Radiant energy – Ion generation – Field ionization type
Patent
1987-08-12
1988-09-27
Anderson, Bruce C.
Radiant energy
Ion generation
Field ionization type
3133591, 31511181, H01J 105
Patent
active
047744146
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a liquid metal ion source suitable as an ion source for a maskless ion implanter, a micro-zone secondary ion mass spectrometer, a micro-zone deposition apparatus or the like. More particularly, the present invention is concerned with a liquid metal ion source suitable for stably extracting ions of at least one element selected from the group consisting of boron (B), phosphorus (P) and arsenic (As) for a long period of time.
BACKGROUND ART
In recent years, a liquid metal ion source has attracted attention, because it can emit an ion beam having a high brightness and a fine diameter of the order of submicrons, which provide a possibility that lithography, doping (implantation), etching, etc. involved in semiconductor processes can be conducted without the use of any mask (i.e., by the maskless method) which has conventionally been required or without resort to any chemical means.
The liquid metal ion source operates according to the following principle. First, a source material (liquid metal) which has been melted by means of resistance heating, electron bombardment, laser radiation or the like is fed to an emitter made of a high-melting material such as tungsten (W), molybdenum (Mo), tantalum (Ta) or silicon carbide (SiC) and having a sharply pointed tip. Application of a negative high voltage to an extraction electrode brings about concentration of an electric field at the tip of the emitter. When a high voltage is further applied and reaches a certain threshold value, the liquid metal located at the tip of the emitter forms a conical protrusion called Taylor Cone, leading to an extraction of ions from the tip.
When such a liquid metal ion source is intended for use in various fields, it is an important requisite that the liquid metal ion source can stably emit an intended ion beam for a long period of time.
Meanwhile, among n-type impurities for silicon semiconductors, the most important elements are arsenic (As) and phosphorus (P) while boron (B) is important with respect to p-type impurities. Phosphorus in the form of a simple substance has a melting point of 44.1.degree. C., and the vapor pressure of P.sub.4 at that temperature is as high as about 24 Pa, which makes it difficult to use phosphorus in the form of a simple substance as a source material for a liquid metal ion source. Similarly, arsenic in the form of a simple substance cannot be used as a ion source, because arsenic in the form of a simple substance has a melting point of 817.degree. C. while its vapor pressure at that temperature is as high as 3.6.times.10.sup.6 Pa. Further, boron in the form of a simple substance is also unsuited as a source material because of its high melting point of about 2400.degree. C.
When an element in a simple substance form which emits an intended ion has a high vapor pressure or a high melting point as mentioned above, the intended element must be converted into an alloy or compound in combination with other elements in order to reduce the above-mentioned difficulties, and the alloy or compound is used as a source material. When the alloy or compound is used as a source material, the emitted ions contain ions of other elements and ions of molecules in combination with other elements besides the intended ion. In such a case, an effectively employed method is one in which a mass spectrometer is provided after the ion source to obtain only the intended ion. In fact, such a method has often been used conventionally. For example, when emission of silicon (Si) ions from a liquid metal ion source is intended, silicon is used as the source material not in the form of a simple substance having a melting point of about 1420.degree. C. but in the form of an alloy thereof with gold (Au), i.e., Au-Si. The melting point of the alloy Au-Si in a eutectic composition form is about 370.degree. C., i.e., much lower than that of silicon. The lowering in melting point advantageously contributes to reduction in electric power consumed during melting as well as reduction in frequency of
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Aida Toshiyuki
Ishitani Tohru
Tamura Hifumi
Umemura Kaoru
Anderson Bruce C.
Hitachi , Ltd.
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