Patent Application
20080057410
Embodiments of the invention are described by way of examples with reference to the accompanying drawings, wherein:
The apparatus 20 includes a light source 26, a photolithographic mask 28, and an optical system of lenses 30. The light source 26 emanates light having a wavelength of approximately 193 nm. The photolithographic mask 28 is placed in a path of the light from the light source 26 so that the light propagates through the photolithographic mask 28 in a z-direction. The lenses 30 are placed in a path of the light after it passes through the photolithographic mask 28. The light radiates onto the photoresist material 22 after passing through the lenses 30.
The photolithographic mask 28 is made of a transparent material and carries a two-dimensional pattern of features in x-and-y directions. The features of the photolithographic mask 28 may attenuate the light or may change its phase to thereby reduce the brightness of the light either partially or entirely, depending on the type of mask that is used. The features of the photolithographic mask 28 result in features being created by the light radiating on the photoresist material 22. The pattern of the features of the photolithographic mask 28 may or may not be exactly replicated on the photoresist material 22, but the lenses 30 generally create a reduction in the overall size of the pattern, so that the pattern created on the photoresist material 22 has a cross-dimension that is approximately one-quarter of the pattern on the photolithographic mask 28.
Z is the thickness of the mesa pattern;
n is the refractive index of glass, namely 1.56;
λ is the wavelength of the light, in this case 193 nm;
The thickness of the mesa pattern 34 is thus 172 nm.
A degree of computer simulation may be required to determine the required width for the phase voxel cavity 42 because the phase voxel cavity 42 may not have a footprint that matches the footprint of the defect 40 exactly. In the present example, the defect 40 is 183 nm wide and 200 nm long and the phase voxel cavity 42 is 344 nm wide and 200 nm long.
Next, at Block 54, a local three-dimensional mask substrate model is constructed on a computer. The model has pattern geometry including the identified defect dimension and topography.
At Block 56 an impacted image reference is built. The impacted reference image is built by optical simulation of the defective mask pattern and is calibrated with the actual impacted image.
At Block 58 an optical model is created based on optimization of the three-dimensional geometry and placement of a phase voxel cavity. The optical model is created for purposes of determining a mask repair design rule.
At Block 60 a high-resolution, high-accuracy quartz etching of a phase voxel cavity is carried out according to the three-dimensional model-based repair design rule.
At Block 62 a mask lithographic imaging impact deposition is carried out.
The process of steps 54, 56, 58, 60, and 62 are carried out until the deposition specification is passed. At Block 64, the mask is shipped for silicon wafer printing.
In one embodiment, a method of repairing a defect detected in the photolithographic mask is described, including forming a phase voxel cavity in the photolithographic mask to compensate for the defect in the photolithographic mask.
The phase voxel cavity may be approximately 172 nm deep, although it will be appreciated that the phase voxel cavity may have any depth, provided that the depth is selected to create the necessary cancellation of light having a predetermined and select wavelength.
In the embodiment described above, the photolithographic mask is a phase mask with a mesa pattern creating a π phase change, the mesa pattern having a defect, and the phase voxel cavity having a depth creating an additional π phase change.
In one embodiment, a method of forming an electronics component is described, including (i) manufacturing a photolithographic mask having a transparent substrate and mask features carried by the transparent substrate, the mask features having a defect, (ii) detecting the defect, (iii) forming a phase voxel cavity in the photolithographic mask, and (iv) directing light through the mask onto the substrate, the mask features causing substrate features on the substrate and the phase voxel cavity compensating for a defect in at least one of the substrate features should the phase voxel cavity be absent.
With specific reference to
The light has a wavelength of approximately 193 nm, in which case the phase voxel cavity is approximately 172 nm deep. Other wavelengths of light and phase voxel cavities may apply in other embodiments.
A photolithographic mask is also described. The photolithographic mask includes a transparent substrate, features carried by the transparent substrate such that a pattern-generating phase shift occurs between light of a pre-selected wavelength propagating through the transparent substrate where the features are compared to where the features are absent, the pattern-generating phase shift being approximately π; and a phase voxel cavity formed in the transparent substrate to create a correcting phase shift of 2π between light propagating through the features and light propagating through the phase voxel cavity. The photolithographic mask in the embodiment described above is a phase mask with a mesa pattern creating a π phase change, the mesa pattern having a defect, and the phase voxel cavity having a depth creating an additional π phase change.
The masks 28A can be used for photolithography or optical lithography. Photolithography or optical lithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a substrate. Often crystalline silicon in the form of a wafer is used as a choice of substrate, although there are several other options including, but not limited to, glass, sapphire, and metal. Photolithography (also referred to as “microlithography” or “nanolithography”) bears a similarity to the conventional lithography used in printing and shares some of the fundamental principles of photographic processes.
Photolithography involves a combination of: (i) substrate preparation, (ii) photoresist application, (iii) soft-baking, (iv) exposure, (v) developing, (vi) hard-baking, and (vii) etching, and various other chemical treatments (thinning agents, edge-bead removal, etc.) in repeated steps on an initially flat substrate.
A part of a typical silicon lithography procedure would begin by depositing a layer of conductive metal several nanometers thick on the substrate. A layer of photoresist—a chemical that hardens when exposed to light (often ultraviolet)—is applied on top of the metal layer. The photoresist is selectively “hardened” by illuminating it in specific places. For this purpose a transparent plate with patterns printed on it, called a photomask or shadowmask, is used together with an illumination source to shine light on specific parts of the photoresist. Some photoresists work well under broadband ultraviolet light, whereas others are designed to be sensitive at specific frequencies to ultraviolet light. It is also possible to use other types of resist that are sensitive to x-rays and others that are sensitive to electron-beam exposure.
A spinner is used to apply photoresist to the surface of a silicon wafer.
Generally most types of photoresist will be available as either “positive” or “negative.” With positive resists, the area that you can see (masked) on the photomask is the area that you will see upon developing of the photoresist. With negative resists it is the inverse, so any area that is exposed will remain, whilst any areas that are not exposed will be developed. After developing, the resist is usually hard-baked before being subjected to a chemical etching stage which will remove the metal underneath.
Finally, the hardened photoresist is etched using a different chemical treatment, and all that remains is a layer of metal in the same shape as the mask (or the inverse if negative resist has been used).
Lithography is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. Its main disadvantages are that it requires a substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.
In a complex integrated circuit (for example, CMOS), a wafer will go through the photolithographic area up to 50 times. For Thin-Film-Transistor (TFT) processing, many fewer photolithographical processes are usually required.
A wafer is introduced onto an automated “wafertrack” system. This track consists of handling robots, bake/cool plates, and coat/develop units. The robots are used to transfer wafers from one module to another. The wafer is initially heated to a temperature sufficient to drive off any moisture that may be present on the wafer surface. Hexa-methyl-disilizane (HMDS) is applied in either liquid or vapor form in order to promote better adhesion of the photosensitive polymeric material, called photoresist. Photoresist is dispensed in a liquid form onto the wafer as it undergoes rotation. The speed and acceleration of this rotation are important parameters in determining the resulting thickness of the applied photoresist. The photoresist-coated wafer is then transferred to a hot plate, where a “soft bake” is applied to drive off excess solvent before the wafer is introduced into the exposure system.
The simplest exposure system is a contact printer or proximity printer. A contact printer involves putting a photomask in direct contact with the wafer. A proximity printer puts a small gap in between the photomask and wafer. The photomask pattern is directly imaged onto the photoresist on the wafer in both cases. The resolution is roughly given by the square root of the product of the wavelength and the gap distance. Hence, contact printing with zero gap distance ideally offers best resolution. Defect considerations have prevented its widespread use today. However, the resurgence of nanoimprint lithography may revive interest in this familiar technique, especially since the cost of ownership is expected to be very low. The cost will be low due to the lack of a need for complex optics, expensive light sources, or specially tailored resists.
The commonly used approach for photolithography today is projection lithography. The desired pattern is projected from the photomask onto the wafer in either a machine called a stepper or scanner. The stepper/scanner functions similarly to a slide projector. Light from a mercury arc lamp or excimer laser is focused through a complex system of lenses onto a “mask” (also called a reticle), containing the desired image. The light passes through the mask and is then focused to produce the desired image on the wafer through a reduction lens system. The reduction of the system can vary depending on design, but is typically on the order of 4×-5× in magnitude.
When the image is projected onto the wafer, the photoresist material undergoes some wavelength-specific radiation-sensitive chemical reactions, which cause the regions exposed to light to be either more or less acidic. If the exposed regions become more acidic, the material is called a positive photoresist, while if it becomes less susceptible it is a negative photoresist. The resist is then “developed” by exposing it to an alkaline solution that removes either the exposed (positive photoresist) or the unexposed (negative photoresist) region. This process takes place after the wafer is transferred from the exposure system back to the wafertrack.
Developers originally often contained sodium hydroxide (NaOH). However, sodium is considered an extremely undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of gate oxides. Metal-ion-free developers such as tetramethyl ammonium hydroxide (TMAH) are now used.
A post-exposure bake is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. The developing chemistry is delivered in a similar fashion to how the photoresist was applied. The resulting wafer is then “hardbaked” on a bake plate at high temperature in order to solidify the remaining photoresist, to better serve as a protecting layer in future ion implantation, wet chemical etching, or plasma etching.
The ability to project a clear image of a very small feature onto the wafer is limited by the wavelength of the light that is used and the ability of the reduction lens system to capture enough diffraction orders from the illuminated mask. Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light with wavelengths of 248 nm and 193 nm, which allow minimum resist feature sizes down to 50 nm.
Optical lithography can be extended to feature sizes below 50 nm using 193 nm and liquid immersion techniques. Also termed immersion lithography, this enables the use of optics with numerical apertures exceeding 1.0. The liquid used is typically ultra-pure, deionised water, which provides for a refractive index above that of the usual air gap between the lens and the wafer surface. This is continually circulated to eliminate thermally-induced distortions. Using water will only allow numerical apertures of up to ˜1.4 but higher refractive index materials will allow the effective numerical aperture to be increased.
Tools using 157 nm wavelength DUV in a manner similar to current exposure systems have been developed. These were once targeted to succeed 193 nm at the 65 nm feature size node but have now all but been eliminated by the introduction of immersion lithography. This was due to persistent technical problems with the 157 nm technology and economic considerations that provided strong incentives for the continued use of 193 nm technology. High-index immersion lithography is the newest extension of 193 nm lithography to be considered. In 2006, features less than 30 nm have been demonstrated by IBM using this technique. Other alternatives are extreme ultraviolet lithography (EUV), nanoimprint lithography, and contact printing. EUV lithography systems are currently under development which will use 13.5 nm wavelengths, approaching the regime of x-rays. Nanoimprint lithography is being investigated by several groups as a low-cost, non-optical alternative. Contact printing has already been established years ago and may yet be revived with the recent strong interest in nanoimprint lithography.
The image for the mask is originated from a computerized data file. This data file is converted to a series of polygons and written onto a square fused quartz substrate covered with a layer of chrome using a photolithographic process. A beam of electrons is used to expose the pattern defined in the data file and travels over the surface of the substrate in either a vector or raster scan manner. Where the photoresist on the mask is exposed, the chrome can be etched away, leaving a clear path for the light in the stepper/scanner systems to travel through.
Optical lithography can be extended to a resolution of 15 nm by using the short wavelengths of 1 nm x-ray lithography for the illumination. This is implemented by the proximity printing approach. The technique is developed to the extent of batch processing. The extension of the method relies on Near Field x-rays in Fresnel diffraction: a clear mask feature is “demagnified” by proximity to a wafer that is set near to a “Critical Condition.” This condition determines the mask-to-wafer gap and depends on both the size of the clear mask feature and on the wavelength. The method is rapid because it uses broadband, and simple because it requires no lenses.
As can be seen in
In the embodiment above the photolithographic mask has two structures with an end-to-end critical dimension that is too large, the phase voxel cavity being formed between the two structures.
In
In the given embodiment, light having a wavelength of 193 nm is used. With a refractive index of 1.56 for glass, the windows 76, 86, and the phase voxel cavities 92 would have to be etched to a depth of 172 nm below the surface of the windows 74 and 84. However, due to an effective phase of the light, the areas within the windows 76 and 86 and the phase voxel cavities 92 are etched to a depth of 160 nm below the surface of the windows 74 and 84. The effective phase is a function of geometry, illumination, etc.
In the embodiment above, the photolithographic mask has a transparent substrate and a nontransparent layer formed on the transparent substrate, the defect being the absence of a portion of the nontransparent layer to leave an area of the transparent substrate exposed, the phase voxel cavity being formed in the area.
While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described since modifications may occur to those ordinarily skilled in the art.
