Referring initially to FIG. 1a, integrated circuits are formed on semiconductor wafers 10 typically made from silicon. The wafers 10 are substantially round and typically have a diameter of approximately 15 to 20 cm. Each wafer 10 is divided up into individual circuit die 15 which contain an integrated circuit. Since a single integrated circuit die 15 is often no more than 1 cm.sup.2, a great many integrated circuit die 15 can be formed on a single wafer 10. After the wafer 10 has been processed to form a number of integrated circuit die on its surface, the wafer 10 is cut along scribe lines 20 to separate the integrated circuit die for subsequent packaging and use.
Formation of each integrated circuit die on the wafer is accomplished using photo-lithography. In general, lithography refers to processes for pattern transfer between various media. The basic photo-lithography system consists of a light source, a photomask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the photomask.
Referring to FIG. 1b, during an intermediate stage in the manufacturing cycle, the wafer 10 is shown to include a film 25 which overlies the wafer 10 and a resist 30 disposed on the film 25. Exposing the resist 30 to light or radiation of an appropriate wavelength through the photomask causes modifications in the molecular structure of the resist polymers to allow for transfer of the pattern from the photomask to the resist 30. The modification to the molecular structure allows a resist developer to dissolve and remove the resist in exposed areas, presuming a positive resist is used. If a negative resist is used, the developer removes the resist in the unexposed areas.
Referring to FIG. 1c, once the resist 30 on the wafer has been developed, one or more etching steps take place which ultimately allow for transferring the desired pattern to the wafer 10. For example, in order to etch the film 25 disposed between the resist 30 and the wafer 10, an etchant is applied over the patterned resist 30. The etchant comes into contact with the underlying film layer by passing through openings 35 in the resist formed during the resist exposure and development steps. Thus, the etchant serves to etch away those regions of the film layer which correspond to the openings in the resist, thereby effectively transferring the pattern in the resist to the film layer as illustrated in FIG. 1d. In subsequent steps, the resist is removed and another etchant may be applied over the patterned film layer to transfer the pattern to the wafer or to another layer in a similar manner.
Presently, there are a variety of known techniques for transferring a pattern to a wafer using photolithography. For instance, referring to FIG. 2, a reduction step-and-repeat system 50 (also called a reduction stepper system 50) is depicted. The reduction stepper system 50 uses refractive optics to project a mask image onto a resist layer 30. The reduction stepper system 50 includes a mirror 55, a light source 60, a filter 65, a condenser lens system 70, a mask 75, a reduction lens system 80, and the wafer 10. The mirror 55 behaves as a collecting optics system to direct as much of the light from the light source 60 (e.g. a mercury-vapor lamp) to the wafer 10. The filter 65 is used to limit the light exposure wavelengths to the specified frequencies and bandwidth. The condenser system 70 focuses the radiation through the mask 75 and to the reduction lens system to thereby focus a "masked" radiation exposure onto one of the circuit die 15.
Since it is complex and expensive to produce a lens capable of projecting a mask of an entire wafer, the reduction stepper system 50, projects an image only onto a portion of the wafer 10 corresponding to an individual circuit die 15. This image in then stepped and repeated across the wafer 10 in order to transfer the pattern to the entire wafer 10 (and thus the name "stepper"). Consequently, the size of the wafer is no longer a consideration for the system optics.
Current reduction stepper systems 50 utilize masks that contain a pattern that is an enlargement of the desired image on the wafer 10. Consequently, the mask pattern is reduced when projected onto the resist 30 during exposure (and thus the name "reduction stepper").
With an ever increasing number of integrated circuit patterns being formed on a circuit die, the importance of properly designing patterns to form structures that are isolated and non-interfering with one another has also increased. Accordingly, when designing a pattern to place on a mask, it is of significant benefit to know in advance the amount of error to expect with respect to the corresponding structures formed on the wafer so that such error can be accounted for in advance.
One known source for errors introduced during the patterning of the resist on a wafer occurs due to diffraction effects caused during the passage of light through the pattern formed on the mask. In particular, light which passes adjacent an edge of a pattern formed on the mask is caused to diffract by the edge thereby scattering the light in multiple directions. As a result the light intensity on the resist is not perfectly binary in nature.
For example, referring to FIGS. 3(a)-3(c) the diffraction affects of light passing through mask 40 having a patterned chrome layer 42 formed thereon is depicted. In particular, as the apertures P1 and P2 in FIG. 3(a) become closer to one another to allow for more tightly packed features on the wafer, the amplitude of light incident on the wafer in regions not corresponding to the apertures P1 and P2 increases. The degree to which the light amplitude in various regions varies due to the diffraction affects can be seen by comparing the light amplitude curve immediately after transmission through the mask in FIG. 3(b) with the light amplitude curve on the wafer in FIG. 3(c). For comparison purposes, an ideal light amplitude curve is also depicted in FIG. 3(c) in dotted lines. From the light amplitude curve shown in FIG. 3(c), the light intensity curve shown in FIG. 3(d) may be derived. In particular, the light intensity is proportional to the square of the resultant amplitude. As the resist is sensitive to the light intensity incident thereon, it can be seen from FIG. 3(d) that the resist will not have the desired sharp contrast resolution in the region between P1 and P2.
In order to reduce the affects of the diffracted light, a phase shift mask (PSM) has been utilized. In a PSM, phase variations are produced in the light that passes through the mask material. The phase variations are achieved by modifying the length that a light beam travels through the mask material.
In particular, referring to FIGS. 4(a)-4(d) two different method of achieving phase shift masking is depicted. In FIG. 4(a) the light passing through region P2 is caused to pass through additional mask material via deposited layer 47. In FIG. 4(b), the light in region P2 traverses through a shorter distance by virtue of etched groove 48. Each of the phase shift masks in FIGS. 4(a) and 4(b) make use of the fact that light passing through the mask material exhibits a wave characteristic such that the phase of the amplitude of the light exiting from the mask material is a function of the distance the light ray travels in the mask material, i.e. thicknesses t1 and t2. By making the thickness t2 such that (n-1) (t2) is approximately equal to 1/2.lambda., where .lambda. is the wavelength of the light in the mask material, and n=refractive index of the added or subtracted material, then the amplitude of the light exiting from the aperture P2 is opposite in phase from the light exiting aperture P1. This is illustrated in FIG. 4(c), showing the effects of diffraction and use of interference cancellation. As mentioned above, the resist is responsive to the intensity of the light at the wafer. Thus, since the opposite phases of light cancel where they overlap and since intensity is proportional to the square of the resultant amplitude, as seen in FIG. 4(d), contrast resolution is significantly improved.
While using a PSM can enhance process latitude at one point in the imaging field, it is also of significance to determine if a PSM can provide enhanced process latitude at all points across a field of view of a lens used in the photolithographic process. In particular, it is of particular interest to determine whether lens aberrations may improve or worsen CD control as compared to masks phase shifting characteristics (also known as binary masks). For example, a lens aberration known as coma which is caused by the distortion of a lightwave as it encounters an optic asymmetry, may add to CD variations across an imaging field during exposure of the resist to light using a PSM. Further, it is of interest also to measure the focus of the lens at the different locations across the imaging field so as to characterize the lens so that known variations can be accounted for during wafer design and manufacture.
Accordingly, there is a strong need in the art for a method of accurately and quickly measuring the affects of coma and focus at various locations in an imaging field.