Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Forming nonplanar surface
Reexamination Certificate
1999-07-01
2003-06-10
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Forming nonplanar surface
C430S311000, C430S325000, C430S328000, C430S330000, C430S394000, C430S432000, C430S942000
Reexamination Certificate
active
06576405
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the fabrication of semiconductor devices as it applies to thick films, and more specifically, to high aspect ratio photolithographic processes.
The use of high energy ion implantation has found a growing number of uses in the fabrication of semiconductor devices, such as retrograde well CMOS, advanced processes used in well engineering, or other high energy implant uses. As the energies used increase through several hundred KeV into the MeV range, ever thicker photoresists are needed to shield those areas to be protected from the implantation. At the same time, the feature size of such devices has become smaller. These competing aims of decreasing scale, down to 0.35 &mgr;m and smaller CMOS processes, and increasing energy, into the MeV range, requires narrower areas of thicker resists, leading to higher aspect ratios, defined here as the thickness of the photoresist to width of the mask line. Producing a photoresist process capable of meeting these conflicting demands is of great importance as process techniques advance.
FIG. 1
is a flowchart of a conventional single-layer photolithographic process used in a typical lower energy ion implantation. First, in step
10
a photosensitive layer of a novolak positive photoresist is formed on the upper most surface of a silicon substrate previously subjected to a hexamethyldisanzane (HMDS) treatment to improve adhesion. The film thickness is typically less than 2.0 microns. This film is then prebaked, or softbaked, in step
20
, customarily at 80-105° C., with a value of 90° C. typically, for anywhere between 30 and 60 seconds. The purpose of the prebake is to drive off the casting solvent in the photoresist. A pattern in the resist film is formed in step
30
utilizing ultraviolet light that is selectively irradiated onto the resist layer using a reticle. In step
40
the film is post exposure baked (PEB) at the higher temperature of 110-120° C., with a value of 115° C. typical, for 30-60 seconds. This post expose bake significantly reduces the standing wave effect in the film from the exposure.
The exposed film is then developed in an aqueous developer, step
50
. Spray, single or double puddle, or hybrid “spuddle” development processes lasting 60 to 90 seconds are common. In the case of a positive resist, the developer removes that portion that has been exposed by the ultraviolet light.
Typically, the now patterned resist film is ultraviolet stabilized after being baked again at 110-120° C. for 30-60 seconds to drive off remaining solvents and chemically crosslink the film in steps
60
and
70
. Not all processes include the deep UV stabilization step and the hardbake of step
60
additionally helps to dry the wafer from step
50
when step
70
is absent. A standard stabilization in a deep UV tool will heat and expose the resist at the same time, typically heating from a temperature of 110° C. to 220° C. with the lamp on during portions of the process. During the stabilization process, the UV and thermal energies work together to remove residual solvents, moisture, and by-products (primarily nitrogen) from the photoactive compound decomposition and to chemically cross link the resist. Once the stabilization is complete, the resist is now ready for the ion implantation of step
80
.
In this conventional process, a pattern is transferred from the reticle into the resist film. The pattern transfer process is limited. The linewidth of the formed pattern is required to have the same linewidth as the pattern on the reticle. For common thicknesses less than 1.5 &mgr;m, this is well know in the art. As the resist thickness increases and/or as the aspect ratios increase, the patterning process becomes extremely difficult. With the application of high energy ion implantation, such as for CMOS or other advanced well processing, thicker photoresist films with high aspect ratios are required, where a high aspect ratio is defined as greater than 2:1.
In one example of a high energy implantation, a retrograde twin well CMOS process forms two types of well structures in the same device. There are n-well and p-well regions in contact with the substrate. These twin well structures are formed by utilizing a high energy (MeV) ion implantation to place the dopant beneath a thin oxide layer and into the underlying silicon. A photoresist film consistent with high energy implantation is required to successfully block the implant species from being implanted in unwanted device regions. These resists may need to be 3-4 &mgr;m or thicker. Combined with the decreasing scale used of critical dimensions, some recent CMOS device design rules require ratios of 4:1 or greater. These challenges tax the capability of any photolithographic process.
A conventional single-layer photoresist process utilized in a high energy ion implantation application produces some very adverse effects. In thicker films, if residual casting solvents are allowed to remain in the resist film, a detrimental sidewall differential and other pattern deformations could occur which dramatically distort the printed image. These phenomena effect both isolated and dense features, but are particularly acute in isolated features. Maintaining acceptable critical dimension stability and uniformity, step coverage, and process latitudes all become increasingly difficult as the aspect ratio increases. Appropriate process conditions are needed to both minimize outgassing at implantation and produce a resist of uniformly hardened through its cross-section.
Proper choices for the parameters of the various bake stages are needed to minimize these problems. Due to the chemical properties of the both the resist and the solvent, the glass transition and decomposition temperatures of the resist along with the volatility and boiling point of the casting solvent must all be considered. Although using higher bake temperatures may cause the solvent to evaporate and defuse more readily, above a certain temperature the resist components will begin to deteriorate. Therefore a difficult balance must be maintained. For the thicker resist layer needed for higher energy implantation, these difficulties become much greater. If the solvent is too volatile, or a bake temperature to high, the solvent may be driven from the outer portions of the resist too rapidly, thereby forming a skin which will seal in the solvents inside the resist film. This last problem can lead to unacceptable resist loss as the ions can wear through the hard outer layer into a soft center that is quickly lost. Also, when bombarded with such high energy ions, an improperly formed resist of the required aspect ratio may collapse. Additionally, this produces a degradation of the other required properties of the resist mentioned above which the art has learned to overcome for thinner resists, such as stability and uniformity for critical dimensions, step coverage, and process latitudes.
There are many other uses for high aspect photoresist in addition to their use in high energy implantation. The following methods have been known as methods of forming thick film photoresists patterns with high aspect ratios:
(A) In U.S. Pat. No. 5,262,281 a method is disclosed in the patterning of thick resists utilized in device and mask manufacture that provides excellent resolution and sensitivity, but this is obtained using a specific composition. Specifically, the composition involves polymers having recurring pendant acid labile a-alkoxyalkyl carboxylic acid ester and/or hydroxyaromatic ether moieties in the presence of a substance that is an acid generator upon exposure to actinic radiation.
(B) In U.S. Pat. No. 5,330,881 discloses a resist patterning process which allows generation of very thick (>3.0 &mgr;m), vertically-walled resist patterns which allow for the subsequent deposition or etching operations. The application for this process is for magnetic thin film heads and other devices requiring high aspect ratios. The invention utilizes a barrier layer in conjunction with a contrast enhancement laye
Adams Jean L.
Buffat Stephen J.
Barreca Nicole
Huff Mark F.
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