8.8 Sspd_chapter 6_part 6_mos ics fabrication technology_to be  (Page 3/4)

In this technology, mass is very critical because mass decides the average Gaussian profile of the dopent. For a given dopent we must choose a given isotope and the most abundant isotope of the dopent. Hence mass analyzer is very important. Also all the dopent must be ionic and not neutral. This is because ionic dopent can be counted and quantitative measure of the dose can be maintained. This will help in reproducibility of the dopent profile. So it is very important that neutral atoms are removed from the ion beam. Therefore ion beam undergoes electrostatic deviation from the straight line path before final implantation. This acts as a trap for the neutral dopent atoms which continue on a straight line path.

The implant dose is measured by putting the wafer deep inside a Faraday Cup. This cup collects the ion current and integrates it over time.

Where I = collected beam current, A = implant area, t = the integration time and q is the charge on the ion.

Conventional ion implanter has much in common with linear accelerators. The ion source can be a solid elemental dopent source or it could be in compound gaseous form such as Arsenie, Bromine or Phosphine diluted by 15% Hydrogen. These are ionized either by electrons due to thermoionic emission or due to plasma gas discharge.

The ions are extracted by a grid at 30kV. Next it is passed through a mass analyzer. In mass analyzer it undergoes magneto-static bending as defined by the following formulation:

Where m = mass of the ion; v = velocity of the ion, B = magnetic field density = α.I where I is the current in the magnetic coil.

Energy imparted to the ion while being accelerated through extraction Voltage on the grid: E = q.V _extraction 6.13

Therefore the final velocity of the ion after emerging from the grid is:

(1/2)mv ² = q. V _extraction

Or v = √(2q V _extraction /m) 6.14

From equations 6.12 and 6.14 we get:

From Equation 6.15 we see that ions coming from the resolving aperture will be of mass m decided by the current I flowing through the magnetic coil of the mass analyzer. Thus by tuning the current ‘I’ we can select the ions which will exit the resolving aperture and constitute the beam current.

As seen in Figure 6.41, after the resolving aperture the ions are further accelerated through a voltage of 0 to 200kV. This accelerated beam of ions are smashed into the wafer. Each impacting ion dislodges thousands of Si atoms and damages the Silicon surface. The total number of ions implanted in the Silicon target near the surface are called the Dose of Implant. This dose is given by Equation 6.11. Hence total dose is the product of ion current and the time of implant.

The standard dose of implant is 10 ¹² /cm ² to 10 ¹⁶ /cm ² . Lower or higher dose is possible. By controlling the dose of implant the dopent profile can precisely be tailored according to our need.

The depth of implanted ion is random but for a large dose of implanted ions, the doping profile is a Gaussian Distribution as shown in Figure 6.42. The heavy ions like Antimony has a very shallow penetration while light ions like Boron have a deep penetration.