Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-11-27
2004-02-03
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S322000, C430S323000, C430S394000, C430S396000
Reexamination Certificate
active
06686102
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
BACKGROUND OF THE INVENTION
This invention is in the field of integrated circuit manufacturing, and is more specifically directed to photolithography processes in such manufacturing. As is fundamental in the field of integrated circuit electronics, the functional capability of an integrated circuit depends substantially upon the number of active components (transistors, resistors, capacitors, etc.) that can be physically realized per unit area of the integrated circuit. It is therefore desirable to fabricate device features that are as small as possible, and as closely packed as possible, to provide not only a high level of functionality for the integrated circuit, but also a high level of circuit performance due to such small feature sizes. For example, many modern integrated circuit devices are fabricated with lateral features that are below one-half micron in width, realizing as many as tens of millions of transistors in a single integrated circuit operating at clock frequencies greater than 100 MHz. It is contemplated that these trends toward smaller and faster devices will continue, to the extent permitted by the state of the art of the manufacturing technology.
Conventional integrated circuit manufacturing technology utilizes photolithography for defining the location and dimensions of lateral features in the integrated circuit. As is fundamental in the art, photolithography is generally carried out by the application of a photosensitive substance, referred to as photoresist, over the film to be patterned. Selective exposure of the photoresist to electromagnetic energy (i.e., light) defines the portions of the film that are to be removed by the developing process, and those locations that are to remain. For purposes of manufacturing efficiency, the photoresist over the full area of one or more of the integrated circuits on the wafer are simultaneously exposed through photomasks, with transparent and opaque regions of the photomasks defining the locations of the photoresist that are exposed or not exposed, respectively. As a result of developing, photoresist is removed from the surface of the wafer, with the remaining regions of the photoresist (as defined by the selective exposure) serving as a mask to the etch of the underlying film, thus defining the features of the integrated circuit. Such masking may also be used in connection with other processes, such as ion implantation. Once the etch is completed, the remaining photoresist mask is then removed from the wafer. The processing of the wafer continues, with deposition of the next film layer and, if desired, photolithographic patterning and etching of this next layer.
According to modern conventional technology, the photomasks are generally in the form of reticles, where the images on the photomask itself are of some multiple magnitude (e.g., 4X) of the feature size to be patterned on the wafer. Exposure of the wafer through the reticle is carried out in combination with a focusing lens system disposed between the reticle and the wafer, so that the patterned exposure is reduced from that on the reticle. Reticles are generally used in connection with stepper exposure systems, in which only one or a few integrated circuit die are exposed at a time; the wafer is then indexed, or “stepped”, to the next position for photo-exposure through the reticle. The larger feature sizes on the reticles, relative to the integrated circuit feature sizes, facilitates the fabrication of the reticles themselves by way of photolithography. Of course, the photomasks may alternatively be so-called 1×photomasks that are placed in proximity to the wafer being patterned. For purposes of this description the term photomask will refer both to 1×photomasks and also to reticles, of both the full wafer and stepper type.
Certain “critical dimension” features in the integrated circuit, such as transistor gate electrodes, contact aperture sizes, and conductor widths and the like, relate directly to the density and performance of the integrated circuit. Typically, minimum width transistor gate electrodes are the most critical features in the integrated circuit layout, given the prevalence of transistors in the integrated circuit and also considering that gate electrode width relates directly to transistor channel length and thus to the gain and switching speed of the device. As such, the ability to reliably define and construct ever-smaller features such as transistor gates is of high importance in the field of integrated circuit design and manufacture.
As noted above, critical dimension features of modern integrated circuits are now on the order of one-half micron or less. Such sub-micron critical dimensions are on the order of the wavelength of the light energy used in the exposure. At these dimensions, the minimum feature size that may be imaged, at a usable depth of focus, depends strongly upon the wavelength of light used; so-called “deep UV” light is currently used to effect the higher resolution imaging required for modern integrated circuits. In modern photolithography processes, the minimum feature size that may be imaged by a photomask is approximately
0.5
λ
NA
,
where &lgr; is the wavelength of the exposing light and NA is the numerical aperture of the lens system of the stepper. The proportionality constant of this resolution ratio (in this example, having the value 0.5) is commonly referred to in the art as k
1
; a similar relationship is provided for depth of focus (having a proportionality constant k
2
). While a large numerical aperture permits the patterning of extremely small features, the depth of focus of the lens system decreases with increasing NA values. Considering the realistic extent to which the topography of the wafer can be made flat during its manufacture, which in turn limits the numerical aperture of the lens system, the minimum feature size that can be patterned by photolithography at a given wavelength reaches a practical limit.
Certain techniques for further reduction in the feature size that may be imaged for a given wavelength are also known in the art. One known technique uses a phase-shift photomask in which adjacent or nearby openings, or apertures, transmit light at opposing phases (i.e., 0° and 180°). As known in the art, light passing through a mask aperture of a size on the same order as the wavelength of the light will be locally coherent. The phase of this locally coherent light depends upon the thickness of the transparent material through which the light passes; as such, phase shift photomask apertures have varying thicknesses relative to one another, to establish the phase shift relationship. The phase shift effect may be used to define extremely small features on the wafer by placing opposite phase apertures on opposite sides of the small feature to be defined. To the extent that diffracted light reaches the photoresist at the location of the feature from both of the opposite phase apertures, the opposing phases will tend to cancel one another. As a result, unintended exposure of critical feature locations is greatly reduced, permitting the formation of these features.
Examples of conventional phase-shift photolithography are described in U.S. Pat. No. 5,045,417, U.S. Pat. No. 5,573,980, and U.S. Pat. No. 5,858,580.
In particular, one conventional approach utilizes two masks in the photolithographic patterning of critical dimension features, such as polysilicon gate electrodes in integrated circuits. While the use of two photomasks, and thus two exposure steps, is of course cumbersome in the manufacture of integrated circuits, the incorporation of phase-shift masking for critical dimension features along with conventional masking for the non-critical dimension features, into a single photomask, has been found to be extremely difficult, and unsuitable for automated mask generation. The above-noted U.S. Pat. No. 5,858,580 describes a known two-photomask photolithographic process. According to this technique, one photom
Fuller Gene E.
Randall John N.
Brady III Wade James
Gamer Jacqueline J.
Huff Mark F.
Sagar Kripa
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