlGaN/GaN heterostructure wafers for high-power HFETs

Undoped 28 nm AlGaN on 1 μm GaN was grown by MOVPE for high power microwave HFETs. Non-destructive methods for rapid characterisation of the layers are described. Double contact mercury probe capacitance voltage measurements gave valuable information on AlGaN layer leakage, thickness and quality, but only gave an estimate of the carrier concentration and pinch-off voltage due to the absence of an ohmic contact to the 2DEG layer. A non-contact resistivity mapping system gave accurate average sheet resistance, but underestimated the cross-wafer variation.

Sheet resistance nonuniformity for ionized titanium deposition

A combination of recipe power and pressure in an ion metal plasma sputter deposition chamber has reduced average within-wafer sheet resistance nonuniformity approximately 40% to a value of 3.1%±0.2%. This result was obtained for a new polished target and nonknurled coil assembly operated over one coil life.

P -type polycrystalline diamond layers by rapid thermal diffusion of boron

P -type diamond layers have been produced in polycrystalline diamond by means of rapid thermal diffusion (RTD) of boron. Using thin film of deposited boron as the source and temperatures near 1600 °C, solid-state diffusion of boron was achieved, forming conducting layers with average sheet resistances of about 356 Ω/square and diffused-layer thicknesses of about 0.6 μm. Ohmic contacts to the diffused layers were formed by depositing molybdenum films before the RTD step so that sintering of contacts occurred simultaneously with the diffusion. The advantages of this approach are that (a) low contact resistances are obtained and (b) only one high-temperature step is involved. Using this technique, molybdenum-diamond Schottky diodes were fabricated and found to have barrier potentials of 2.6 V and ideality factors of 1.6.

Large area deposition of ITO films by cluster type sputtering system

The preparation of large area ITO films for transparent electrodes of liquid crystal display (LCD) devices by a cluster type sputtering system (Anelva C-3500) was carried out. Effects of sputtering pressure and introduced oxygen gas flow rate at a low substrate temperature of 10°C on crystal structure, electrical property and etching properties were measured. Amorphous films could be obtained at a sputtering pressure higher than 2.6 Pa and an introduced oxygen gas flow rate lower than 0.8 sccm. During the measurement of etching properties of these films, an etching rate > 80 nm/s and a uniformity of about ±20% were obtained. The sheet resistance could not be reduced to a practical level for LCD devices even after annealing. To obtain a uniform and high etching rate and a low sheet resistance at a practical level, the parameters should be optimized. Although the film was not completely amorphous, an average value of 27 nm/s and a uniformity of ±23.5% for the etching rate was obtained within an area of 370 × 470 mm on a 400 × 500 mm substrate and an average sheet resistance of about 30 ω/□ was also obtained after annealing.

High current gain InGaP/GaAs heterojunction bipolar transistors grown by multi-wafer gas-source molecular beam epitaxy system

We report for the first time on the high quality and highly uniform InGaP/GaAs heterojunction bipolar transistors with a carbon-doped base grown by a multi-wafer gas-source molecular beam epitaxy (GSMBE) system, which was developed by ourselves. A large-area uniform growth of p-type GaAs and InGaP has been realized. The variation in layer thickness of these layers was less than ± 1.8% across three 3-inch wafers. A large-area (200 × 200 μm 2) heterojunction bipolar transistor (HBT) with a base-sheet resistance of 350 Ω showed a high current gain of 190. This is effectively the highest value considering the base sheet resistance, and is comparable to the best data of AlGaAs/GaAs HBTs reported so far. The small signal current gains showed an excellent uniformity of 3% across one of five 2-inch wafers. The wafer-to-wafer variation of current gains for the two 2-inch wafers, in the same growth run, was less than 3%. The base sheet resistances also showed an excellent uniformity of 0.86%. In addition the average base sheet resistances of the two 2-inch wafers was the same. The device also showed an excellent high-frequency characteristics. The cut-off frequency ( f t ) of 61 GHz, and the maximum-oscillation frequency ( f max ) of 94 GHz were obtained for the transistor having a base dopant concentration of 4 × 10 19 cm −3. These promising results demonstrate the high potential capability of multi-wafer GSMBE for the production of InGaP/GaAs HBTs.

Ion implant standard

The Greater Silicon Valley Implant Users Group (GSVIUG) has been working with the American Society for Testing Materials (ASTM) and the National Institute of Standards and Technology (NIST, previously known as NBS) to develop a standard for ion implantation. This standard would address two critical needs of the industry: (1) standard reference material (SRM) for certifying and calibrating equipment, and (2) a recommended standard procedure for fabricating such a standard. The SRM would be a wafer that had been implanted with a specified species, energy and dose. Its average sheet resistance and uniformity would be certified to be within certain tolerances. It would be used to verify the performance of the four-point probe and sheet resistance mapping equipment and to calibrate analytical characterization techniques such as secondary ion mass spectroscopy (SIMS), Rutherford backscattering spectroscopy (RBS) and spreading resistance profiling (SRP). An unannealed sample could be used to calibrate Thermawave, ellipsometer and other optical tools and to verify annealing systems. The standard procedure would prescribe the recommended steps to produce the same wafer in the fab. This would help determine if an implanter is operating correctly at the specified conditions. We have reviewed all the previous implant round robins and studies and solicited input from implant vendors and implant service organizations in recommending a set of implant conditions for a proposed standard. Particular attention was paid to the requirement of stability over time and minimum variation with ambient temperature. Fabrication specifications for species, energy and dose are detailed. Plots are also presented for the sensitivity to various parameters that could influence the results, such as substrate resistivity, screen oxide thickness, anneal time and temperature and measurement conditions.

The sheet resistance of 3 μm thick Al-1%Si films deposited by batch mode d.c. magnetron sputtering

The room temperature sheet resistance of 3 μm thick d.c. magnetron sputtered Al-1%Si films has been shown to vary with the type of substrate used and the substrate position in a batch deposition system. The presence of a 1 μm thick low pressure chemically vapour deposited oxide on an Si(100) substrate has been shown to cause the average sheet resistance of the deposited film to decrease. It is proposed that the variation in sheet resistance is linked to the impurity distribution in the deposited film.

Conformal implantation for trench doping with plasma immersion ion implantation

Taking advantage of the moderate beam divergence angle of plasma immersion ion implantation (PIII), we demonstrated conformal doping of boron in high aspect ratio Si trenches. A uniformly doped p + layer all around the side walls and bottom of trenches with an aspect ratio of 6:1 was observed with scanning electron microscopy (SEM) after junction staining. To test the limit of this conformal doping technique, an extremely high aspect ratio macroscopic trench model (0.5 mm wide and 12.5 mm deep) made with Si wafer slices was implanted using similar PIII conditions. After rapid thermal annealing, sheet resistance along trench top and sidewalls was evaluated with the four-point probe method. The average sheet resistance of trench sidewalls was about twice of that of the trench top. The deviation of the sidewall sheet resistance was ±60% with respect to the mean value.

High-density high-reliability tungsten interconnection by filled interconnect groove metallization

A novel multilevel metallization scheme has been developed that uses tungsten for the primary level of interconnection. Grooves that are 1- mu m wide and 2.0- mu m deep, corresponding to a level of line conductor interconnection, are pattern-etched into a planar layer of oxide dielectric. They are filled by isotropically depositing a blanket layer of pure tungsten using low-pressure chemical vapor deposition (CVD), followed by etch-back of the tungsten to produce a filled-interconnection-groove (FIG) conductor structure. Fabricated FIG conductors display an average sheet resistivity of 48 m Omega /square, comparable to conventional aluminum conductors. In addition, FIG metallization provides excellent planarity and greatly improved electromigration strength, and it facilitates the use of stacked vias.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A New Paint‐On Diffusion Source

A new paint‐on diffusion source has been developed utilizing chemically stable polymethylsiloxane resins, n‐butylacetate as a solvent, and dopant compounds such as carborane polymers for boron, triphenylphosphate for phosphorus, and for arsenic. These paint‐on diffusion sources were spun on silicon wafers by conventional photoresist spinners. Ozone was used at 200°C for the decomposition of paint‐on diffusion sources into doped oxide films. Doped oxide densifications or diffusions were performed in the same furnace tube as the decomposition in order to prevent the moisture absorption of the undensified doped oxide. Doped oxide diffusions yielded average sheet resistances with standard deviations of 1.71% for boron and 1.65% for arsenic.

An investigation of the surface self-diffusion coefficients of pure copper and silver by the grain boundary grooving technique

Transparent conducting electrodes (TCE) with increased robustness and stability were produced from a composite of silver nanowires (AgNW) and reduced graphene oxide (GO) using ultrasonic spray coating. Ultrasonic spray coating is suited for large area depositions and thin films of AgNW and reduced GO TCEs with consistent optoelectronic properties were produced using this method and the reduction of GO was kept environmentally friendly by using ascorbic acid (AA) as the reducing agent. TCEs produced in this manner were highly conductive and had an average sheet resistance of 5.3 Ω/□ and an average transmittance of 64.9% ― the best TCE produced in this manner had a figure of merit of 190. Here, AgNWs were protected from humidity induced degradation and the effect of capillary instability at elevated temperatures on the nanowires was retarded by the introduction of a GO–AA over layer. A τ value of 1.5×10−9 was found to signify the onset of capillary instability induced failure in the AgNW TCE and from experimentally supported calculations, it was predicted that the lifetimes of nanowire TCEs under thermal degradation could be extended significantly by keeping the gaseous environment oxygen free.