Shifeng Lu

Two-dimensional ultrashallow junction characterization of metal-oxide-semiconductor field effect transistors with strained silicon

Strained Si has been realized as one of the most promising candidates of next generation complementary metal-oxide-semiconductor technology. Since the carrier mobility can be significantly increased with strained Si lattice, the device speed can be further increased without reducing the critical dimensions. However, ultrashallow junction engineering becomes more challenging due to much complicated dopant diffusion behavior. We have used scanning capacitance microscopy and dopant selective etching to characterize such differences by comparing the devices fabricated with strained Si channel and with conventional unstrained Si. The devices we used are p-type channel complementary metal-oxide-semiconductor field effect transistors fabricated with 130 nm technology, with strained Si channel built on SiGe pseudosubstrate. Significant differences were observed in the formation of source/drain (S/D) extensions. The junction profile shows abrupt transition from S/D extension to S/D comparing with unstrained Si. Me...

Characterization Techniques for Evaluating Strained Si CMOS Materials

The electron and hole mobility of Si complementary metal on oxide field effect transistors (CMOS) can be enhanced by introducing a biaxial tensile stress in the Si channel. This paper outlines several key analytical techniques needed to investigate such layers. Raman scattering is used to measure the strain in the Si channel as well as to map the spatial distribution of strain in Si at a lateral resolution better than 0.5 μm. Atomic force microscopy (AFM) is used to measure the surface roughness. Transmission electron microscopy (TEM) is used to reveal dislocations in the structure, the nature of the dislocations and the propagation of the dislocations. Secondary ion mass spectrometry (SIMS) is used to monitor the Ge content profile in the structure and the thickness of each layer. In the long term, inline nondestructive techniques are desired for epi‐monitoring in manufacturing. Two techniques, spectroscopic ellipsometry (SE) and x‐ray reflectivity (XRR), have shown promise at this stage.

Secondary ion mass spectrometry characterization of source/drain junctions for strained silicon channel metal–oxide–semiconductor field-effect transistors

As complementary metal–oxide–semiconductor (CMOS) devices approach the sub-100-nm dimensions in accordance with Moore’s Law, several major technical barriers exist with the formation of ultrashallow junctions. Strained silicon CMOS devices have the advantages of higher carrier mobility and high current drive. The use of silicon germanium substrates for strain in the silicon channel presents many challenges for CMOS integration including maintaining the channel strain and effect on shallow source/drain (SD) junctions. Low energy secondary ion mass spectrometry (SIMS) has been used to study boron and arsenic diffusion behavior in strained silicon and in SiGe. In addition, diffusion of germanium from the relaxed SiGe into the strained silicon layer will be discussed in relationship with SD implant and annealing. SIMS experimental results will also be compared to theoretical simulation results.

High depth resolution Secondary Ion Mass Spectrometry (SIMS) analysis of Si1−xGex:C HBT structures

Low energy Secondary Ion Mass Spectrometry (SIMS) was employed to study graded-base Si1−xGex:C heterostructure bipolar transistors (HBTs) structural properties. Using an O2+ beam at 500 eV with normal incident angle, Ge profiles can be quantified by minimizing the influence of matrix effect. This is achieved with a correction based on the linearity of the ratio (IGe/ISi) with respect to the ratio (x/1−x). The SIMS results showed an excellent correlation with the graded Auger Ge profile in 5–20% range. Moreover, SIMS analysis revealed an enhanced Ge diffusion with the carbon incorporation, which was used to suppress base boron diffusion during the rapid thermal annealing (RTA) process. Based on the SIMS Ge profile, device simulation can be used to design Ge profile shape in order to optimize process throughput without impact on the device performance. The high depth resolution SIMS data is essential for Si1−xGex:C HBTs structural characterization and process optimization.

Root‐Cause Analysis and Statistical Process Control of Epilayers for SiGe:C Hetero‐Structure Bipolar Transistors

The SiGe:C hetero‐structure bipolar transistor (HBT) has turned into a key technology for wireless communication. This paper describes various critical analytical techniques to bring up and maintain the SiGe:C epi‐process. Two types of analysis are critical, (1) routine monitoring SiGe base and Si cap thickness, doping dose, Ge composition profile, and their uniformity across the wafer; and (2) root‐cause analysis on problems due to non‐optimized process and variation in process conditions. A transmission electron microscopy (TEM) technique has been developed allowing a thickness measurement with a reproducibility better than 3 A. Charge‐compensated low‐energy secondary ion mass spectrometry (SIMS) using optical conductivity enhancement (OCE) allows a Ge composition measurement to a required precision of 0.5 at. %.

Characterization of High-k Dielectric Films with Tunneling AFM

In this paper, we report the leakage current characterization of HfO 2 high-k dielectric thin films by using tunneling AFM, which utilizes a conducting AFM probe to detect current passing through the sample and the probe while simultaneously acquiring a topographic image. We have studied tunneling current behavior of HfO 2 films by characterizing the hot spots, which are characterized by excessive local leakage current, as well as the overall current distribution. Tunneling AFM results show sensitive dependence of tunneling current with variation of film thickness. The current distribution can be described approximately by a log-normal distribution, which is consistent with the characteristics of the thickness variation. Furthermore, the film structure and thickness were also characterized with TEM and spectroscopic ellipsometry.

2D Dopant Profiling for Advanced Process Control

As the CMOS device dimensions continue to shrink, it is more and more critical to control the process parameters during mass production of advanced VLSI chips in order to achieve high yield and profitability. 2D dopant characterization is one of the critical techniques to resolve manufacturing excursions. A quick access to dopant distribution, especially precise delineation of p-n junction would readily provide critical information for many manufacturing issues, as well as device design and process development. Here we present our approaches to some of those issues with available techniques. The main techniques we used are dopant selective etching (DSE) and scanning probe microscopy based electrical measurements including scanning capacitance microscopy (SCM) and scanning spread resistance microscopy (SSRM). These techniques provided complementary results and showed strengths in solving different issues. We have successfully delineated junction of CMOS devices with 0.13 μm technology with source/drain extensions. Other applications, including diode leakage, well-well isolation, and buried layer delineation with the combination of these methods are presented.

AB-INITIO MODELING OF C-B INTERACTIONS IN SI

We present the results of ab-initio calculations for the structure and energetics of small boron-carbon (BCI) as well as carbon-carbon (C2I) clusters in Si , a continuum model for the nucleation, growth, and dissolution of the clusters, and experimental investigation by SIMS. The modeling results suggest that these clusters may play a role in controlling B diffusion in Si and SiGe systems and the experimental results seem to support the modeling findings. I. INTRODUCTION SiGeC has several beneficial materials properties over SiGe which include the compensation of film strains by adjustment of the Ge/C ratio, enhanced thermal stability, increased critical film thickness, suppressed transient enhanced diffusion (TED) of boron [1], and preservation of the narrowed band gap of strained SiGe. One particular device application of SiGeC films is the construction of heterojunction bipolar transistors (HBT) using Si/SiGeC/Si heterostructures. SiGeC HBTs have increased performance (higher frequency) due to the smaller band gap [2] and stability of the base profile. The currently prevalent explanation of suppression of diffusion of boron by carbon is that carbon reduces the free silicon interstitial (I) concentration by forming a CSi complex, resulting in fewer B-Si complexes, which are believed to be responsible for B TED in Si [1,3]. Reference [1] further showed that the diffusion of carbon incorporated in silicon well above its solid solubility causes an undersaturation of silicon self-interstitials, which further retards boron diffusion. In this work, we further explore the atomic mechanisms for the effect of carbon on B diffusion through ab-initio investigation of C-B interactions by focusing on C-B split interstitial pairs (CBI). For the first time the results of our ab initio calculations indicate that carbon and boron can interact directly by forming CBI pairs. For the modeled case, CBI is the most important cluster containing B, while the predominant cluster capturing and storing Si interstitials is the C2I cluster. Our modeling predicts that the CBI concentration may not be sufficient to surpass the effect of carbon in reduction of Si interstitials by forming C2I clusters, and thus that the direct interactions between C and B by forming C-B pairs may play a secondary role in suppressing B diffusion. However, a comparison to experiment indicates that BCI complexes might play an even more central role than C2I clusters (our ab initio calculations have error bars of ~0.3 eV). II. COMPUTATIONAL METHODS AND ASSUMPTIONS VASP (the Vienna Ab-initio Simulation Package) [4] was used for the calculations in this work. The ultra-soft pseudopotentials with a plane wave basis supplied with VASP and the generalized gradient approximation (GGA) were used. A supercell of 64 Si atoms and a cutoff energy of 21 Ry corresponding to the cutoff energy of the C pseudopotential were chosen for all calculations. Diffusion barriers were determined using the nudged elastic band method (NEBM) [5] implemented in VASP,

Materials and Physical Properties of Novel High-k and Medium-k Gate Dielectrics

Rapid shrinking in device dimensions calls for replacement of SiO 2 by new gate insulators in future generations of MOSFETs. Among many desirable properties, potential candidates must have a higher dielectric constant, low leakage current, and thermal stability against reaction or diffusion to ensure sharp interfaces with both the substrate Si and the gate metal (or poly-Si). Extensive characterization of such materials in thin-film form is crucial not only for selection of the alternative gate dielectrics and processes, but also for development of appropriate metrology of the high-k films on Si. This paper will report recent results on structural and compositional properties of thin film SrTiO 3 and transition metal oxides (ZrO 2 and HfO 2 ).