Collimated sputtering of semiconductor and other films

Details Collimated sputtering of semiconductor and other films Collimated sputtering of semiconductor and other films

1998-07-14

2002-03-26

Pham, Long (Department: 2823)

Semiconductor device manufacturing: process

Coating with electrically or thermally conductive material

To form ohmic contact to semiconductive material

C438S792000

Reexamination Certificate

active

06362097

ABSTRACT:

BACKGROUND
The present invention relates generally to the formation of semiconductor and other films and, in particular, the formation of such films using collimated sputtering techniques.
Sputtering describes a number of physical techniques commonly used, for example, in the semiconductor industry for the deposition of thin films of various metals and other materials on a substrate such as a glass plate or a semiconductor wafer. In general, physical vapor deposition (PVD) sputtering techniques involve producing a gas plasma of ionized inert gas, such as argon (Ar). Particles of the ionized gas are attracted to a “target” material by an applied electric field in an evacuated chamber. The ionized particles collide with the target. As a result of the collisions, free atoms or groups of atoms of the target material are ejected from the surface of the target, essentially converting the target material to the vapor phase comprised of free atoms or molecules. In the low pressure range, near 1 milliTorr (mTorr) of Ar pressure, most of the free particles which escape the target surface in the direction of the substrate strike the substrate without intervening collisions with the Ar gas and form a thin film on the surface of the substrate being processed.
One common sputtering technique is magnetron sputtering in which a magnetic field is used to concentrate sputtering action in the region of the magnetic field so that the target sputtering occurs at a higher rate and at a lower process pressure. The target, which is biased electrically with respect to the substrate and the chamber, functions as a cathode.
Although direct current (DC) techniques for supplying a target with an electric potential are effective with a metallic or conducting target material, DC magnetron sputtering generally has not been employed in the fabrication of thin semiconducting films for commercial electronic devices. Rather, radio frequency alternating current (AC) power commonly is utilized for supplying the electric potential to an insulating target material, such as intrinsic silicon (Si). A high frequency potential, however, generates a plasma effect so as to bombard the film with charged particles of the plasma during the film's deposition and growth, thereby causing damage to the film.
The electronic and other properties of Si differ considerably depending on the degree of crystallinity of the material (see FIG.
1
). In particular, factors affecting the electronic and optical properties of silicon include the degree of crystallinity, the grain size and distribution, and the orientation of the grains. Other factors affecting the properties of Si films include the defect distributions in the grains, on the grain boundaries and in the interstitial phase, as well as physical defects such as voids or pinholes, which are caused by poor nucleation at the beginning of film growth. Such factors affect properties such as the final density of the film, the film's index of refraction, its ability to absorb light, and the conductivity of carriers, among others.
For example, thin films of sputtered silicon are generally amorphous when deposited in a vacuum by sputtering at low temperature. Amorphous films are referred to as “alpha” or “a” type films and have a low fraction of crystalline composition. Amorphous silicon (a—Si) sputtered films also generally have high defect distributions which limit their performance. The defects can significantly reduce the electron and hole flux in devices such as transistors and solar cells.
Although the electron mobility of a—Si is generally sufficient for thin film transistor (TFT) applications such as a TFT flat panel liquid crystal display (TFT-LCD), the minority carrier mobility and lifetime are severely limited by trapped charge, by the concentration of vacancy defects, and by other grain boundary-related defects. Similarly, reactive sputtered Si in hydrogen has excessive electron spin resonance so that the band-gap and mobility are not useful for solar cell or other optical detection devices. Thus, due to the defect distributions and a low degree of crystallinity, sputtered Si films generally have a high deposited sheet resistivity and are not useful for electronic applications requiring high conductivity. Consequently, sputtered a-Si thin films are not used in commercial applications such as TFT LCDs or thin solar voltaic films.
Poly-crystalline silicon (poly-Si) also is used in many electronic devices, including the fabrication of TFT-LCD devices. Depending on the degree of crystallinity, the size and orientation of the crystals, the kind of defects and their distribution in the crystals and on the boundaries, poly-Si films can have very high carrier mobility and good transistor properties for use in integrated microelectronic devices. Previously developed techniques, such as a silane and disilane plasma enhanced chemical vapor deposition (PECVD), can provide the desired electronic properties such as electron mobility, but require a relatively high substrate temperature of approximately 550 degrees Celsius (° C.) or higher. Similarly, annealing processes used to form poly-Si from an a-Si precursor film also require substrate temperatures which are above 650° C. for annealing on the order of days and near 850° C. for annealing on the order of hours. Such high temperatures prevent those techniques from being used with low temperature substrates like the glass substrates used in LCD applications or low cost plastic substrates.
Although the use of poly-Si is desirable in many applications, previous chemical vapor deposition (CVD) techniques and PVD techniques have not provided a combination of useful deposition rates, uniform wide area substrate capability and useful degrees of crystallinity for commercial applications. For example, it is difficult to provide highly uniform thin films on large glass substrates having dimensions on the order of about 550 millimeters (mm) by 650 mm and larger.
To deposit such poly-Si or micro-crystalline Si, a number of difficulties must be overcome. Those difficulties include the energetic and kinetic barriers to crystal nucleation, in other words, the difficulties in starting crystal growth, particularly at temperatures below the annealing point of the material to be deposited and at small crystal grain sizes. Even the time and temperature conditions used for annealing result in initial grains having diameters of hundreds of angstroms (Å), poor quality and high defect density, rather than high quality grains of 50 to 100 Å in diameter.
Many semiconductor devices also include one or more dielectric layers. For example, a metal-oxide-semiconductor (MOS) transistor includes a metal gate, an oxide or other dielectric layer, and a high mobility semiconducting layer such as Si. The performance of the transistor junction depends, in part, on the quality of the semiconductor to dielectric interface. Defects at the interface and within several hundred Å of the interface can cause a decrease in switching performance of the transistor. In addition, the breakdown threshold of the dielectric limits the minimum thickness of the gate dielectric. Since the conductance of a transistor is proportional to the gate capacitance, it is inversely proportional to the thickness of the dielectric layer. A thinner dielectric layer results in higher current. A typical silicon nitride thickness, which has been used as a dielectric layer in TFT-LCDs, is in the range of about 1500-2000 Å. Thinner dielectric films, however, have not been possible at low temperatures without annealing due to dielectric breakdown.
Sputtering also generally has not been used in the fabrication of optical thin films, such as laser mirror coatings, because the threshold of ablation depends on the vacancies, the trapped charge concentration and the physical defects such as pinholes and voids. Such defects scatter light and degrade optical signals which is particularly problematic in low light or high resolution applications.
Summary
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