Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into... – Ion implantation of dopant into semiconductor region
Reexamination Certificate
1997-06-16
2001-03-13
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
Ion implantation of dopant into semiconductor region
C438S527000
Reexamination Certificate
active
06200883
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of ion implantation.
DISCUSSION OF PRIOR ART
Ion implantation is one of the standard processes employed in the manufacture of integrated circuit devices to modify the electrical properties of defined regions of a substrate of semiconductor material by doping these regions with a selected concentration of impurity atoms. The technique involves generating a beam of ions containing a preselected atomic specie and directing the beam towards a target substrate. The depth of implant of the atomic specie depends inter alia on the energy of the specie at implantation which is in turn dependent on the ion beam energy.
There is an increasing demand for ion implantation to be conducted at lower implant energies so that junctions between regions of different conductivity type can be formed in the semiconductor substrate at shallow depths below the substrate surface. Such shallow junctions require implantation at implant energies of less than 10 keV and sometimes less than 2 keV. An implanter which can operate down to low implant energies is the Applied Materials Precision Implant xR80 described in British Patent Specification No. 2307095.
It is important, when forming junctions between regions of different conductivity type in semiconductor wafers that the junctions are formed at a predictable depth beneath the surface of the wafer. It is also often desirable for the concentration of dopant atoms in the region above the junction to be relatively uniform, and to fall away as sharply as possible at the junction itself.
A problem with ion implantation techniques is that a proportion of the ions entering the substrate at the desired implant energy penetrate into the substrate to a depth greater than would be expected from the calculations of the probability of experiencing a collision with an atom of the substrate material. This phenomenon is believed to arise because some ions entering the substrate are “channelled” along axes and planes of symmetry in the crystal. This channelling effect can produce significant concentration levels of the dopant specie beyond the intended junction depth so that the effective junction depth is greater than intended.
FIG. 1
of the accompanying drawings illustrates the effect. This Figure is a graphical representation of the atomic concentration of a desired dopant (here boron) against depth below the surface of the substrate. In the absence of channelling, a junction would be formed at a depth d. However, because of channelling, boron atoms extend in a “tail”
10
beyond depth d so that the actual junction is at depth d+&Dgr;d.
This problem is especially significant if it is intended to form the junction at a relatively shallow depth using low implant energies, as the extent of channelling is greater at lower energies.
One prior art method of reducing the effect of channelling on the concentration profile of the dopant ion is to perform a pre-implant of relatively higher energy silicon ions. This silicon pre-implant has the effect of amorphising the crystalline silicon of the substrate at and beneath the surface of the substrate so that channelling is reduced during subsequent implanting of dopant ions. The energy of the silicon pre-implant is chosen so that the damaged region
11
caused by the implanted silicon extends beyond the intended junction depth to minimise damage in the doped region above the junction.
A problem of this procedure is, however, that the damaged region
11
extending into the region of the substrate beneath the junction, cannot be fully repaired by the subsequent heat treatments of the substrate, for dopant activation, so that some residual damage from the silicon pre-implant remains which can reduce the performance of the resulting devices. Also, the need to conduct a pre-implant with a different material significantly reduces the productivity of the implant machine.
SUMMARY OF THE INVENTION
The present invention provides a method of implanting ions containing a selected atomic specie through a surface of a semiconductor substrate to form a junction between regions of different conductivity type in the substrate at a desired depth below said surface, comprising the steps of selecting in accordance with said desired depth a maximum implant energy of said selected atomic specie and the total dose of said atomic specie to achieve a desired concentration of said specie implanted in the region in front of said junction, and implanting a part of said dose with said atomic specie at below said maximum implant energy before completing said dose at said maximum energy.
With this procedure part of the intended dose of dopant ions is first implanted at a relatively lower energy at which any channelling tail would not extend beyond the junction depth. This initial part of the dose at low energy does, however, cause amorphising of the surface layer of the substrate. As a result, when the dose is completed at the full implant energy, to achieve the desired junction depth, these higher energy ions are implanted through an already amorphised zone so that their channelling probability is very much reduced.
The effect of this procedure is to reduce greatly the channelling tails of implanted ion concentration profiles so that shallow junctions can be formed more reliably. Importantly, no damage to the substrate is caused outside the intended target region.
The required dose may be implanted in at least two steps at different implant energies for the atomic specie. In fact, three or more discreet energy steps may be employed with the energy of each step increasing to the maximum energy for the final step.
Instead, the implant energy may be progressively increased to said maximum, either by continuously increasing the implant energy as the implant progresses, or by incrementing the energy in a large number of relatively small steps.
Preferably, at least 5% of said dose is implanted initially at less than 50% of said maximum energy. In fact, between 5% and 20% of said dose may be implanted initially at less than 50%, and preferably less than 30%, of said maximum energy.
Up to 50% of said dose may be implanted initially at up to 70% of said maximum energy.
The procedure is especially useful for relatively low energy implants, for example where said maximum energy is no greater than 10 keV, or in some cases no greater than 5 keV.
Apart from selecting the energies at which the dose implanted to reduce the number of implanted ions penetrating in the substrate beyond said desired depth, the distribution of energies may also be selected to produce a profile of the concentration of implanted ions in a region of the substrate in front of the junction which is closer to a desired profile. Typically the desired profile may be a concentration which is uniform with depth.
The invention may also be regarded as a method of forming by ion implantation in a semiconductor substrate a junction between regions of different conductivity at a desired depth below a surface of said substrate, comprising implanting ions through said substrate surface at a first energy to amorphise said semiconductor material beneath said surface in front of the desired junction depth and then implanting ions at a higher energy through said amorphised semiconductor.
The energy of implantation of the selected atomic specie may be altered by adjusting the acceleration or deceleration parameters of the implanter so that the same ion beam is used at different final energies.
However, said part of said dose may be implanted by implanting ions having a higher mass to charge ratio than ions implanted to complete said dose. Then, said higher mass to charge ratio ions may be implanted at the same energy as said ions implanted to complete the dose. For example boron may first be implanted using a beam of BF
2
+
ions and subsequently changing to B
+
ions.
In another aspect, the invention provides a method of implanting ions containing atomic species of a selected conductivity doping type through a surface of a semiconductor substra
Adibi Babak
Foad Majeed Ali
Taylor Mitchell C.
Applied Materials Inc.
Mulpuri Savitri
Tennant Boult Wade
LandOfFree
Ion implantation method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ion implantation method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ion implantation method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2467457