Formation of ultra shallow junctions in silicon germanium by pulsed laser annealing

CHAPTER 1 – INTRODUCTION 1.1 LASER THERMAL PROCESSING Laser thermal processing (LTP) is a paradigm shift in the formation of shallow junctions in silicon. The sub-processes of LTP are amorphization, dopant implant, laser irradiation, liquid melt and liquid phase epitaxy. During laser irradiation the amorphous region is liquefied allowing the dopant atoms to distribute homogeneously throughout the melt by diffusion. Upon rapid solidification, the dopants largely retain the even distribution and produce an abrupt junction that is box shaped, with the depth of the junction defined by the position of the amorphizing implant. Additionally, due to rapid conductive cooling, the liquid phase epitaxy is non-equilibrium, resulting in the substitutional incorporation of the dopant atoms into the crystalline lattice in excess of equilibrium solid solubility. Since the invention of light amplification by stimulated emission of radiation or LASER in 1960, lasers have been used in a variety of ways to process semiconductors. From as early as 1968 lasers were used to modify the electrical resistivity of semiconductors. 1 By 1976 lasers were used to remove lattice damage caused by ionimplantation and to electrically activate dopants in a process termed laser annealing.2 In 1978 re-crystallization of an amorphous silicon film was achieved by irradiation with a single laser pulse.3,4 More recently lasers have been used to melt doped 1 amorphous layers to define the depth of electrically active junctions.5 For the current text, the process of laser irradiating a doped amorphous layer to achieve full melt and subsequent epitaxial re-growth resulting in the depth of the electrical junction being defined by the original amorphous layer thickness is termed laser thermal processing. The following five sections further describe laser thermal processing in terms of amorphization, dopant implantation, laser irradiation, liquid melt and liquid phase epitaxy. 1.1.1 PRE-AMORPHIZING IMPLANT The depth of the amorphous layer controls the depth of the melt. Implanting the substrate with a high dose of a low energy, high mass ion, forms a shallow amorphous layer. Amorphization is a result of the damage introduced to the crystalline lattice during ion implantation. 6 For crystalline silicon, amorphization requires displacement of approximately 10~12 % of the lattice atoms. Formation of an amorphous layer progresses gradually, as damage to the crystal lattice is accumulated with each implanted ion . The number of ions per unit area required to amorphize a layer is referred to as the threshold dose. Amorphizing implants must have an implant dose higher than the threshold dose. The threshold dose for an ion with low mass is larger than that for ions with a high mass. The depth of the amorphous layer is determined by the energy of the implant ion. Ions with low implant energy do not travel as deep into the substrate as highenergy implants; consequently, low energy implants result in shallower amorphization depths. 2 1.1.2 DOPANT IMPLANT The dopant is implanted into the amorphous surface layer at a low energy and high dose. Implantation of the dopant is consistent with conventional ion implantation7. The implant energy of the dopant ions needs to be sufficiently low to confine the dopant to the amorphous layer, thus maintaining an electrical junction defined by the amorphous depth. Additionally, the dose of the dopant needs to be high enough, in order to provide sufficient population of carriers to achieve a low sheet resistance. 1.1.3 LASER IRRADIATION Lasers provide a high power-density energy source that is both monochromatic and coherent, which can be controlled to achieve nanosecond pulse durations. For laser thermal processing several types of lasers have been used, differing primarily in wavelength (e.g., XeCl 3088, 9 , frequency doubled Nd:YAG 53210 and Nd: YAG 1064 nm). The optical properties of the material; specifically reflectivity, absorption and absorbance are wavelength-dependant. Reflectivity is the amount of incident energy that is reflected from the surface of the material relative to the incident intensity. Absorption determines the degree to which the amplitude of the radiation is damped as a function of distance into the sample. Output of the laser is directed through optics that homogenize, focus and define the cross-sectional area of the laser irradiation. In the irradiated area of the sample, 3 absorption of laser energy by the silicon may result in emission of phonons that are expressed as thermal heating11. Absorption occurs when a photon hits an electron and the electron absorbs the energy and as a consequence moves up to a higher energy state. The electron can transition between energy bands by a direct transition or indirect transition. A direct transition produces a photon that is reflected, while an indirect transition produces a photon and a phonon. It is the phonon that produces the effect of heating. For amorphous silicon, the transition mechanism is predominately indirect, thus resulting in phonon induced lattice heating. 1.1.4 LIQUID MELT Heating from the laser provides the thermal budget for melting, diffusion and electrical activation. Amorphous silicon melts at approximately 200 ± 50 K lower than that of the crystalline silicon. 12 Laser thermal processing utilizes this melting point difference to establish a process window. As the laser energy density is increased, a threshold is reached at which the amorphous silicon begins to melt. Further increase in the laser energy density increases the melt depth, until the entire amorphous layer is melted and the original amorphous/crystalline interface is reached. The melt does not proceed into the crystalline material until sufficient thermal energy is added to raise the liquefied amorphous silicon temperature to the melting point of crystalline silicon. The difference in laser energy density required to completely melt the amorphous layer and the laser energy density required to begin melting the crystalline silicon is termed the process window.10 4 Dopants distribute homogeneously throughout the liquid melt. The diffusion coefficient of dopant atoms in liquid silicon is approximately eight orders of magnitude greater than that in crystalline silicon. For example the diffusion coefficient of boron in liquid silicon is [3.3 ± 0.4]x10-4 cm2/s 13 , while the diffusion coefficient in crystalline silicon is approximately 1.0x10-15 cm2/s. The large diffusivity of the dopant atoms in the liquid region results in a rapid redistribution that tends to homogenize the dopant distribution throughout the melt. In contrast, the amount of diffusion that occurs in the solid is negligible in comparison; thus in terms of dopant distribution, an abrupt box-like junction is formed at the maximum melt depth14. 1.1.5 LIQUID PHASE EPITAXY Liquid phase epitaxy (LPE) is the commensurate growth of a solid from a liquid, based on the crystal structure of the seed or substrate material. The process of liquid phase epitaxy begins as one atom, in order to minimize free energy, attaches itself to a metastable lattice site on the surface of the seed material. Subsequently, other atoms attach to the ledge or kink site formed by the original atom making it more stable. Atoms continue to attach to the newly formed ledge sites until an atomic layer is formed. This process continues until the liquid phase has been completely regrown. Liquid phase epitaxy results in the incorporation and re-distribution of dopant atoms, as the melt solidifies. For a laser pulse with duration less than 18 ns, the melting and solidification times are less than 100 ns in silicon, resulting in rapid regrowth of the 5 melt. Rapid regrowth is driven by the large thermal gradient that exists in the wafer due to the surface being at the melting temperature of amorphous silicon (1480 ± 50 K)12, while the back side of the wafer is at ambient temperature (~298 K). This thermal gradient drives liquid phase epitaxy of the silicon with melt front velocities of 2 m/s to 4.5 m/s15. Large regrowth velocities can increase the segregation coefficient (k = Cs/Cl) of dopants by several orders of magnitude above equilibrium values. The segregation coefficient increases as the regrowth velocity exceeds the diffusion velocity of the dopant impurities in the liquid. The increase in the segregation coefficient results in a higher concentration of dopants incorporated into the solid. Hence, high regrowth velocities achieve higher dopant concentrations. Laser thermal processing utilizes high regrowth velocities to produce junctions that are highly doped with correspondingly low sheet resistance values. 1.2 EXCIMER LASER ANNEALING Excimer laser annealing (ELA) was thoroughly investigated in the 1970–1980s and is a long known approach for annealing of ion implanted Si. Today there is a renewed interest from the semiconductor community for a possible application of ELA to sub-70 nm technology. ELA offers considerable advantages over the conventional rapid thermal annealing (RTA). It provides the possibility to obtain ultra-shallow junctions, to remove completely the implantation defects and to form more abrupt dopant profiles with higher dopant activation efficiency. The annealing of implantation induced damage during ELA occurs due to melting of the damaged region and subsequent liquid-phase re- 6 crystallization of the melt. The process is characterized by extremely short times of phase transformation, with typical laser pulses of 10–30 ns and the melting time within the microsecond range, which results in a very localized effect of ELA on the material, where all the modifications take place mainly within the melting region. Because of this highly non-equilibrium nature, a very high dopant incorporation efficiency can be obtained with the dopant concentration exceeding the solid solubility level.16 Pulsed excimer laser annealing has been proven to be effective for reducing the poly-Si gate depletion effect to less than 1 Å. High active boron concentration and low resistivity can be achieved without boron penetration as can be seen from the work by Wong et al. 17 Gate oxide quality is preserved, and ELA is found to be more compatible with HfO2 gate dielectric than RTA due to the ultrashort melt time. Therefore, ELA is promising for application to sub-65-nm bulk-Si CMOS technologies.17 1.3 LASER ANNEALING FOR ULTRA-SHALLOW JUNCTIONS Future semiconductor technology nodes are not achievable with current processing technologies. Shrinking of device dimensions is limited by the ability to fabricate doped junctions that are shallow, abrupt and highly activated. In conventional processing, achieving high activation is at odds with achieving a shallow junction. High activation is generally achieved by high thermal budgets, which cause the dopant to diffuse, thus increasing the final junction depth. In contrast, a low thermal budget limits diffusion of the dopant but yields a low activation. Consequently, new processing 7 techniques need to be developed which are capable of producing device features in compliance with the requirements of future technology nodes. A potential candidate for reaching future technology nodes is laser annealing. Laser annealing, in which the laser melts the surface layer of silicon and causes the dopants to be distributed uniformly within the melted region, produces abrupt, highly activated and ultra-shallow junctions. The degree of melting is determined by the extent of laser absorption and rate of heat dissipation, which are dependent on the substrate properties. Laser annealing with extremely low energy is required to guarantee enough process windows. Laser annealing offers several advantages over conventional processing techniques: 1) the junction depth is defined by the amorphous/crystalline interface. Laser annealing, combined with pre-amorphization implant (PAI) has been reported to control ultrashallow junction depth. Since an amorphous silicon layer has approximately 300°C lower melting temperature than that of crystalline silicon, laser annealing energy can be reduced by PAI. 2) the amorphous region is regrown by liquid phase epitaxy providing an even distribution of the dopant across that junction, resulting in a box-like junction. The amorphous silicon thickness and melted thickness are precisely controlled by PAI and the dopant diffusion occurs within the melted layer during very short laser annealing period, so that a box-like profile is formed. 8 Fig. 1.1: Sheet resistance versus junction depth for p-type dopants activated by rapid thermal annealing, Levitor, Flash, SPER and LTP. The solid solubility line represents an impurity concentration of 2.0x1020 /cm3, which is obtained at a temperature of 1050 oC for boron.18 Fig. 1.2: Vertical junction abruptness for p-type dopant gradients achieved by rapid thermal annealing, Levitor, Flash, SPER and LTP..18 9 According to the International Technology Roadmap for Semiconductors (2007)19, the source/drain extensions shallower than 14 nm will be required. In addition, the sheet resistance must be lower than 900 and 400 ohms/sq for PMOS and NMOS, respectively. Rapid-thermal-annealing (RTA)-based technology with low-energy ion implantation is the standard for Ultra-Shallow Junction (USJ) formation. However, the obtainable junction depth and sheet resistance with these technologies will not meet the requirements for upcoming technology nodes. For RTA-based annealing methods, wafer heating and cooling times are on the order of seconds. Consequently, it is difficult to restrain the thermal diffusion of dopants, particularly the diffusion of boron. In addition, the sheet resistance is limited by the thermal equilibrium solid solubility of dopants. Various methods to form USJs have been reported as alternatives, including atomic layer doping, plasma doping, flash lamp annealing and laser annealing. These methods have their own advantages and disadvantages. For example, the junction depth obtainable by atomic layer doping satisfies the demands of some of the next generation devices; however, its sheet resistance is too high for practical applications. Laser annealing combined with low-energy ion implantation has been reported to be an effective method for simultaneously achieving a shallow junction depth and a low sheet resistance. However, for actual production, the following problems still remain; the leakage current caused by residual defects and the difficulties of integration into the device fabrication processes.20 10 1.4 ADVANCED FRONT-END PROCESSES FOR THE 45NM CMOS TECHNOLOGY NODE For the PMOS transistor, co-implantation or cocktail implants of combinations of germanium (Ge), boron (B), fluorine (F) and carbon (C) have been shown to hold great promise in improving the abruptness, increasing the activation and limiting the diffusion of boron. For the 45 nm technology, diffusion-less anneal will be paramount. One potential annealing technology is Laser Thermal Processing (LTP). The specific implant conditions will also need to be examined in order to achieve the maximum improvement as RTP-based solutions are not necessarily transferable. The results of a recent experiment for a millisecond laser anneal are shown in figures 1.3 (a) and (b) for 1c1015cm  2   and 2c1015cm 2, 500eV B implants. The sheet resistance decreases with higher Ge+ PAI implant energy, from 790 Ω/sq at 2 keV Ge+ to   548 Ω/sq at 10 keV Ge+ for a boron dose of 1c1015 cm 2. The Ge+ dose has little effect on the measured sheet resistance, as the data for 5keV Ge+ in Fig. b demonstrate. This is because for the Ge energies used here, the amorphous layer thickness does not increase   significantly above a dose of 1c1015 cm 2. The weak sensitivity to dose and the fact that the two B doses have different optimum pre-amorphization conditions (see Fig. b) point to the importance of the relative positioning of both implants. 11 The lowest Rs is now produced using a 10 keV Ge+ pre-amorphization implant, different from RTP anneal. This is because the damage evolution is very different between a regular RTP and a millisecond anneal. For the latter, all the implant damage beyond the amorphous/crystalline interface remains. It is well known that this damage can prevent activation through clustering or even de-activate B. For a deeper amorphous layer, the damage overlap with the B profile will be reduced, thus leading to better activation and lower Rs. Fig. 1.3: (a) Sheet resistance as a function of Ge pre-amorphisation implant energy. (b) Sheet resistance as a function of Ge implant dose for 5 keV Ge with 500 eV B implants at 1 ✁ 1015 cm✂2 and 2 ✁ 1015 cm✂2 . 21 Fig. 1.4 shows the B and Ge SIMS profiles for three different conditions: (A) 10keV Ge+ at 1c1015 cm-2 with 0.5 keV B+ at 1c1015 cm-2, (B) 2keV Ge+ at 5c1014 cm-2 with 0.5 keV B+ at 1c1015 cm-2, 12 (C) 10keV Ge+ at 2c1015 cm-2 with 0.5 keV B+ at 2c1015 cm-2. Compared to the 2keV Ge profile, the 10keV Ge gives a significantly improved abruptness, 2.3 nm/dec for the highest B and Ge doses (case C). With Xj < 17 nm, Rs < 760 Ω/sq and abruptness [...]... causes a great challenge in maintaining the integrity of the strained-Si layer during shallow- melt laser annealing They have shown that non-melt laser annealing produces dopant profiles of negligible diffusion and improved activation in the strained-Si/SiGe substrate From their work, it can be concluded that non-melt laser annealing is an attractive process in the dopant activation of thermally less conductive... objective of this work is to study the activation of p-type ultra- shallow implants on blanket Si (100) by submelt laser annealing while making use of the fact that the melting point of the SiGe alloy is lower than that of c-Si, thus giving a process window within which the melt depth and hence junction depth is controlled by the thickness of the SiGe layer instead of the laser fluence The application of sub-melt... depth are independent (within limits) of the laser fluence This makes use of the fact that the melting point of the SiGe alloy is lower than that of c-Si, thus giving a process window within which the melt depth is controlled by the thickness of the SiGe layer instead of the laser fluence The main objective is to ensure that the laser annealing should not lead to 18 overmelt of the crystalline region,... is shown that for the formation of ultra- shallow junctions it is essential to combine ELA with ultralow energy ion implantation to avoid producing a deep tail of active boron, preventing the possibility to form abrupt and ultra- shallow junctions The impact of sub-melt laser annealing on embedded Si1-x,Gex and source/drain defectivity was studied by E Rosseel et al25, by means of transistor data and... region, and to maintain the dopants within the SiGe epitaxial layer for ultra- shallow junction formation 1.7 THESIS OUTLINE AND ORIGINAL CONTRIBUTIONS The objective of this document is to describe the electrical and physical characteristics of junctions formed by annealing of silicon- germanium layers doped with boron Chapter 1 presents the motivation for the work by reviewing the trends in scaling and highlights... technique since junction depths would then be controlled by the laser fluence instead of the pre-amorphized depth.26, 27 Some work has been carried out to understand and reduce the formation of extended defects that influence boron diffusion in silicon during conventional annealing techniques like RTA However, none of these techniques have been applied to laser annealing. 28 Based on the above results by previous... of combinations of germanium (Ge), boron (B), fluorine (F) and carbon (C) have been shown to hold great promise in improving the abruptness, increasing the activation and limiting the diffusion of boron For the 45 nm technology, diffusion-less anneal will be paramount One potential annealing technology is Laser Thermal Processing (LTP) The specific implant conditions will also need to be examined in. .. After the implantation into the thin SiGe epitaxial layer, laser annealing was carried out using a 248 nm KrF excimer laser with a pulse duration of 23 ns and a 17 repetition rate of 1 Hz, at laser fluence ranging from 0.55 to 0.95 J/cm2 to activate the dopants and create ultra- shallow junctions with low resistance Fig 1.6: Process flow showing various stages of fabrication In this proposed scheme,... FOR LASER ANNEALING The excimer laser used in the experiment is the Novaline 100 Excimer laser at the Singapore Institute of Manufacturing Technology (SIMTECH), which is a 248 nm KrF laser with pulse duration of 23 ns This laser system has a homogenized, uniform and flat-top beam profile and is equipped with a CCD camera for sample alignment and monitoring, and a high resolution (1 µm) xyz moving stage... limited to a shallow amorphized top layer) On the other hand, the dwell time can be drastically reduced and this is done using techniques such as flash and laser annealing where typical dwell times are in the order of msec for flash and < 1 msec for laser annealing In the recent work by K.K Ong et al,22 it is shown that laser annealing has a strong dependence on the thermal conductivity of the substrate ... process windows Laser annealing offers several advantages over conventional processing techniques: 1) the junction depth is defined by the amorphous/crystalline interface Laser annealing, combined... and ultra-shallow junctions The degree of melting is determined by the extent of laser absorption and rate of heat dissipation, which are dependent on the substrate properties Laser annealing. .. correspondingly low sheet resistance values 1.2 EXCIMER LASER ANNEALING Excimer laser annealing (ELA) was thoroughly investigated in the 1970–1980s and is a long known approach for annealing of ion