P. Roitman

Effect of implant dose on formation of buried-oxide Si islands in low-dose SIMOX

There are still important issues that remain on the microstructure of low-dose SIMOX that may affect its quality and performance as an SOI material. These issues include the presence of crystalline defects in the top Si layer and the presence of Si islands in the buried oxide (BOX) which can severely degrade dielectric properties. While the reasons for crystalline defect formation have been recently determined, the mechanism(s) of Si island formation in the BOX are still unclear. Such understanding could assist in improved processing for fabricating high quality BOX layers in low-dose SIMOX. In this paper we report on the effect of implant dose on the microstructural changes found during Si island formation.

Defect formation mechanism causing increasing defect density during decreasing implant dose in low-dose SIMOX

Silicon-on-insulator material synthesized by oxygen implantation (SIMOX) is a leading candidate for advanced large scale integrated circuit applications due to thickness uniformity and moderate defect density. In the past few years, there has been a significant reduction of the defect density by optimizing processing conditions. Studies on defect formation mechanisms may suggest further modification of the processing conditions for both production cost and material quality. Recently, it was shown that through-thickness defects (TTDs) in high-dose SIMOX originated from as-implanted defects, dislocation half-loops (DHLs). On the other hand, a high density (/spl sim/10/sup 8/ cm/sup -2/) of defects in very low dose (0.25/spl times/10/sup 18/ cm/sup -2/) SIMOX has been observed by Nakashirna et al. (1993), but the origin of these defects has not been understood. In this paper we report on the effect of implant dose on defect formation mechanisms, and propose a defect formation mechanism in the very low-dose regime for the first time. It was found that the stacking faults generated during the initial stage of annealing are the origin of TTDs in the low-dose (<0.5/spl times/10/sup 18/ cm/sup -2/) regime, while as-implanted defects (DHL) are the origin of TTDs in the high-dose regime (>1.3/spl times/10/sup 18/ cm/sup -2/). Several approaches of process modification have been suggested for economical production of low-defect-density SIMOX.

High Resolution Electron Microscopy of Defects in High-Dose Oxygen Implanted Silicon-On-Insulator Material

High resolution electron microscopy (HREM) has been used to study the atomic arrangement of defects formed during high-dose oxygen implantation of silicon-on-insulator material. The effect of implantation parameters of wafer temperature, dose, and current density were investigated. Wafer temperature had the largest effect on the type and character of the defects. Above the buried oxide layer in the top silicon layer, HREM revealed that microtwins and stacking faults were created during implantation from 350–450°C. From 450–550°C, stacking faults were longer and microtwinning was reduced. From 550–700°C, a new type of defect was observed which had lengths of 40 to 140 nm and consisted of several discontinuous stacking faults which were randomly spaced and separated by two to eight atomic layers. We have referred to them as “multiply faulted defects” (MFDs). Beneath the buried oxide layer in the substrate region, the defects observed included stacking faults and ( 113 ) defects. The results indicated that some parts of the ( 1131 defects can assume a cubic diamond structure created through a twin operation across (115) planes. Details of the structure and formation mechanisms of MFDs and other defects will be discussed.

Effect of annealing conditions on precipitate and defect evolution in oxygen implanted SOI material

Summary form only given. The effects of annealing temperature, time, and ambient on the evolution of defects and precipitates in the superficial silicon layer in SIMOX material are discussed. Precipitates and defects were studied with various electron microscopy techniques, including high-resolution imaging. Annealing temperatures from 1250 degrees C to 1300 degrees C for two hours in a nitrogen ambient produced qualitatively the same general structure in the samples. This consisted of a precipitate-free upper region of the superficial layer and a high density of precipitates in the lower region of the layer. Dislocations frequently ran laterally between adjacent precipitates, but some dislocations also ran upward to the wafer surface from precipitates closer to the surface of the superficial layer. The buried layer had a waviness of about the same size as the precipitates.<<ETX>>

Effect of single-step, high-oxygen-concentration annealing on buried oxide layer microstructure in post-implant-amorphized, low-dose SIMOX material

Fabrication of high-dose SIMOX (typically 1.8/spl times/10/sup 18/ cm/sup 2/ at 200 keV) is a maturing materials technology with increasing commercial usage. However, lower-dose SIMOX (2 to 4/spl times/10/sup 17/ cm/sup 2/) has the potential to be more economical, as well as allow device designers a choice of oxide thickness, but film uniformity and quality must be as good or better than standard high-dose material. A variety of approaches to produce low-dose SIMOX have been used which include: low dose implant plus ITOX (internal thermal oxidation), which uses a second high temperature anneal with high oxygen concentration (Nakashima et al. 1996; Mrstik et al. 1995); multiple energy implants (Alles, 1997); lower energy implantation (Anc et al. 1998); rapid ramping to the high temperature anneal range (Ogura, 1998); N pre-implantation (Meyappan et al. 1995); and very-low dose, post-implant amorphization prior to high temperature annealing (Holland et al. 1996; Bagchi et al. 1997). For the last technique, it was reported there were changes in the precipitation mechanisms that control BOX development. The first was elimination of multiply-faulted defects as sites for preferred nucleation and growth of oxides which form a discontinuous upper layer of precipitates in untreated material. The second was enhanced diffusion of oxygen along defects and phase boundaries in the amorphized region to the single BOX layer that was developing. In this research, we extend the work on post-implant-amorphized low-dose SIMOX by reporting effects of a single-step high oxygen concentration anneal on its BOX microstructure.

Mechanisms of formation of buried-oxide in low-dose SIMOX

There is increasing interest in low-dose SIMOX as a substrate material. Circuits fabricated on such wafers need to have a high-quality buried-oxide (BOX) with flat, uniform interfaces and density of pipes and of Si islands as low as possible. Si islands have been reported to be responsible for increased electrical leakage current through the BOX, or in extreme cases, its dielectric breakdown. While the thickness of such Si islands in high-dose SIMOX spans only 3-5% of the BOX thickness, in low-dose material they may span 50%, or more, of the total BOX thickness. As a result, the reduction of the effective thickness of BOX could lead to degradation of dielectric properties. Thus, the understanding of BOX microstructural development is an important issue for low-dose SIMOX. Here we are reporting the mechanism of the development of BOX microstructure for annealed SIMOX as a function of implantation dose.

Effect of single vs. multiple implant processing on defect types and densities in SIMOX

In this paper we describe the origin and characteristics of the defect structures in contemporary SIMOX and show how their densities are controlled by the processing method and conditions.<<ETX>>

Effect of post-implantation amorphization on microstructural development of the buried oxide layer in low-dose SIMOX material

Summary form only given. In processing of SIMOX material, understanding the formation of the buried-oxide (BOX) layer and the effect of processing parameters is critical to production of high quality material. Most studies have focused on higher dose SIMOX material, typically 1.8/spl times/10/sup 18/ cm/sup -2/, but since the BOX is relatively thick (/spl sim/400 nm), the defects, such as Si islands, have a small effect on electrical characteristics while the density of Si pipes, which short the top Si layer and the substrate, is very low. As dose is decreased, however, the pipe density can increase with a lower dose limit at which a continuous BOX can form. Recently, Holland et al. (1996) reported that pre-amorphization of the as-implanted region prior to annealing can extend the lower limit. It was proposed that rapid diffusion of oxygen along grain boundaries in the recrystallized layer promoted formation of a continuous BOX during annealing. To examine this phenomenon, we report a comparison of microstructural development during annealing of the BOX for untreated and post-amorphized implant low-dose SIMOX.

Effect of implantation energy on the defect formation in SIMOX

Silicon-on-insulator material synthesized by oxygen implantation (SIMOX) offers the advantages of thickness uniformity and moderate defect density. This makes it a leading candidate for integrated circuit fabrication in the deep submicron (0.25 /spl mu/m) regime. Low-dose SIMOX has high densities of crystalline and BOX defects. While the parameters for reduction of these defects have not been realized fully, it can be reasonably stated that the as-implanted oxygen profile has a crucial role in the subsequent development of microstructure. Implantation at lower doses (<200 keV) is potentially attractive from the perspective of reduced implant damage and a tighter oxygen profile. Possible additional benefits are in terms of reduced equipment cost and the ability to realize SIMOX with ultra-thin (/spl les/200 /spl Aring/) top-silicon layer. We have compared the effect of two implant energies, namely 200 keV and 120 keV, on the defect formation.

Small signal modeling of the MOSOS capacitor

Summary form only given. The high-frequency small-signal sinusoidal steady-state response of the 2-terminal MOSOS (metal-oxide-silicon-oxide-silicon) capacitor is solved numerically using perturbation analysis of the basic semiconductor equations. The model includes the effects of a floating semiconductor region, interface traps and nonuniform doping. Simulations show a difference between the high-frequency and quasi-static solutions for accumulation. This is due to a dipole layer which exists in the semiconductor film. The internal small-signal currents which exhibit the dipole layer are presented.<<ETX>>