2.5.2 Knock-in Implantation - A Feasibility Study for the Production of Ultra Shallow Profiles



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2.5.2 Knock-in Implantation - A Feasibility Study for the Production of Ultra Shallow Profiles

Implantation through a screen oxide is known to knock oxygen atoms into the silicon crystal. These oxygen atoms (recoils) build a layer in the crystalline silicon which is rich in oxygen within a few lattice distances. We use this knock-in effect to produce ultra shallow profiles in the sub Å range [Wim91a], [Orl91b], [Sub91].

 

Figure 2.5-4 shows the basic scheme for the knock-in implantation. A layer of a material (target ) which is rich in some dopant species , e.g. borosilicate glass (BSG) with 6% Boron, is deposited on the substrate, e.g. silicon. The ion bombardment with a projected range preferably within the deposited layer kicks out some of the dopants (recoils). Such recoils can be knocked into the substrate directly (Figure 2.5-4 left) or can kick out other dopants (Figure 2.5-4 right). The knocked-in species remain within a thin layer (Å) at the surface of the substrate. The concentrations achieved can be a few of dopant for implantation dose of bombarding species (Figure 2.5-5).

 

The simulations were performed with the Monte Carlo module of PROMIS. The Monte Carlo ion implantation module is able to calculate implantations of any bombarding species into any target which is assumed to be amorphous and may contain species , , ... The trajectories of any recoil can be followed. One bombarding ion is likely to produce several dozen recoils.

We have simulated implantations through glass targets containing a certain percentage of boron, phosphorus and germanium with silicon and germanium as bombarding species. For the practical application we need to know how deep the knocked-in profiles for different recoil species and different bombarding species are, and how we can control the peak concentration and the depth of the knocked-in profiles.

  

The knocked-in amount of dopant increases linearly with the percentage of in the layer and also linearly with the dose of the bombarding species . We have implanted different doses of silicon into a layer of BSG/PSG containing some percentage of boron/phosphorus into a silicon substrate. The dopant concentration at the glass/ interface in Figure 2.5-6 is divided by the implanted silicon dose. Concentrations as high as several can be gained with reasonable silicon doses of .

Figure 2.5-7 shows the dependence of the interface concentration on the implantation energy using as bombarding species for knocked-in boron, phosphorus and germanium profiles. The concentrations are normalized by the implanted silicon dose (.

The knocked-in profiles are very shallow in the silicon substrate. The profiles decrease by two orders of magnitude within a few nanometer from the glass/ interface (depth = ), and decrease more moderate deeper into the substrate. To explain this behavior we would have to analyze energy and angle distribution of the knocked-in ions at the interface.

A typical knocked-in boron profile is a few nanometers shallower than a knocked-in phosphorus profile. Using a target containing boron and phosphorus, e.g., BPSG, in a practicable ratio differential knock-in technique can be applied to produce ultra shallow n-p-n junctions (Figure 2.5-8 left) by just one implantation step.

 

Another application consists of combining the differential knock-in technique with band-gap engineering [Sub91]. While forming the deposited target we might also add germanium together with other dopants. During the implantation process germanium will be knocked-in along with the dopants into silicon. During the subsequent anneal (RTA) the germanium is incorporated into the silicon lattice forming a lattice affecting the transistor performance by the altered band-gap structure. The knock-in technique might be an alternative to the expensive molecular beam epitaxy (MBE) technique.

For effectively influencing the band gap structure we require germanium concentrations of the same order of magnitude as the silicon atomic density . Typical knocked-in germanium concentrations are below , therefore, the knock-in method for band gap engineering is limited to non-glass target layers.

However, the application of glasses as target materials is questionable, since a huge amount of oxygen is knocked into the crystalline silicon substrate (see Figure 2.5-9). For a silicon dose of the simulation shows that the oxygen concentration in the silicon target is as high as within the top few nanometers and for several .

Knocked-in profiles are very shallow and can be controlled in the concentration. Surface concentrations of several can be obtained with reasonable implantation doses. The depth of the profiles is typically in the Å range and cannot be influenced easily. Silicate glasses are not applicable as target material, because of the inacceptable high oxygen concentration.

For the application as knock-in source for ultra shallow profiles other target materials have to be found, which are easily removable from the crystalline silicon. Applications of knock-in techniques where the target material need not be removed for further processing are possible. For instance to improve the adhesive strength and the electrical properties of contacts.

 



next up previous contents
Next: 3 Diffusion Up: 2.5 Applications Previous: 2.5.1 Analytical Ion Implantation



Martin Stiftinger
Wed Oct 19 13:03:34 MET 1994