Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
1996-11-04
2003-05-13
Duda, Kathleen (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S318000, C430S950000, C438S902000
Reexamination Certificate
active
06562544
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for, and the processing of, semiconductor substrates. In particular, the invention relates to the patterning of thin films during substrate processing.
Since semiconductor devices were first introduced several decades ago, device geometries have decreased dramatically in size. During that time, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), meaning that the number of devices that will fit on a chip doubles every two years. Today's semiconductor fabrication plants routinely produce devices with feature sizes of 0.5 &mgr;m or even 0.35 &mgr;m, and tomorrow's plants will be producing devices with even smaller feature sizes.
A common step in the fabrication of such devices is the formation of a thin film on a substrate by chemical reaction of gases. When patterning thin films, it is desirable that fluctuations in line width and other critical dimensions be minimized. Errors in these dimensions can result in variations in device characteristics or open-/short-circuited devices, thereby adversely affecting device yield. Thus, as feature sizes decrease, structures must be fabricated with greater accuracy. As a result, some manufacturers now require that variations in the dimensional accuracy of patterning operations be held to within 5% of the dimensions specified by the designer.
Thin films are often patterned by etching away portions of the deposited layer. Modern substrate processing systems often employ photolithographic techniques in the patterning process. Typically, such photolithographic techniques employ photoresist or other light-sensitive material. In conventional processing, photoresist is first deposited on a substrate. A photomask (also known simply as a mask) having transparent and opaque regions that embody the desired pattern is positioned over the photoresist. When the mask is exposed to light, the transparent portions permit the exposure of the photoresist in those regions, but not in the regions where the mask is opaque. The light causes a chemical reaction in exposed portions of the photoresist. A suitable chemical, chemical vapor or ion bombardment process is then used to selectively attack either the reacted or unreacted portions of the photoresist. This process is known as developing the photoresist. With the remaining photoresist acting as a mask, the underlying layer may then undergo further processing. For example, material may be deposited, the underlying layer may be etched or other processing carried out.
Modern photolithographic techniques often involve the use of equipment known as steppers, which are used to mask and expose photoresist layers. Steppers often use monochromatic (single-wavelength) light, enabling them to produce the detailed patterns required in the fabrication of fine geometry devices. As a substrate is processed, however, the topology of the substrate's upper surface becomes progressively less planar. This uneven topology can cause reflection and refraction of the monochromatic light, resulting in exposure of some of the photoresist beneath the opaque portions of the mask. As a result, this uneven surface topology can alter the patterns transferred by the photoresist layer, thereby altering critical dimensions of the structures fabricated.
Reflections from the underlying layer also may cause a phenomenon known as standing waves. When a photoresist layer is deposited on a reflective underlying layer and exposed to monochromatic radiation (e.g., deep ultraviolet (UV) light), standing waves may be produced within the photoresist layer. In such a situation, the reflected light interferes with the incident light and causes a periodic variation in light intensity within the photoresist layer in the vertical direction. Standing-wave effects are usually more pronounced at the deep UV wavelengths used in modern steppers than at shorter wavelengths because many commonly used materials are more reflective at deep UV wavelengths. The use of monochromatic light, as contrasted with polychromatic (e.g., white) light, also contributes to these effects because resonance is more easily induced in monochromatic light. The existence of standing waves in the photoresist layer during exposure causes roughness in the vertical walls formed when the photoresist layer is developed, which translates into variations in line widths, spacing and other critical dimensions.
One technique helpful in achieving the necessary dimensional accuracy is the use of an antireflective coating (ARC). An ARC's optical characteristics are chosen to minimize reflections occurring at interlayer interfaces. The ARC's absorptive index is such that the amount of monochromatic light transmitted in either direction is minimized, thus attenuating both transmitted incident light and reflections thereof. The ARC's refractive and reflective indexes are fixed at values that cause any reflections, which might still occur, to be cancelled by incident light. This cancellation is accomplished by ensuring that reflected light is 180° (or 540° or another odd multiple of 180° ) out-of-phase with respect to the incident light.
FIG. 1A
illustrates another phenomenon often encountered in photolithography, known as footing. In a traditional photolithographic process, a layer
110
, which is to be patterned, is deposited or grown on a substrate
120
. In a traditional patterning process, a photoresist layer
130
is first deposited on layer
110
. Photoresist layer
130
is then developed (i.e., patterned). This pattern is exemplified in
FIG. 1A
by a gap
140
. Once photoresist layer
130
has been developed, the exposed areas of layer
110
may then be subjected to further processing, such as doping, etching or the like.
As is illustrated in
FIG. 1A
, after photoresist layer
130
is patterned, residual photoresist material may remain in junction areas
150
and
160
. This residue, or footing, can cause variations in line width. Footing is underexposed photoresist material, which may remain at the foot of the vertical walls that are formed during the developing of photoresist layer
130
. Footing is caused by the existence of amino groups (NH
4
+
) at the surface of layer
110
, and therefore is related to the amount of nitrogen contained in layer
110
. Amino groups are slightly basic, and can form bonds with the photoresist material (which is slightly acidic) at a bottom portion of photoresist layer
130
. When this occurs, the affected photoresist material is desensitized to radiant energy. Given this reduced photosensitivity, the bottom portion of photoresist layer
130
resists developing completely, and so may remain after the photoresist layer is developed. Some desensitized areas, such as areas in the center of gaps and large open areas, can be fully exposed by simply increasing the exposure's duration.
However, an exposure of longer duration may not be effective in exposing desensitized photoresist material in areas such as junction areas
150
and
160
. Radiant energy, after passing through an opening, will vary in intensity with the angle from the opening's centerline. On average, the radiant energy's intensity falls as the angle from the opening's centerline increases, relative to the intensity maximum that exists at the opening's centerline. This is in accordance with Young's theory, which predicts this type of diffraction phenomenon. Thus, a longer-duration exposure may alter the resulting line width, but does not avoid the formation of footing.
The opening at the top of a gap, such as gap
140
, may create such variations in intensity'within the gap. Because it is at an angle from the opening's centerline (i.e., from the center of gap
140
), the photoresist in junction areas
150
and
160
receives less radiant energy due to the optical mechanics of gap
140
, even though the photoresist in the center of gap
140
is fully exposed. This, in turn, may result in the photoresist
Cheung David
Feng Joe
Huang Judy H.
Yau Wai-Fan
Applied Materials Inc.
Duda Kathleen
Townsend and Townsend and Crew
LandOfFree
Method and apparatus for improving accuracy in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for improving accuracy in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for improving accuracy in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3079024