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Study of Nickel Silicide Processes for Advanced CMOS Applications

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Study of Nickel Silicide Processes for Advanced CMOS Applications Curanovi~, Branislav 2 ~ Annual Microelectronic Engineering Conference, May 2003 Study of Nickel Suicide Processes for Advanced CMOS Applications Branislav Curanovid, Student Member, IEEE PMOS transistors. In general, today’s CMOS processes Abstract—A study has been performed to determine have many uses for silicides, and are predicted by the critical mechanisms involved in formation of nickel International Technology Roadmap for Semiconductors suicides. The process parameters under investigation were (ITRS) [11 to be very important and relevant for several silicidation temperature, presilicide N5~ implant, and the years into the future. presence of a titanium capping layer. Contact sheet The choice of metals for suicide formation is resistance phosphorous-implanted silicon was measured and determined to degrade with high silicidation becoming crucial, as certain metals have started to temperature. Titanium capping was found to improve approach physical limits associated with their properties. contact resistance and compensate for the effects of high There are three metals that are most frequently used in temperature treatment for polycides. The nitrogen modern suicide formation: titanium, cobalt, and nickel. incorporation via implant shows a degradation in Titanium suicide (TiSi2) has been a long-time resistivity for both silicides and polycides. dominant suicide technology, but has reached extinction in modern deep sub-micrometer CMOS applications. Index Terms—Ni silicidation, nitrogen implant, RTP Although it offers low resistivity when properly formed, TiSi2 exhibits several critical failures. One such failure is the silicon-diffusion dominated formation of silicides I. INTRODUCTION that can lead to problems of bridging. Secondly, TiSi2 exhibits a strong narrow-line effect due to incomplete S microelectronicILICIDES haveindustrybeen usedto forcircumventmany yearsthe inhighthe transition from a high-resistivity C49 phase to the low- resistivity seen when contacting metal to poly gates and resistivity C50 phase. The narrow-line effect has source/drain regions of conventional MOSFET necessitated the elimination of TiSi2 from sub-O.25ftm structures. The suicide process involves incorporation of CMOS technology. Cobalt silicide (CoSi2) has replaced metal into silicon being contacted to reduce the TiSi2 as the pre-eminent contact technology in most resistivity of the contact. Different metals are chosen to advanced CMOS applications. Cobalt suicide offers create suicides based on their properties such as many advantages over TiSi2, most importantly avoiding processing temperature, dry etch capabilities, wet etch narrow-line effects and bridging problems. However, chemicals necessary, resistivity, dominant diffuser species, etc. CoSi2 is also highly sensitive to ambient contamination The use of suicides on polysilicon gates (resulting in during silicidation, and exhibits a high consumption rate of silicon. “polycides”) and S/D contacts (self-aligned silicides — “salicides”) has shown to be necessary in small (sub- To remedy deficiencies of CoSi2, nickel silicide micrometer) devices that eventually become limited in (NiSi) has emerged as a candidate for most advanced their performance by the resistance seen at the silicide applications. Nickel silicide offers a lower source/drain and the delay seen at the gate. Other very consumption rate of silicon, no narrow-line effects, and important potential uses of suicides have presented generally low resistivity (often reported in —15-18 themselves when considering dual-gate devices, when i.i≤2cm2 range) [2]. However, the detriments of NiSi contact between n+ and p+ polysilicon gates is include high sensitivity to ambient contamination and necessary. Suicides are essentially an enabling poor thermal stability [3]. These problems must be technology in this case, as their use prevents the investigated in order to advance the science formation of weak diodes between the NMOS and This study investigates the properties and mechanisms of NiSi formation. Silicidation is performed on doped This work is part of capstone design requirement for a B.S. degree crystalline silicon and doped polysilicon to simulate the in Microelectronic Engineering at the Rochester Institute of effects of silicidation on source/drain (S/D) and gate Technology (RIT), Rochester, NY. The results of the project were first presented as part of the 21” Annual Microelectronic Engineering CMOS regions, respectively. Suicide formed on Conference, May 2003 at RIT. crystalline silicon is used to determine the specific B. Curanovic is with the Microelectronic Engineering Department, contact resistivity (Pc) using the transfer length method RIT, (e-mail Branisla~’.Curanovic(~iicec.org) (TLM). Polycide is formed on a blanket polysilicon 92 Curanovid, Branislav 2l°~ Annual Microelectronic Engineering Conference, May 2003 film, and its sheet resistance is measured using 4-point to the substrate, or are formed by chemical means probe methods. Experiment parameters are a presilicide (during cleans, for example). During silicidation, the N2+ implant (expected to improve suicide thermal diffusing metal ions may penetrate the interfacial oxide stability), anneal temperature (expected to affect and begin silicide formation below the oxide layer. resistivity), and presence of a capping layer during However, presence of ambient contaminants (most suicide formation (expected to decrease the sensitivity notably oxygen and steam) during silicidation to ambient contamination). encourages growth of Si02 on top of the silicide, eventually blocking the suicide process and leading to II. THEORY formation of metal oxides, To combat this process, capping layers such as titanium (Ti) or titanium nitride A. Nickel (TiN) are employed. These capping layers are deposited Properties of nickel are shown in Table 1: immediately following the metal used in silicidation (Ni) and are removed following the silicide formation. The TABLE I cap acts to either react with oxygen in the ambient (as (T).1I°PAT NTCIPT PUnPPRTTFc Ti) or to passively block oxygen diffusion (as TiN). Capping layers are commonly used in CoSi2 formation, Property Value and do not greatly complicate processing. Atomic number 28 3) Self-aligned Silicidation: As is the case with other Atomic weight 58.6934 amu metals, nickel will only react with silicon, leaving Density of bulk solid 8.908 g cm~3 Melting point 1455°C materials such as SiO2 or silicon nitride (Si3N4) intact. Boiling point 2913°C This allows formation of nickel salicides — self-aligned Thermal conductivity @ 25 °C 91 W m~ K” silicides, formed only in the regions where nickel and Electrical resistivity @ 25 °C 7 ~iD cm silicon (single-crystal or polycrystalline) are in direct Pauling electronegativity 1.88 Pauling units contact. Nickel, along with any capping layers, can be removed from regions where silicide is not formed (such B. Nickel Suicide (NiSi) Formation as transistor isolation) by an etch in H2S04:H2O2 1:2 Silicides in general are formed by placing metals mixture (also known as Piranha etch). The Piranha etch (such as tungsten, tantalum, titanium, cobalt, nickel) in does not remove any suicides formed, and properties of direct contact with silicon, followed by a high- the silicide remain unchanged. temperature heat treatment. Often performed as rapid C. Resistivity Measurements thermal processing (RTP), the heat treatment creates a The relative success of silicidation can be determined metal/semiconductor compound of varying physical by measuring the sheet resistance (R5) of polycided properties. gates and the specific contact resistivity of contact The properties of a silicide film are heavily dependent regions. The measurements techniques used in this study on processing conditions. As such, there are several are the transfer length method (TLM) for contact important aspects to formation of nickel silic ides: resistivity and four-point probe method for sheet 1) Temperature of Silicidation: Nickel monosilicide resistance. (NiSi) has been identified [4j to form at temperatures as low as —400 °C. This relatively low temperature of formation is beneficial in decreasing the thermal budget of a CMOS process. However, the distinction of nickel suicide phase being formed is crucial — at approximately 750 °C and above, a higher-resistivity (-‘-50 jii2cm2) nickel disilicide (NiSi2) phase forms. The precursor to formation of NiSi2 is silicon agglomeration in the NiSi Lacing beFLI~en pont~cts11111~clj thro~.1gh p4). film, occurring at temperatures of —-700 °C. Both silicon jLId~l I d2 I I d3 I I d4 I agglomeration and formation of disilicide result in 1) Transfer Length Method: Originally proposed by increased resistivity and degraded device performance. Shockley [5), a TLM test structure (Fig. I) consists of Because of this problem, thermal stability of nickel several contacts of unequal spacing. monosilicide is regarded as a primary roadblock in By applying a voltage bias across the contact pads adaptation of NiSi to modern CMOS devices. and the resulting current, the total resistance is used to 2) Surface and Ambient Contamination: Silicide determine contact resistance (Rc), as shown in Fig. 2: formation is slowed down or even blocked by any silicon oxides (SiO2) located at the metal/silicon interface. These interface oxides are often films native 93 Curanovi~, Branislav 21°’ Annual Microelectronic Engineering Conference, May 2003 substrate using a shadow mask. The silicon wafer was RT then cleaved into six pieces that were
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