1. Field of the Invention
The present invention relates generally to the alignment of a substrate in a lithographic projection apparatus. More specifically, it relates to alignment of the substrate after some process layers have been deposited above an alignment mark.
2. Description of the Related Art
The term, “patterning means”, “patterning structure” or “mask” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
A very important criterion in semiconductor manufactures is the accuracy with which the successive layers printed on the substrate are aligned with each other. Mis-alignments of the layers are referred to as overlay errors, for all the many layers required to make an integrated circuit the overlay errors must be kept within tight limits for the resulting device to function correctly. To correctly align the substrate to the mask and consequently minimize overlay errors, alignment marks, which generally take the form of diffraction gratings are etched in the bare silicon substrate. These alignment marks (referred to as “zero marks”) are aligned to corresponding marks provided on the mask using a variety of techniques, including through the lens (TTL) alignment systems and off-axis alignment systems. An example of the latter is described in EP-A-0 906 590 (P-0070). However, once a few process layers have been deposited or grown on the substrate, the zero marks etched in the bare substrate often become obscured and are no longer visible to the radiation used in the alignment process. Even if not completely obscured, the growth of layers on top of the alignment marks can be uneven, leading to a shift in the apparent position of the alignment mark. To enable alignment after the zero marks have been obscured, further alignment marks are printed during the deposition of suitable layers of the device. The subsequent marks, referred to as non-zero marks, are however subject to damage during subsequent process steps and will also accumulate overlay errors from previous process layers. When etching a blanket aluminum layer to define the interconnects of the integrated circuit, it is preferred to align to the original zero marks but to do this requires that the overlying aluminum layers, and possibly also dielectric layers, be removed. Such clearout steps are undesirable.
A technique known as Impulsive Stimulated Thermal Scattering (ISTS) for measuring acoustic and thermal film properties, such as elastic constants and thermal diffusion rates, has been described in various publications such as J. A. Rogers et al., Appl. Phys. Lett 71 (2), 1997; A. R. Duggal et al. J. Appl. Phys. 72 (7), 1992; R. Logan et al., Mat. Res. Soc. Symp. Proc. 440, pg 347, 1997; L Dhar et al., J. Appl. Phys. 77 (9), 1995; and J. A. Rogers et al. Physica B 219 & 220, 1996. In this method, two excitation pulses overlapping in time and space are incident on a sample at slightly different angles. The two pulses interfere and heat the sample in a pattern corresponding to the interference pattern between them. The local heating sets up vibrations in the crystal structure of the sample which act as a diffraction grating to a probe pulse incident on the sample shortly after the excitation pulses. The diffraction of the excitation pulse is measured to give an indication of the property being investigated in the sample.