Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2003-11-07
2004-11-16
Wells, Nikita (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492210, C250S42300F, C250S425000
Reexamination Certificate
active
06818909
ABSTRACT:
This application is a division of application Ser. No. 10/307,499, filed Dec. 2, 2002, now U.S. Pat. No. 6,686,601 the priority of which is hereby claimed.
The present invention is concerned with ion implantation apparatus and particularly with ion sources for such apparatus.
In the manufacture of semiconductor devices and integrated circuits it is necessary to modify the semiconductor substrate material (particularly silicon) by diffusing or implanting therein atoms or molecules of selected dopants to produce regions in the semiconductor substrate of selected varying conductivity and having majority charge carriers of different polarities.
Accuracy and control of the doping of the semiconductor substrate using ion implantation has become increasingly important with the continuing reduction in feature sizes of integrated circuit structures.
Typical dopant materials used in this process are boron, phosphorus, arsenic and antimony, but there is also a requirement for implanting of other ions, including, for example, as well as erbium, magnesium, indium and refractory metal ions, and aluminium ions into such substrates to produce faster switching for bipolar transistors.
As to aluminium ion implantation, this is evidenced by, for example, U.S. Pat. No. 4,999,309 in which implantation and diffusion of aluminium ions into silicon monolithic integrated circuits is described to form high performance PNP transistors and PN junction capacitors.
Various apparatuses have been devised and various methods and processes have been carried out to produce and implant aluminium ions.
One such method of producing aluminium ions has used aluminium chloride (AlCl
3
) as the source material for the aluminium ions, and this has been successful in producing a high beam current; furthermore the life of the source has been found to be as much as between 50 and 70 hours before the source requires replacement or replenishment. However, it has also been found that use of aluminium chloride does result in extended periods, beyond what is acceptable, for both tuning the ion source initially and subsequently servicing the ion source. Tuning times have been found to be typically more than one hour, while downtime due to the need to carry out servicing of the ion source (due to the hygroscopic nature of aluminium chloride) has been of the order of four hours. Such ‘downtimes’ are costly in terms of loss of production time and are perceived as unacceptable. The presence of water due to the fact that aluminium chloride is hygroscopic also significantly loads the vacuum system of an implanter.
Another method of producing aluminium ions has involved the use of solid aluminium oxide with silicon tetrafluoride being used as a reactant gas. In carrying out this method, silicon tetrafluoride is passed over heated aluminium oxide and a plasma is created in an ionization chamber to dissociate and ionize the products of the reaction, and, in so doing, produce the aluminium ions. While it has been found that the tuning time for such a procedure was acceptable (of the order of five minutes), it has also been found that the source life was less than adequate (i.e. less than twenty hours), and that there was significant flaking exhibited on the walls of the ionization chamber and significant erosion of the filament cathode, typically formed of tungsten.
More importantly, it has also been found by experiment that silicon ions formed during the ionization process contaminate the beam due to the presence of ions of Si
14
28
which gives a charge/mass ratio and mass which is close to that of the aluminium ions. This can lead to incorrect dosage of Al
13
27
ions being measured due to the similarity of the charge/mass ratio of the aluminium and silicon ions; the Si
14
28
ions are, in some situations, not effectively filtered by the mass analyzer/selector of the ion implantation apparatus and the presence of the silicon ions therefore contributes to the overall measured current delivered to the substrate, thus, while contributing to the delivered ion current, providing a false reading of the aluminium ions actually delivered to the substrate. Once implanted into the silicon substrate, silicon ions are no longer detectable but disrupt the proper distribution of implanted Al ions. The mass resolution capability of the ion implanter can be increased to reduce this effect. However, increase in mass resolution reduces the useful current of aluminium ions to a level rendering the useable ion beam insufficient for volume production of substrates.
Thus, known processes for implanting aluminium ions into semiconductor substrates suffer from a number of drawbacks.
In consequence, it is perceived that there is a need to provide an improved source of and method of supply of aluminium ions. Additionally, it is desirable to achieve this with increased source life and reduced servicing requirement and particularly service downtime.
Interaction of nitrogen trifluoride (NF
3
) with solid aluminium oxide (Al
2
O
3
) and solid aluminium nitride (AlN) under specific controlled conditions has been found and reported to produce a significantly increased yield of aluminium ions coupled with reduced start-up or tuning times, less contamination, and longer running times between shorter service intervals than has been exhibited in the prior art.
Nitrogen trifluoride has been used in a number of applications in the manufacture of semiconductors.
One use of nitrogen trifluoride has been previously suggested in U.S. Pat. No. 5,700,580 for the purpose of forming a nitride spacer over an underlying oxide layer and etching the nitride layer using an atmosphere of ionized fluorocarbon followed by a further step in which an atmosphere of NF
3
is combined with an ionized halogen containing compound.
The use of nitrogen trifluoride has also been suggested in ion implantation technology.
In U.S. Pat. No. 5,073,507, a process of producing beryllium ions and beryllium fluoride ions is disclosed in which boron trifluoride and beryllium are ionized in an ionization chamber to produce the required ions. In this disclosure, nitrogen trifluoride is suggested inter alia as an alternative to boron trifluoride.
In U.S. Pat. No. 5,707,424, there is disclosed an adsorption/desorption system for storing gases in which there is disclosed a range of storage media, which may include inter alia alumina, which may be supplied in porous form for storing a range of gases, including inter alia nitrogen trifluoride.
Nitrogen trifluoride has also been proposed as a medium for cleaning plasma chambers. In U.S. Pat. No. 5,620,526, there is disclosed a process for cleaning of a plasma chamber in which nitrogen trifluoride plasma is firstly passed at a first pressure through a plasma chamber to scour any oxide material that has been previously deposited on walls of the chamber and a second treatment plasma medium is then passed at a lower pressure through the chamber to further scour the walls. In the second stage, nitrogen trifluoride plasma is inter alia one of the gases that may be used. A final, third, stage of cleaning the chamber is undertaken using nitrogen trifluoride plasma.
In European Patent Application EP 0945892 A2, there is disclosed a method for in-process cleaning of an ion source during its operation, using a cleaning gas (such as nitrogen trifluoride) with a source of ionisable dopant gas such as phosphine (PH
3
) and arsine (AsH
3
). The cleaning gas dissociates to scour the plasma chamber of any phosphorus or arsenic deposited on the chamber walls and to reduce the likelihood of such deposits being formed. The nitrogen trifluoride dissociates into its respective ions to combine with the phosphorus or arsenic ions and the positive ions are then accelerated toward a mass analyzer magnet of the implantation system whereat ions having an inappropriate charge-to-mass ratio are blocked from proceeding further.
Use of nitrogen trifluoride as a reactive or etchant gas has also been suggested in U.S. Pat. No. 6,001,172 wherein a process and system are disclosed for producing dopa
Allen Andrew
Banks Peter Michael
Clarke Neil L.
Dobson Matthew Peter
Murrell Adrian
Dykema Gossett PLLC
Hughes James P.
Tennant Boult Wade
Wells Nikita
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