Salicide technology

The incorporation of metal silicides is quite necessary as we meet the basic requirements for contact metallization: low specific resistivity, low contact resistivity to both n- and p- type silicon, high thermal stability and good processibility. For the various generations of MOS devices, the contacts technology have evolved from the so-called POLYCIDE (it means silicdes deposited on the poly-Si) to today’s standard salicides (self-salign silicided).

A salicide structure consists of a metal silicide formed atop the lines of polysilicon that make up the gates and the local interconnects, It is also form on the source/drain regions. The salicide on the source/drain regions can reduce the sheet resistance of the path between the metal contact and the channel edge. The salicide technology has an advantage that it can be formed without any extra mask. A salicide process is executed as follows [2]:

1. The metal which used to form the silicide is deposited on silicon.

2. The wafer is heated and the metal reacted with silicon to form the silicide

3. The unreacted metal is selectively removed by using an etchant that does not attract the silicide, the silicon substrate, and the SiO2. Then, the salicide process is completed.

Among the metal silicides, TiSi2, CoSi2, and NiSi have been widely studied due to their good thermal stability and low electrical resistivity. TiSi₂ has been the most studied due to its wide usage for CMOS metallization. CoSi2 began to replace the TiSi2 at a gate length of 0.25 μm and below. This replacement is due to increasing difficulties in forming low-resistivity TiSi2 as the dimension grew smaller. The CoSi2 technology is mature by now, however, it is anticipated that NiSi will take over for CMOS technologies when gate length at 70 nm and below.

1.1.1.1 Titanium Silicide

TiSi₂ is known to occur in two different crystallographic structures, C49 and C54 TiSi₂, and latter is a stable phase. Therefore, C54 TiSi2 is desired for the electronic devices, not only due to its stability, but also due to the fourfold lower resistivity than C49 TiSi₂, the resistivity of C49 TiSi2 is around 60-80 μohm-cm, and that of C54 TiSi2 is only around 15-20μ ohm-cm [2]. TiSi₂ can reduce native oxide layers, making the TiSi₂-contacted shallow junction exhibits low contact resistance. When it is used being the gate electrode, it is an effective getter for the hydrogen atoms, and this improves the hot-electron reliability [3].

However, some drawbacks of Ti have been reported that Ti may react with the implanted dopant to form the metal compounds, and this will prevent the shallow junction formation [4].

The grain size of TiSi2 has to be larger than 0.2μm to achieved lower resistance. When the gate line width is smaller than 0.2μm, TiSi₂ application would be in trouble. And TiSi₂ has bridge effect (as show in Fig 1.3), the creep-up phenomenon during the formation of TiSi2

between the gate and the source/drain during annealing process, causing the device failure [5].

1.1.1.2 Cobalt Silicide

CoSi₂ is another attractive material for salicide process. The cobalt (Co) is transformed to CoSi2 by a two step process. Co and Si are reacted to form CoSi at 450℃ first. After removing the unreacted Co, a second thermal step at 700℃ is performed for the CoSi₂ formation. The CoSi2 offers the benefits of lower resistivity, lower thermal budget, and higher thermal stability than TiSi₂. Furthermore, bridging effect is not a problem for CoSi₂. But it still has some disadvantages. Cobalt is very reactive and forms cobalt oxide easily when it contacts with air or moisture, so it often needs passivation layer. And the formation of CoSi₂ consumes a relatively larger amount of Si to achieve an equivalent silicide sheet resistance than other silicides, as discussed in [3].

1.1.1.3 Nickel Silicide

In recent years, NiSi is another promising candidate to replace the CoSi₂ for the contact metal in submicron device applications. [6], some main advantages of NiSi are listed as follows:

1. NiSi has a lower formation temperature than TiSi2 or CoSi2. 2. NiSi has a low electrical resistivity.

3. NiSi has a low Si consumption.

4. NiSi also doesn’t have the creep-up phenomenon [7].

Because of the high solubility of Ni in Si, NiSi can be formed at low temperature(~400℃)[8]

and low activation energy of about 1.5 eV [9]. NiSi has a slightly lower resistivity (10-15 μ ohm-cm) than both C54 TiSi2 and CoSi2 (15-20 μohm-cm) have. There are two reasons contribute to the lower consumption of Si. First, the Si density of NiSi is lower than that of CoSi2, and this attribute makes the reduction for Si consumption more then 10%. Second, because the resistivity of NiSi is lower than that of CoSi₂, for the same sheet resistance, the thickness of NiSi film can be thinner. NiSi can be formed at narrower poly lines than cobalt silicide is, and it doesn’t show a resistance increasing for narrow lines. All in all, NiSi film has the properties of low NiSi/Si contact resistance, wide process window (for example, even for a narrow Si line down to 100 nm width, NiSi can be formed at low annealing temperature 400℃ for 30 s [10]), and low film stress [11]. The main hurdle in using NiSi for contact metallization and local interconnection has been the poor thermal stability, if the NiSi suffered above 650℃, NiSi film might agglomerate which would produce leakage current [12].

Fortunately, the annealing temperature we adopt are below 650℃, the agglomerating problem of NiSi film can be ignored. Because of above reasons, NiSi film is the most suitable materials for our study. The comparison of the characterization of the Ti, Co and Ni is listed at Table 1.1.