Abstract: Recent advances in ion implantation

Abstract

The progress in solid state technology is largely determined by our ability to control and modify certain materials parameters in a well defined manner. Semiconductors are a typical example, where minute traces of impurities in the order of parts per million and less will influence the electrical properties markedly.The concept of introducing such dopants into semiconductors by means of high energetic particles was discussed many years ago. In 1954, Shockley1 submitted a patent describing the ’’Forming of Semiconductor Devices by Ionic Bombardment.’’ As the semiconductor technology matured, developing shallower and shallower device structures, the control aspect became vitally important. And it is precisely in the field of control that ion implantation offers major advantages over standard diffusion technologies, e.g.,(a) Accurate control of the total amount of impurity transferred to the wafer by measurement of the accumulated charge during implantations.(b) A high degree of areal uniformity across the wafer obtained by mechanical or electrical scanning (uniformity better than 1%).(c) Accurate control of the depth distribution by a well defined accelerating potential. Thus a wide range of dopant profiles for device configurations is possible.In contrast to standard diffusion profiles, which are frequently close to error‐function types (with the maximum concentration at the surface), the impurity profiles in implanted structures are of full‐Gaussian or truncated‐Gaussian type with the maximum concentration at a mean projected range RP and with a standard deviation ΔRP. Substantial progress has been made in recent years in understanding the physical phenomena concerning the interaction of high‐energy particles with target atoms. Investigations by various authors2–6 have given rise to theoretical models enabling one to make reasonably accurate predictions of implant profiles as a function of ion species, target material, implant energy, and dose. These models also permit some estimate of the radiation damage and its distribution. The radiation damage is sometimes a less desirable side effect of this new technology and, in most applications, requires post‐implant annealing. This effect is strongly dependent on species, dose, energy, and certain process details. A typical example is discussed by Mader and Michel7 for high‐dose As implantations into Si as required for emitter, subcollector, or source–drain application. In these experiments, As was predeposited into a shallow surface layer (∠500 Å) by an implant step and subsequently diffused into the Si. Pronounced differences in the resulting defect patterns are found for implants into bare silicon and through screen silicon oxide films. In the latter case, oxygen atoms are injected into the silicon by knock‐on effects between the high energetic As projectiles and the oxygen. The high‐temperature heat treatment8,9 serves a multiple purpose: to ’’activate’’ the implanted species electrically, that is to permit impurities like As, B, or P to be incorporated into substitutional lattice sites; to anneal out the radiation damage; and, in special cases, even to diffuse the impurities away from a residual radiation‐damaged zone deeper into the bulk region, a consideration particularly important for emitter processes in bipolar devices. Excellent electrical junction characteristics have been obtained under such conditions.10Ion implantation is expected to be strongly directional and to result in considerably less lateral spread than standard diffusions in a planar technology. Furukawa et al.11 have studied these effects theoretically, and experiments by Pan and Fang12 have confirmed their calculations on narrow‐gate field effect transistor (FET) devices by use of self‐alignment concepts.In fact, it was in the area of FET’s that ion implantation was first used successfully to manufacture active silicon devices. Since they require low doping concentrations and shallow distributions, FET’s are ideally suited to take advantage of the features of ion implantation.Ion implantation opened up the possibility of selective channel‐doping and individual threshold adjust, thus enabling one to build enhancement and depletion devices on the same chip. It further permits the reduction of parasitic capacitances, increasing the speed performance of FET’s to the point where they can compete favorably with bipolar transistors. Fang and Crowder13 have reported on a microwave FET structure, which has been operated up to 14 GHz. By combining an n‐channel enhancement‐driver device with a depletion‐load FET, Fang and Rupprecht14 have measured turn‐on delays as low as 115 ps per stage for an 11‐stage ring oscillator.It was mentioned before that ion implantation is normally accompanied by radiation damage. Some examples follow to illustrate how, in certain cases, these effects can be utilized beneficially. For many years, FET devices have been known to be very susceptible to the gate insulator integrity. Large threshold variations can be measured because of fast surface states at the insulator–silicon interface, or because of mobile charges (e.g., Na ions). Normally, the surface‐state density can be effectively reduced by a hydrogen annealing step. In the presence of a silicon nitride–silicon oxide gate structure, relatively high temperatures are required to diffuse the hydrogen through the nitride layer. Various workers in the field therefore suggested the use of implantation. An interesting side effect, according to Ku,15 was the simultaneous reduction in mobile ions. The present assumption is that the mobile ions become pinned on traps in the silicon oxide, which are created by the radiation damage rather than by a chemical effect. This model is supported by the observations of Goetzberger,16 who found a similar phenomenon by implanting sodium directly into the gate oxide. Another example in which radiation damage has been utilized is the gettering of fast‐diffusing metals to improve the electrical junction characteristics. Buck et al.17 for instance, reported on striking reductions in the density of bright spots on silicon photodiodes.Summary: This paper has concentrated on the advances made in the semiconductor field by the use of ion implantation, this being the area in which the greatest impact has been felt so far. There are, however, many other disciplines in solid state technology—such as metallurgy, magnetic bubbles, insulators, and integrated optics—where ion implantation has just started to make contributions and where a wide open field is to be explored.