Patent Application
20130193350
As technology nodes shrink, in some integrated circuit designs spacing between features in a semiconductor device decreases and issues such as parasitic capacitance become more prevalent. Low k dielectric materials are used as interlayer material to reduce parasitic capacitance and increase speed in circuit components. A conventional method of creating a low k dielectric material is to cure a wafer using ultraviolet radiation to increase the porosity of the wafer. The amount of light incident on the wafer determines the increase in porosity.
Uneven curing produces electrical property variations across the wafer surface. These variations cause large deviations between subsequently cured wafers.
Curing is measured using percent shrinkage with respect to the original wafer thickness. As the degree of curing increases, the percent shrinkage increases because more material is removed from the wafer to increase porosity. Some techniques form wafers with high percent shrinkage in the central region and lower percent shrinkage on the outer portion because the amount of light passing through the quartz plate decreases toward the outer portion of the quartz plate.
One or more embodiments are illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout. It is emphasized that, in accordance with standard practice in the industry various features may not be drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features in the drawings may be arbitrarily increased or reduced for clarity of discussion.
The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are of course, merely examples and are not intended to be limiting.
In some embodiments, wafer curing apparatus 100 further includes an ultraviolet light source 102. Ultraviolet light source 102 emits light in the ultraviolet spectrum to cure wafer 110. Ultraviolet light source 102 is a metal halide lamp having a wavelength range ranging from about 200 nm to about 450 nm. This range, in some embodiments, is narrower, e.g., from 200 nm to 450 nm. In some embodiments, ultraviolet light source 102 includes a xenon lamp, one or more ultraviolet light emitting diodes, or another suitable ultraviolet light source. Higher intensity ultraviolet light sources cure wafers more rapidly, but can exacerbate non-uniformity problems such as variations in percent shrinkage across a wafer surface.
Ultraviolet light source 102 is positioned at least partially within reflector 104. Reflector 104 has a reflective inner surface in order to collect the light emitted by ultraviolet light source 102 and direct the light to plate 106. Reflector 104 is a concave parabolic reflector. Light reflected off the inner surface of reflector 104 is directed parallel to an axis of reflector 104. In at least some embodiments, all light reflected off the inner surface of reflector 104 is directed parallel to an axis of reflector 104. In some embodiments, reflector 104 has an elliptical shape or other suitable shape.
In some embodiments, a secondary reflector 112 is positioned between reflector 104 and plate 106, as shown in
Plate 106 is a window of a chamber housing wafer 110 during the curing process. Plate 106 is arranged as a light incident surface of the chamber and transmits ultraviolet light from ultraviolet light source 102 incident on plate 106 into an interior of the chamber. In the embodiment of
Antireflective coating 108 is over an incident surface of plate 106. Antireflective coating 108 acts to reduce reflection and redirection of ultraviolet light on the incident surface of plate 106. An incident angle of ultraviolet light at the outer portion 116 of plate 106 is significantly larger than the incident angle in a central portion 114 of plate 106. Antireflective coating 108 modifies the refractive index difference of the light incident surface, to prevent ultraviolet light from being reflected or refracted away from wafer 110. Based on a reduced amount of reflect or refracted light, antireflective film 108 increases the amount of light transmission through plate 106 and onto wafer 110 at the outer portion 116 of plate 106 resulting in more uniform curing of wafer 110.
Wafer 110 is a dielectric wafer. Wafer 110 is silicon dioxide. In other embodiments, wafer 110 is fluorine-doped silicon dioxide, or other suitable dielectric materials. The curing process reduces the dielectric constant, k, of wafer 110. A lower dielectric constant helps reduce parasitic capacitance between features formed in wafer 110.
(D2−R2) (π/4) (2)
where D is the diameter of plate 106 and R is the diameter of central opening 202.
The inclusion of antireflective coating 108 results in more uniform curing of wafer 110. It was found that percent shrinkage uniformity increased by about 18% across a single wafer using antireflective coating 108 versus uncoated plate arrangements. The more uniform curing results in more uniform electrical properties of wafer 110. The higher rate of transmission in an arrangement including antireflective coating 108 also increases curing efficiency because the intensity of light incident on the wafer outer portion 116 is higher than in uncoated plate arrangements. It was found curing efficiency increases by about 10% over uncoated plate arrangements. In at least some embodiments, higher efficiency increases production speed and reduces costs.
Using antireflective coating 108 provides better results in wafer to wafer analysis as well. It was found the standard deviation of percent shrinkage on wafer to wafer testing improved by about 50% over uncoated plate arrangements. The reduced standard deviation allows for improved process optimization because the products formed have more uniform characteristics. In at least some embodiments, reduced variation between wafers also increases production efficiency because more wafers are likely to pass quality control tests.
One aspect of the description relates to a wafer curing apparatus having an ultraviolet light source, a plate for transmitting ultraviolet light, an antireflective coating on the plate and a wafer for receiving light transmitted through the plate and the antireflective coating has an opening in a central portion thereof. Another aspect of the description relates to a method of curing a wafer by emitting ultraviolet light from an ultraviolet light source, transmitting the ultraviolet light through a plate having an antireflective coating thereon and irradiating a wafer with the light transmitted through the plate, where the antireflective coating as an opening in a central portion thereof.
The above description discloses exemplary elements, but they are not necessarily required to be arranged in the order described. Embodiments that combine different claims and/or different embodiments are within the scope of the disclosure and will be apparent to those skilled in the art after reviewing this disclosure.
