A remote carbon method for fabricating strained SiGeC:B layers
08/01/2006
Advanced RF BiCMOS technologies that use NPN heterojunction bipolar transistors (HBT) require the lowest possible base resistance, which is achieved with boron doping. Ultra-thin (sub-40 nm) strained layers of silicon germanium (SiGe) typically have boron concentrations pushing solid-solubility limits. To reduce boron out-diffusion, carbon is added to the SiGe lattice to stuff interstices in the crystal, and a new “remote carbon method” (RCM) for the growth of boron-doped SiGe carbon (SiGeC:B) achieves improved boron confinement and enhanced film conductivity. The correspondingly improved carrier transport and dopant diffusion characteristics translate directly into enhanced flexibility in HBT design and manufacturing.
Reducing and controlling boron outdiffusion is vital for the manufacturing of NPN HBTs with ultra-thin (sub-40nm) strained layers of silicon germanium (SiGe), especially with boron concentrations pushing solid-solubility limits in efforts to achieve the lowest possible base resistance (Rb). To reduce boron outdiffusion, carbon is added to the SiGe lattice to bring about a localized undersaturation of silicon interstitials (I) [1, 2] to prevent Bi-I pairing, which together possess greater diffusivity than boron alone. The limiting mechanism is through a sacrificial Ci-I pairing, instead of Bi-I, which has been shown to influence up to a 10× reduction in boron outdiffusion, allowing fabrication of ultra-narrow base regions in NPN HBTs.
The narrow base translates into a reduced base transit time (τb) and improved unity gain cutoff frequency (Ft). However, the addition of carbon into the boron-doped SiGe lattice has been shown to possess degenerative qualities related to the following: increased film sheet resistance (Rs), increased base recombination current (IrB) in NPN HBTs, and unwanted compensation of the built-in biaxial compressive strain in pseudomorphic SiGe films for some applications.
This improved pseudomorphic strained SiGe growth technique will gain importance as device speeds continue to climb by providing an additional degree of freedom for optimal engineering of base resistance, current gain, maximum oscillation frequencies, and noise figures. The method segregates the carbon and the boron in the film, which results in a number of important benefits: less carbon needed to prevent boron outdiffusion, a more compressively strained lattice, significant reduction in the film sheet-resistance, and reduction in electron-hole recombination. These film attributes translate directly into enhanced flexibility in HBT design and manufacturing.
Device electrical tests reveal a concurrent reduction in base resistance (Rb) and base current (IB), which couples into increased current gain (β) and maximum frequency of oscillation (Fmax). Subsequent work has explored improved device figures of merit, such as Fmax and minimum noise (NFmin) in HBTs.
RCM processing
Figure 1 depicts the relative positions of the as-grown (before thermal anneal) carbon and boron profiles within a box SiGe structure for the typical complete carbon method (CCM) as well as for the RCM. Samples contained 1019-1020cm-3carbon concentrations with boron held constant at ~1.2×1020cm-3 and germanium held constant at ~20.5% in box-like SiGe structures of 36nm ±1nm in width. The as-grown boron profile width (measured at 1×1019cm-3) is 24nm. SIMS was used to measure peak concentrations of B, C, and Ge. X-ray diffraction (XRD) was used to measure average %Ge, film thickness, and the SiGeC lattice parameter as carbon was increased. Thermal anneals were performed on each sample at 930°C for 90 sec.
 Figure 1. SIMS profiles of pseudomorphically grown a) SiGe:B with continuous carbon method (CCM), and b) SiGe:B with remote carbon method (RCM). |
RCM augments the intrinsic diffusion limiting attributes of carbon. Figure 2 shows the improved efficiency of RCM over CCM for mitigation of boron outdiffusion as carbon is increased from 1019 to 1020cm-3. The as-grown profile width for both methods was 24nm. RCM requires ~3.5×1019cm-3 peak carbon to prevent boron spreading, while CCM requires peak carbon approaching 1020cm-3, with ~2nm of spread still evident. Therefore, RCM resulted in less boron spread and required 70% less carbon to accomplish the objective.
The enhanced diffusion limiting efficiency with RCM might be explained by the Ci-I and Ci-Bi interactions. With CCM, carbon resides substantially in the same lattice space as boron, whereas with RCM the carbon and boron are segregated. Therefore, a plausible explanation is that while carbon mitigates boron diffusion by substituting the Ci-I pairing, there might be a prevalent Ci-Bi pairing with CCM, which adds to the outward flux during thermal anneals. RCM hypothetically reduces this component, resulting in a more efficient “sink” for silicon interstitials. Additionally, RCM presents a net flux of carbon that acts in opposition to the outward flux of boron.
RCM device results
RCM minimizes the hole transportation degradation that occurs with carbon incorporation. The 49-point four-point probe (FPP) sheet resistance (Rs), measured in a direction parallel to the (100) Si/SiGe growth interface, is inversely proportional to the conductivity (σ). Efforts to measure carrier mobility through the Hall Effect are problematic due to the large variation in the Hall scattering factor, rH, which occurs with various mole fractions of boron, carbon, and germanium in the strained SiGe lattice [4]. However, previous studies provide ample information to carefully deduce the effects of carbon on mobility in SiGe.
Figure 3 presents the within-wafer (WIW) average of FPP measurements versus peak carbon for both the RCM and CCM methods, where the WIW 1 standard deviation of all samples was <2%. When comparing results at equivalent values of peak carbon, RCM results in an 8% to 16% reduction in Rs. The difference grows to ~25% when comparing Rs over the “useful” range of carbon (that which mitigates boron diffusion), as indicated in Fig. 3.
Previous studies [3, 4] have shown that activation of boron in strained SiGeC is relatively unaffected for carbon mole fractions <2%. Therefore, the improved σ with RCM is likely not from increased NA but is instead an improved μh due to a reduction in substitutional carbon incorporation, and therefore reduced alloy scattering (τalloy) [4].
RCM minimizes strain compensation and in so doing reduces the hole effective mass and interband scattering, thus further augmenting carrier transport. Previous research with CCM has shown that carbon will lattice-compensate the germanium content by a ratio of approximately 1:10, with ratios ranging from 1:8 to 1:12; thus, a carbon level of 1×1020cm-3 with CCM should lattice-compensate ~2% Ge.
When making a direct comparison of peak carbon concentration versus %Ge (as measured by XRD), there is an increase in “apparent Ge content” for films grown by RCM, which illustrates there is less overall lattice and strain compensation compared to CCM. However, when comparing the useful range of carbon of both methods, RCM results in a lattice compensation that is 1/7th to 1/10th that of CCM. Since RCM requires less carbon to mitigate boron diffusion, the SiGe lattice parameter and compressive strain are least affected by this method. Thus, the carrier transport properties are enhanced by a reduction in lattice scattering and lifting of lh and hh energy bands.
Process and design flexibility with RCM
RCM imparts design flexibility for incremental improvements to device performance due to the enhancements in carrier transport and dopant diffusion limitation. RCM provides another lever for tuning such parameters as Rb, β, and Fmax/Ft ratio without serious degradation to collector-emitter breakdown (BVCE0) and early voltage (VAF) to name only a few.
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The table shows some early device test results on HBTs with SiGe films grown by RCM. The device base sheet resistance (Rsb) is reduced by 23% for RCM vs. CCM, which tracks the results obtained on blanket films (Fig. 3). Thus, Fmax and the Fmax/Ft ratio are increased ~12%, benefiting lumped analog ICs where Fmax/Ft ratios >1.2:1 are desirable, and distributed amplifiers where Fmax is the primary limitation [5].
CM links enhanced boron confinement, increased β due to reduced IB, and reduced Rb in a single “lever” for both process and device designers. In these experiments, current gain was increased by 33%, and concurrently Rb was reduced by 23%. The increased β was due to a reduction in IB, likely due to increased electron minority carrier lifetime (τnb), which reduces base recombination current (IrB ∝ 1/τnb). The implication for high-speed devices with low noise is exciting for device designers. Since boron diffusion is more effectively mitigated, device designers also have the option to pump in additional boron to achieve even lower values of base resistance, while maintaining the narrow base and thus Ft.
Conclusion
Experimental results highlight the significant improvements afforded by the RCM for fabrication of SiGe HBTs with ultranarrow base regions and with low base resistance. RCM offers a lever for the simultaneous reduction in Rb and IB, which translates into device design flexibility and performance enhancement through increased Fmax and β, with serious potential for low noise applications. RCM’s enhanced film properties, specifically with respect to improved hole transport and minority carrier lifetimes, are due to a reduction in lattice scattering and enhanced biaxial compressive strain.
References
1. L. Lanzerotti, J. Sturm, E. Stach, et al., “Suppression of Boron Transient Enhanced Diffusion in SiGe HBT by Carbon Incorporation,” Applied Physics Letters, Vol. 70, No. 23, pp. 3125-3127, 1997.
2. M. S. Carroll, C. L. Chang, J. C. Sturm, “Complete Suppression of Boron Transient-Enhanced Diffusion and Oxidation-Enhanced Diffusion Using Localized Substitutional Carbon Incorporation,” Applied Physics Letters, Vol. 73, No. 25, 1998.
3. Chia-Lin Chang, “Properties and Applications of Crystalline SiGeC Alloys,” PhD Thesis, Princeton University, 1998.
4. Erich Kasper, Klara Lyutovich, Properties of Silicon Germanium and SiGe:Carbon, INSPEC, The Institution of Electrical Engineers, 2000.
5. John D. Cressler, Guofu Niu, Silicon-Germanium Heterojunction Bipolar Transistors, Artech House, 2003.
Darwin Enicks is a process engineering section manager for diffusion/epi/SiGe processes at Atmel Corp., 1150 E. Cheyenne Mountain Blvd., Colorado Springs, CO 80906; ph 719/540-1753, [email protected].