SiGe Base Research Articles

Growth of High Frequency SiGe Heterojunction Bipolar Transistors Structures

The growth of heterojuntion bipolar transistor structures using chemical vapor deposition has been investigated. Generation of defects in selectively or nonselectively grown collector layers using arsenic as the dopant has been studied. Minimizing the defect density in SiGe base layers by optimizing the growth rate has also been investigated in detail. High resolution reciprocal lattice mapping, atomic force microscopy and secondary ion mass spectrometry have been used as the main characterization tools.

Process Control of Epi-Layers for SiGe:C Hetero-Structure Bipolar Transistors

ABSTRACTThe SiGe:C hetero-structure bipolar transistor (HBT) has turned into a key technology for wireless communication. This paper describes the metrology tools for SiGe epitaxy process control. Two types of analysis are critical, (1) routine control of SiGe base and Si cap thicknesses, location and thickness of the doping layer, doping dose, Ge composition profile, and their uniformity across the wafer; and (2) root-cause analysis on non-routine problems. This is achieved by developing a transmission electron microscopy (TEM) technique allowing a thickness measurement with a reproducibility better than 3 Å. Charge-compensated low-energy secondary ion mass spectrometry (SIMS) using an optical conductivity enhancement (OCE) allows a Ge composition measurement to a required precision of 0.5 at. %.

A Base-Collector Architecture for SiGe HBTs using Low-Temperature CVD Epitaxy Combined with Chemical-Mechanical Polishing

A new collector technology intended for an integrated high-speed SiGe heterojunction bipolar transistor (HBT) is reported. The collector was fabricated by selective epitaxial growth (SEG) using chemical vapor deposition at 770°C under reduced pressure (20 torr) using SiH2Cl2 as silicon precursor and PH3 as n-type dopant source. Chemical-mechanical polishing (CMP) was applied to the overgrown SEG collector in order to achieve a smooth surface in level with the surrounding oxide. Finally, a SiGe base doped with boron was deposited using non-selective epitaxial growth (NSEG) at 650°C. The topography of the collector is inspected after each process step by atomic force microscopy and the topography of the completed collector/base stack indicates that this structure is promising for fabrication of the emitter window. A further advantage with the CMP procedure is the elimination of phosphorous segregation as evidenced by secondary ion mass spectroscopy of the base-collector stack.

Materials and technology issues for SiGe heterojunction bipolar transistors

This paper reviews progress in SiGe heterojunction bipolar technology, with particular emphasis on the influence of materials issues on the technology performance. Low-temperature epitaxy and transient enhanced diffusion are identified as the main materials issues. Three epitaxy approaches are described, namely differential epitaxy, selective epitaxy, and combined selective and non-selective epitaxy. Differential epitaxy has the advantage of a simple growth process, selective epitaxy the advantage that an extrinsic base implant is not needed, and combined expitaxy the advantage that the Si collector and SiGe base can be grown in a single step. Transient enhanced diffusion of the boron in the base is shown to be a constraint on transistor performance, and the incorporation of a carbon concentration of ≈1×10 20 cm −3 in the base is shown to ease this constraint. Preliminary results on SiGe HBTs on wafer bonded SOI substrates are reported. The SOI transistors have characteristics comparable to those of bulk transistors, but the advantage of a gain that is a factor of approximately two higher.

Dopant diffusion control by adding carbon into Si and SiGe: principles and device application

The incorporation of low concentrations of carbon (<10 20 cm −3) into the SiGe region of a heterojunction bipolar transistor (HBT) can significantly suppress boron outdiffusion caused by subsequent processing steps. This effect can be described by coupled diffusion of carbon atoms and Si point defects. We discuss the increase in performance and process margins in SiGe heterojunction bipolar technology by adding carbon. SiGe:C HBTs demonstrate excellent static parameters, exceeding the performance of state-of-the-art SiGe HBTs. Carbon also enhances the high frequency performance, because it allows one to use a high B doping level in a very thin SiGe base layer without outdiffusion from SiGe, even if applying post-epitaxial implants and anneals. Finally, we demonstrate the first modular integration of SiGe:C HBTs into a 0.25 μm, epi-free, dual-gate CMOS platform.

Estimated effect of germanium and carbon on theEarly voltage of a Si1-x-yGexCy heterojunctionbipolar transistor

We have presented a generalized method of calculatingthe Early voltage VA of aSi/Si1-x-yGexCy/Si heterojunctionbipolar transistor (HBT) and successfullyexplained the published measured values of VA with and withoutincorporation of C. The investigation was carried out on thedependence of VA of an N-p-N HBT on different compositions, namely Ge and C, incorporated into the base and on their variousdistribution profiles such as box, trapezoidal and triangular, foran exponential base doping. It is found that the triangularcompositional profile yields the largest value of VA ascompared to box-type and trapezoidal profiles for fixed amounts of individual composition in the base. The highest grading,which corresponds to a triangular profile, generates the highestvalue of effective intrinsic-carrier concentration local to theCB junction causing very little change in minority carrierconcentration gradient in the base with VBC. The smallchange in minority carrier concentration is responsible for theincreased value of VA. Furthermore, a large value ofVA is obtained for rising and falling exponential basedoping profiles relative to the box doping profile with the sameamount of base doping. The value of VA is found to increasewith the incorporation of small amounts of C in the SiGe base dueto its ability to prevent boron out-diffusion as reported for a fabricated device. Further improvement in the value of VAis accounted for due to the sharp fall-off of the C profile at thecollector-base junction as observed from the SIMS profile of afabricated device. For the same amount of strain between Si andSiGe, and between Si and SiGeC, the latter yields a larger valueof VA.

Calculation of figures of merit of Si/Si 1− x− yGe xC y/Si HBTs and their optimization

Incorporation of C in the SiGe base of a heterojunction bipolar transistor (HBT) prevents outdiffusion of B across the junctions, improves film stability by reducing strain, and leads to better process integration. This paper investigates theoretically how the values of figure of merit such as current gain ( β), cut-off frequency ( f T) related inversely to the base transit time ( τ b), and the Early voltage ( V A) change with a small amount (<1%) of C incorporation in the base. For this purpose, we have derived generalized expressions for these quantities using a box-type, trapezoidal and triangular material (Ge and C) profile and an exponential doping profile. The variation of saturation-drift velocity with Ge mole fraction has been used to calculate τ b. A comparison between the calculated values, keeping the strain the same in both SiGe and SiGeC bases, indicates that all values of figure of merit are enhanced with C incorporation. The value of V A increases in SiGeC HBTs with small amounts of C because of prevention of B outdiffusion across the collector–base junction and sharp fall off of the C profile at the same junction.

An analytical model of the inverse base width modulation effect in SiGe graded heterojunction bipolar transistors

The analytical model of the collector current ideality factor degradation at high Vbe in modern SiGe graded base heterojunction bipolar transistors (HBTs) has been developed for the first time. It is subsequently used for the analysis of the inverse base width modulation (IBWM) effects in SiGe HBTs with respect to collector current degradation, current gain premature roll-off and SiGe base transit time. Comparing the IBWM effects calculated for HBTs with various base impurity concentration doping profiles and the Ge grading, we have found that the careful optimization of SiGe base parameters may substantially minimize negative influence of the IBMW effects on the HBT electrical parameters.

High injection effects on noise characteristics of Si BJTs and SiGe HBTs

We present a physically based comparison of the current spectral densities in a SiGe heterojunction bipolar transistors (HBT) and a Si bipolar junction transistor (BJT) of identical geometry and doping levels, based on the direct evaluation of emitter, base and collector current fluctuations. An ensemble Monte Carlo (EMC) simulator self-consistently coupled with a 2D Poisson solver has been employed for the calculations. In the studied bias range, the largest reduction of the RF current noise values in the HBT as compared with the BJT derives from the spectral density of base current fluctuations, S J B , and from the spectra of the cross-correlation between emitter and base current fluctuations, S J B J E . This is due to the fact that the base current in the HBT is strongly reduced as a consequence of the lower gap of the SiGe base. At low injection, the collector spectral density S J C exhibits a typical shot noise response while S J B is governed by thermal noise. At high injection, the presence of hot carriers in the base–collector junction (which are less important in the HBT than in the BJT due to the SiGe/Si hetero-interface), the high carrier concentration in the base and the base push-out provokes the deviation of S J C from the pure shot behavior. Under these conditions, the S J B term can be neglected in the total noise analysis of the HBT for lower values of J C, than in the BJT due to the Ge content benefits.

Growth of SiGe heterojunction bipolar transistor using Si 2H 6 gas and Ge solid sources molecular beam epitaxy

N–p–n Si/SiGe/Si heterostructure has been grown by a disilane (Si 2H 6) gas and Ge solid sources molecular beam epitaxy system using phosphine (PH 3) and diborane (B 2H 6) as n- and p-type in situ doping sources, respectively. X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) measurements show that the grown heterostructure has a good quality, the boron doping is confined to the SiGe base layer, and the Ge has a trapezoidal profile. Post-growth P implantation was performed to prepare a good ohmic contact to the emitter. Heterojunction bipolar transistor (HBT) has been fabricated using the grown heterostructure and a common-emitter current gain of 75 and a cut-off frequency of 20 GHz at 300 K have been obtained.

Characterisation of emitter/base leakage currents in SiGe HBTs produced using selective epitaxy

SiGe heterojunction bipolar transistors have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by non-selective epitaxy for the SiGe base and Si emitter cap. E/B leakage currents are compared with cross-section TEM images to identify sources of leakage currents associated with the epitaxy. In addition, the influence of the position of the extrinsic base implant with respect to the polysilicon emitter on the leakage currents is studied. The emitter/base leakage currents are modelled using Shockley–Read–Hall recombination, trap-assisted tunnelling and Poole–Frenkel (PF) generation. The position of the extrinsic base implant is shown to have a strong influence on the leakage currents. The PF effect dominates the emitter/base leakage current in transistors in which the collector area is smaller than the polysilicon emitter. This result is explained by penetration of the emitter/base depletion region into the p + polysilicon extrinsic base at the perimeter of the emitter. These leakage currents are eliminated when the collector area is increased so that the extrinsic base implant penetrates into the single-crystal silicon at the perimeter of the emitter.

Investigation of deep traps in silicon–germanium epitaxial base bipolar transistors with a single polysilicon quasi self-aligned architecture

The electrical properties of Si/Si 1− x Ge x bipolar transistors have been analysed at temperatures ranging from 77 to 500 K. The investigated SiGe base transistors were fabricated using a BiCMOS single-polysilicon quasi self-aligned process, where base implant had been replaced by selective epitaxy on the base active area. At low temperature, static current–voltage measurements show a degradation of base current ideality, whereas collector current remains ideal over the whole temperature range. By studying forward and reverse currents at emitter–base and base–collector junctions, we have established that deep levels were involved in conduction phenomena at these junctions. Detailed measurements using capacitance transient spectroscopy (with different reverse and filling pulse voltages and different filling pulse durations) have revealed the presence of two deep levels along the periphery of the emitter. These deep levels have been found with identical characteristics at both junctions. It is demonstrated that these traps are most probably induced by the extrinsic base implantation.

Leakage current mechanisms in SiGe HBTs fabricated using selective and nonselective epitaxy

SiGe heterojunction bipolar transistors (HBTs) have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by nonselective epitaxy for the p/sup +/ SiGe base and n-Si emitter cap. DC electrical characteristics are compared with cross-section TEM images to identify the mechanisms and origins of leakage currents associated with the epitaxy in two different types of transistor. In the first type, the polysilicon emitter is smaller than the collector active area, so that the extrinsic base implant penetrates into the single-crystal Si and SiGe around the perimeter of the emitter and the polycrystalline Si and SiGe extrinsic base. In these transistors, the Gummel plots are near-ideal and there is no evidence of emitter/collector leakage. In the second type, the collector active area is smaller than the polysilicon emitter, so the extrinsic base implant only penetrates into the polysilicon extrinsic base. In these transistors, the leakage currents observed depend on the base doping level. In transistors with a low doped base, emitter/collector and emitter/base leakage is observed, whereas in transistors with a high doped base only emitter/base leakage is observed. The emitter/collector leakage is explained by punch through of the base caused by thinning of the SiGe base at the emitter perimeter. The emitter/base leakage is shown to be due to a Poole-Frenkel mechanism and is explained by penetration of the emitter/base depletion region into the p/sup +/ polysilicon extrinsic base at the emitter periphery. Variable collector/base reverse leakage currents are observed and a variety of mechanisms are observed, including Shockley-Read-Hall recombination, trap assisted tunneling, Poole-Frenkel and band to band tunneling. These results are explained by the presence of polysilicon grains on the sidewalls of the field oxide at the collector perimeter.

The influence of BF/sub 2/ and F implants on the 1/f noise in SiGe HBTs with a self-aligned link base

A study is made of 1/f noise in SiGe heterojunction bipolar transistors (HBTs) fabricated using selective growth (SEG) of the Si collector and nonselective growth (NSEG) of the SiGe base and Si emitter cap. The transistors incorporate a self-aligned link base formed by BF/sub 2/ implantation into the field oxide below the p/sup +/ polysilicon extrinsic base. The influence of this BF/sub 2/ implant on the 1/f noise is compared with that of a F implant into the polysilicon emitter. Increased base current noise S/sub IB/ and base current are seen in transistors annealed at 975/spl deg/C, compared with transistors annealed at 950 or 900/spl deg/C. At a constant collector current, both the BF/sub 2/ and F implants reduce S/sub IB/, whereas at a constant base current, only the BF/sub 2/ implant reduces S/sub IB/. This result indicates that the BF/sub 2/ implant decreases the intensity of the base current noise source whereas the F implant decreases the base current. The proposed explanation for the increased 1/f noise is degradation of the surface oxide by viscous flow at 975/spl deg/C under the influence of stress introduced during selective Si epitaxy. The influence of the BF/sub 2/ implant on the noise is explained by the relief of the stress and hence the prevention of viscous oxide flow.

Self-aligned selective-epitaxial-growth SiGe HBTs: process, device, and ICs

We review the fabrication process and device technologies of a high-speed self-aligned selective-epitaxial-growth (SEG) SiGe-base heterojunction bipolar transistor (HBT), and its potential application in ICs for optical-fiber-link systems. A 0.54-μm-wide SiGe base, self-aligned to the 0.14-μm-wide emitter to reduce collector capacitance, was selectively grown by using a UHV/CVD system. A self-aligned stacked metal/in situ doped poly-Si electrode technology enables low parasitic resistance, and allows the intrinsic base profile to be kept shallow, so it is well-suited to a SiGe-base HBT. A 2-μm-wide insulator refilled trench was introduced to reduce substrate capacitance. This SiGe HBT makes it possible to obtain 95-GHz cutoff frequencies and 97-GHz maximum oscillation frequencies, and ultra-high-speed emitter-coupled-logic (ECL) circuits with an 8-ps gate delay. The technology was applied in ICs required for optical-fiber-link systems, including a 1/8 static frequency divider with a maximum operating frequency of up to 50 GHz, a time-division multiplexer and a demultiplexer operating at 40 Gb/s, a preamplifier with a bandwidth of 35 GHz, an AGC amplifier core with a bandwidth of 32 GHz, and a decision circuit operating at 40 Gb/s.

Comparison of SiGe and SiGe:C heterojunction bipolar transistors

We compare the performance of heterojunction bipolar transistors with pure SiGe (SiGe HBTs) with those incorporating C-doped SiGe base layers (SiGe:C HBTs). The transistors were produced in a single-polysilicon technology with implanted, epi-free wells. Doping the SiGe layers with low C concentration (∼0.2%) allows us to use a higher base boron dose than for C-free HBTs, without B outdiffusion from the heteroepitaxial layer. As a result, the device RF performance can be significantly improved. The higher base doping of SiGe:C HBTs increases the peak fmax from around 50 up to more than 80 GHz, and reduces the minimum noise figure (at 10 GHz) from >3 to 2 dB and ring oscillator delays from 21–22 to 12–14 ps. The SiGe:C HBTs also exhibit leakage currents, which are sufficiently low for reliable IC application.

On the design and fabrication of novel lateral bipolar transistor in a deep-submicron technology

The performance of a npn Lateral Bipolar Transistor (LBJT), based on a minimally modified submicron MOSFET without gate oxide, was studied by means of simulations and measurements on fabricated devices. Large-tilt angle, single-sided implants were successfully used to control the breakdown voltage by tailoring an asymmetric collector doping profile and providing a lightly doped region on the collector side to increase the breakdown voltage. This was accomplished by using a base/gate pedestal as the mask for the large-tilt angle, single sided implantation in a self-aligned fashion. The individual collector–base and emitter–base junctions were found to be of excellent quality but, as expected due to the lack of carrier confinement in the base of devices built on bulk silicon, the common emitter current gain was lower than one. Two options enhancing the device performance were studied using simulators: building the LBJT on Silicon-on-Insulator (SOI) or introducing SiGe into the base. The SOI device promises to provide better performance and ease of processing when compared to the SiGe base device.

Process design for SiGe-HBTs prepared using cold-wall UHV/CVD

We have studied the influence of Si/SiGe/Si profile on electrical characteristics of a self-aligned SiGe-base heterojunction bipolar transistor (HBT) with a poly-crystalline silicon (polysilicon) emitter. Particular attention has been given to a collector-base undoped SiGe intermediate layer. This undoped layer acts as a spacer for boron out-diffusion of the p +-type SiGe to the n −-type Si collector, during heat treatment after the SiGe base formation. The SiGe-base was selectively grown by a cold-wall ultra-high vacuum/chemical-vapor-deposition (UHV/CVD) technique. Using both low temperature oxide (LTO) as a passivation film, and in-situ phosphorus-doped emitter polysilicon for suppressing of boron out-diffusion, a 15-nm-thick undoped layer was an insufficient boron out-diffusion spacer. This was confirmed by measuring the cut-off frequency, f T, characteristics. With a 30-nm-thick spacer layer, the f T of the SiGe base bipolar transistor increased from 38 to 63 GHz.

SiGe HBTs and ICs for optical-fiber communication systems

An ultra-high-speed selective-epitaxial-growth (SEG) SiGe-base heterojunction bipolar transistor (HBT) with self-aligned stacked metal/in-situ doped poly-Si (IDP) (referred to as SMI) electrodes has been developed. A 0.54-μm-wide SiGe base self-aligned to the 0.14-μm-wide emitter, which reduces collector capacitance, was selectively grown by using a UHV/CVD system. SMI electrode technology, which enables low parasitic resistance, allows the intrinsic base profile to be kept shallow, so it is well suited to a SiGe-base HBT. A 2-μm-wide BPSG/SiO 2 refilled trench was introduced to reduce substrate capacitance by reducing its sidewall element. This makes it possible to obtain a 95-GHz cut-off frequency and ultra-high-speed emitter-coupled-logic (ECL) circuit with an 8.0-ps gate-delay. As applications for these SiGe HBTs, various ICs for optical-fiber-link systems have been developed. These include a 1/8 static frequency divider with a maximum operating frequency of up to 50 GHz, a time-division multiplexer and a demultiplexer operating at 40 Gb/s, a preamplifier with a bandwidth of 35 GHz, an AGC amplifier core with a bandwidth of 32 GHz, and a decision circuit operating at 40 Gb/s.

A 60-GHz f/sub T/ super self-aligned selectively grown SiGe-base (SSSB) bipolar transistor with trench isolation fabricated on SOI substrate and its application to 20-Gb/s optical transmitter ICs

A 60-GHz cutoff frequency (f/sub T/) super self-aligned selectively grown SiGe-base (SSSB) bipolar technology is developed. It is applied to 20-Gb/s optical fiber transmitter ICs. Self-aligned bipolar transistors mutually isolated by using a BPSG-filled trench were fabricated on a bond-and-etchback silicon-on-insulator (SOI) substrate to reduce the collector-substrate capacitance C/sub CS/. The SiGe base was prepared by selective epitaxial growth (SEG) technology. A 0.4-/spl mu/m wide emitter was used to reduce the junction capacitances. The maximum cutoff frequency f/sub T/ and the maximum frequency of oscillation f/sub max/ were 60 and 51 GHz, respectively. By using this technology, Si-ICs for an optical transmitter system were made, such as a selector (a multiplexer without input and output retiming D-type flip-flops (D-F/Fs)), a multiplier, and a D-F/F. An internal high-speed clock buffer circuit achieves stable operation under a single clock input condition in the selector and the multiplier ICs. Their stable operation was confirmed up to 20 Gb/s. The selector IC for data multiplexing operates at over 30 Gb/s.