# Patent

## Title
Secure photomask with blocking aperture

## Abstract

A secure photomask including a substrate having one or more pattern layers formed thereon and a blocking aperture disposed below the one or more pattern layers that prevents at least one of unauthorized use and copying of the photomask.

## Background
RELATED APPLICATIONS
This application is a continuation-in-part of U.S. patent application Ser. No. 11/788,473, filed Apr. 20, 2007, the contents of which are incorporated herein by reference.


FIELD OF THE INVENTION
The present invention generally relates to photomasks and optical lithography for fabrication of integrated circuits. The present invention also generally relates to a method of manufacturing integrated circuits using photomasks.
BACKGROUND OF THE INVENTION
Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are typically made from very flat pieces of quartz or glass with a layer of chrome on one side. Etched in the chrome is a portion of an electronic circuit design. This circuit design on the mask is also called “geometry.” A typical photomask used in the production of semiconductor devices is formed from a “blank” or “undeveloped” photomask. As shown in FIG. 1, a typical blank photomask 5 is comprised of three or four layers. The first layer 11 is a layer of quartz or other substantially transparent material, commonly referred to as the substrate. The next layer is typically a layer of opaque material 12, such as Cr, which often includes a third layer of antireflective material 13, such as CrO. The antireflective layer may or may not be included in any given photomask. The top layer is typically a layer of photosensitive resist material 14. Other types of photomasks are also known and used including, but not limited to, phase shift masks, embedded attenuated phase shift masks (“EAPSM”) and alternating aperture phase shift masks (“AAPSM”). These types of phase shift masks are characterized by design features including opaque regions and partially transparent regions through which the phase of light is shifted by, for example, approximately 180°. Examples of such photomasks are described in U.S. Pat. Nos. 6,682,861, U.S. Pat. No. 6,933,084, U.S. Patent Publication No. 2005-0026053 and U.S. Pat. No. 7,049,034 to Photronics, Inc., the contents of which are incorporated by reference herein.
The process of manufacturing a photomask involves many steps and can be time consuming. In this regard, to manufacturer a photomask, the desired pattern of opaque material 12 to be created on the photomask 5 is typically defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E-beam) or laser beam in a raster or vector fashion across the blank photomask. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737 to Collier. Each unique exposure system has its own software and format for processing data to instruct the equipment in exposing the blank photomask. As the E-beam or laser beam is scanned across the blank photomask 10, the exposure system directs the E-beam or laser beam at addressable locations on the photomask as defined by the electronic data file. The areas of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble.
In order to determine where the e-beam or laser should expose the photoresist 14 on the blank photomask 5, and where it should not, appropriate instructions to the processing equipment need to be provided, in the form of a jobdeck. In order to create the jobdeck the images of the desired pattern are broken up (or fractured) into smaller standardized shapes, e.g., rectangles and trapezoids. The fracturing process can be very time consuming. After being fractured, the image may need to be further modified by, for example, sizing the data if needed, rotating the data if needed, adding fiducial and internal reference marks, etc. Typically a dedicated computer system is used to perform the fracturing and/or create the jobdecks. The jobdeck data must then be transferred to the processing tools, to provide such tools with the necessary instructions to expose the photomask.
After the exposure system has scanned the desired image onto the photosensitive resist material 14, as shown in FIG. 2, the soluble photosensitive resist material is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material 14′ remains adhered to the opaque material 13 and 12. Thus, the pattern to be formed on the photomask 5 is formed by the remaining photosensitive resist material 14′.
The pattern is then transferred from the remaining photoresist material 14′ to the photomask 5 via known etch processes to remove the antireflective material 13 and opaque materials 12 in regions which are not covered by the remaining photoresist 14′. There is a wide variety of etching processes known in the art, including dry etching as well as wet etching, and thus a wide variety of equipment is used to perform such etching. After etching is complete, the remaining photoresist material 14′ is stripped or removed and the photomask is completed, as shown in FIG. 3. In the completed photomask, the pattern as previously reflected by the remaining antireflective material 13′ and opaque materials 12′ are located in regions where the remaining photoresist 14′ remain after the soluble materials were removed in prior steps.
In order to determine if there are any unacceptable defects in a particular photomask, it is necessary to inspect the photomask. A defect is any flaw affecting the geometry. This includes undesirable chrome areas (chrome spots, chrome extensions, chrome bridging between geometry) or unwanted clear areas (pin holes, clear extensions, clear breaks). A defect can cause the circuit to be made from the photomask not to function. The entity ordering the photomask will indicate in its defect specification the size of defects that will affect its process. All defects of that size and larger must be repaired, or if they cannot be repaired, the mask must be rejected and rewritten.
Typically, automated mask inspection systems, such as those manufactured by KLA-Tencor or Applied Materials, are used to detect defects. Such automated systems direct an illumination beam at the photomask and detect the intensity of the portion of the light beam transmitted through and reflected back from the photomask. The detected light intensity is then compared with expected light intensity, and any deviation is noted as a defect. The details of one system can be found in U.S. Pat. No. 5,563,702 assigned to KLA-Tencor.
After passing inspection, a completed photomask is cleaned of contaminants. Next, a pellicle may be applied to the completed photomask to protect its critical pattern region from airborne contamination. Subsequent through pellicle defect inspection may be performed. In some instances, the photomask may be cut either before or after a pellicle is applied.
Before performing each of the manufacturing steps described above, a semiconductor manufacturer (e.g., customer) must first provide a photomask manufacturer with different types of data relating to the photomask to be manufactured. In this regard, a customer typically provides a photomask order which includes various types of information and data which are needed to manufacture and process the photomask, including, for example, data relating to the design of the photomask, materials to be used, delivery dates, billing information and other information needed to process the order and manufacture the photomask.
After the manufacturing steps described above are completed, the completed photomask is sent to a customer for use to manufacture semiconductor and other products. In particular, photomasks are commonly used in the semiconductor industry to transfer micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. The process for transferring an image from a photomask to a silicon substrate or wafer is commonly referred to as lithography or microlithography. Typically, as shown in FIG. 4, the semiconductor manufacturing process comprises the steps of deposition, photolithography, and etching. During deposition, a layer of either electrically insulating or electrically conductive material (like a metal, polysilicon or oxide) is deposited on the surface of a silicon wafer. This material is then coated with a photosensitive resist. The photomask is then used much the same way a photographic negative is used to make a photograph. Photolithography involves projecting the image on the photomask onto the wafer. If the image on the photomask is projected several times side by side onto the wafer, this is known as stepping and the photomask is called a reticle.
As shown in FIG. 5, to create an image 21 on a semiconductor wafer 20, a photomask 5 is interposed between the semiconductor wafer 20, which includes a layer of photosensitive material, and an optical system 22. Energy generated by an energy source 23, commonly referred to as a Stepper, is inhibited from passing through the areas of the photomask 5 where the opaque material is present. Energy from the Stepper 23 passes through the transparent portions of the quartz substrate 11 not covered by the opaque material 12 and the antireflective material 13. The optical system 22 projects a scaled image 24 of the pattern of the opaque material 12 and 13 onto the semiconductor wafer 20 and causes a reaction in the photosensitive material on the semiconductor wafer. The solubility of the photosensitive material is changed in areas exposed to the energy. In the case of a positive photolithographic process, the exposed photosensitive material becomes soluble and can be removed. In the case of a negative photolithographic process, the exposed photosensitive material becomes insoluble and unexposed soluble photosensitive material is removed.
After the soluble photosensitive material is removed, the image or pattern formed in the insoluble photosensitive material is transferred to the substrate by a process well known in the art which is commonly referred to as etching. Once the pattern is etched onto the substrate material, the remaining resist is removed resulting in a finished product. A new layer of material and resist is then deposited on the wafer and the image on the next photomask is projected onto it. Again the wafer is developed and etched. This process is repeated until the circuit is complete. Because, in a typical semiconductor device many layers may be deposited, many different photomasks may be necessary for the manufacture of even a single semiconductor device. Indeed, if more than one piece of equipment is used by a semiconductor manufacturer to manufacture a semiconductor device, it is possible more than one photomask may be needed, even for each layer. Furthermore, because different types of equipment may also be used to expose the photoresist in the different production lines, even the multiple identical photomask patterns may require additional variations in sizing, orientation, scaling and other attributes to account for differences in the semiconductor manufacturing equipment. Similar adjustments may also be necessary to account for differences in the photomask manufacturer's lithography equipment. These differences need to be accounted for in the photomask manufacturing process.
Conventional integrated circuit manufacturing processes using photomasks require monitoring and adjustment on a frequent basis to optimize the quality of the final product and prevent errors from occurring. In this regard, information regarding, among other things, structural features of the photomask, the integrated circuit to be manufactured using the photomask, process parameters, and performance characteristics of the photomask, must be gathered, stored and regularly updated during the manufacturing process. This often requires numerous interruptions of the overall manufacturing process to conduct steps such as simulation of the photomask performance and sampling of the integrated circuit to determine whether adjustments to the process are required. For example, a sampling procedure may reveal that one or more manufacturing tools must be adjusted to compensate for errors or defects in the integrated circuit. The time required to sample the integrated circuit and perform other monitoring steps, analyze the results, and make the necessary adjustments to the manufacturing process may result in substantial delays, thereby generating a lower yield and a reduced overall profit margin for the integrated circuit manufacturer.
Also, it is important to provide protections against unauthorized actions with a photomask, either prior to, during, or after a manufacturing process using the photomask. The photomask is generally considered to be an “unsafe” or “unsecure” medium for electronics design information. This is for three primary reasons: a) the photomask is the first physical and measurable record of the electronic design information; b) the electronics design information, once recorded on the mask, cannot be encrypted or scrambled (it is possible however to manipulate the mask content in selective ways with mask repair equipment); and c) once the photomask is recorded on a wafer through wafer exposure by an optical imaging system, the wafer pattern reveals additional measurable information about the device pattern. In the extreme case, the device can be fabricated in whole in or in part by an unauthorized source if the masks are in the parties' possession.
As an example, photomasks used to fabricate military, government or personal information sensitive devices must be assured against manipulation or copy. Also, such protections is desirable for photomasks used to fabricate commercial devices in highly competitive applications, such as, for example, dynamic random access memories (DRAMs) and flash memories. An added concern in these applications is the relative simplicity of the patterns and the amount of competitive information that can be acquired through even visual inspection of the photomasks or patterned wafers.
Protection may also be necessary for photomasks used to evaluate new equipment or test new modules of technology in a multi-company or multi-party environment. This is especially important given the pervasive use of multi-company consortia for early research and development. In particular, the initial evaluation of equipment for new process nodes requires the most advanced mask information in a nominally uncontrolled environment such as a fabrication facility for equipment owned by a third party.
Providing protection for photomasks used to generally build devices in “wafer foundries” is also important. Securing and tracking intellectual property within the IC production process remains a key concern for end users.
Actions of unauthorized users that mask owners and other interested parties may seek to prevent include exposure of the mask on a wafer by a wafer exposure system, reverse engineering of the mask content by scanning the features of the mask with an electronic imaging system such as a mask inspection system, and manipulation of the mask content for purposes of changing the electrical properties of devices being fabricated by the mask. In the case of unauthorized mask exposure, such exposure may be used to partially or completely fabricate the device intended to be manufactured using the mask, reverse engineer the device or otherwise discover the elements of the device.
Photomasks are provided with some protection against unauthorized use using several conventional methods. For example, mask information may be scanned with a mask inspection or mask measuring system and compared to an electronic version of the design database. This can be used to test against mask manipulation when compared to a verified database. However, this protection method cannot prevent unauthorized exposure of the mask or physically secure the content on the mask. Alternatively, the layers of a full device can be separated to minimize the chance of reverse engineering the device content. In this case, half the device layers might be sent to foundry A while the other half might go to foundry B. Another protection method includes escorting the mask through the process by a designated security official. In general, there is no way at present to physically validate and secure the content of a mask at the point of use.
SUMMARY OF THE INVENTION
A secure photomask according to an exemplary embodiment of the present invention comprises: a substrate comprising one or more pattern layers formed thereon; and a blocking aperture disposed below the one or more pattern layers that prevents at least one of unauthorized use and copying of the photomask.
In at least one embodiment, the blocking aperture is an electrochromic film.
In at least one embodiment, the electrochromic film comprises tungsten oxide.
In at least one embodiment, the secure photomask further comprises an electric charge delivery element that delivers electric signals to the electrochromic film to switch the electrochromic film from being substantially opaque to substantially transparent and from being substantially transparent to substantially opaque.
In at least one embodiment, the one or more pattern layers comprise at least one antireflective layer.
In at least one embodiment, the one or more layers comprise at least one opaque layer.
In at least one embodiment, the one or more layers comprise at least one partially transparent layer.
In at least one embodiment, the blocking aperture is disposed between the substrate and the pattern layers.
In at least one embodiment, the blocking aperture is disposed below the substrate.
A secure photomask blank according to an exemplary embodiment comprises: a substrate comprising one or more layers formed thereon; and a blocking aperture disposed below the one or more layers that prevents at least one of unauthorized use and copying of a photomask formed from the photomask blank.
In at least one embodiment, the one or more layers comprise at least one antireflective layer.
In at least one embodiment, the one or more layers comprise at least one opaque layer.
In at least one embodiment, the one or more layers comprise at least one partially transparent layer.
In at least one embodiment, the blocking aperture is disposed between the substrate and the one or more layers



BRIEF DESCRIPTION OF THE DRAWINGS
The above and related objects, features and advantages of the present invention will be more fully understood by reference to the following, detailed description of the preferred, albeit illustrative, embodiment of the present invention when taken in conjunction with the accompanying figures, wherein:
FIG. 1 represents a blank or undeveloped photomask of the prior art;
FIG. 2 represents the photomask of FIG. 1 after it has been partially processed;
FIG. 3 represents the photomask of FIGS. 1 and 2 after it has been fully processed;
FIG. 4 is a flowchart showing the method of using a processed photomask to make or process a semiconductor wafer;
FIG. 5 shows the process of making a semiconductor using a wafer stepper;
FIG. 6 is a block diagram of a system for monitoring an integrated circuit manufacturing process according to an exemplary embodiment of the present invention;
FIG. 7 is a plan view of a photomask according to an exemplary embodiment of the present invention;
FIGS. 8A-8D are cross sectional views of a photomask according to an exemplary embodiment of the present invention;
FIGS. 9A-9D are cross-sectional view of a photomask according to an exemplary embodiment of the present invention;
FIG. 10 is a flowchart showing a method for monitoring a photomask according to an exemplary embodiment of the present invention;
FIG. 11 is a flowchart showing a method for monitoring a wafer manufacturing process using a feedback loop according to an exemplary embodiment of the present invention; and
FIG. 12 is a flowchart showing a method for inspecting a photomask using a feedback loop according to an exemplary embodiment of the present invention.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Various exemplary embodiments of the present invention are directed to a photomask having electronics and detection capability integrated into the photomask substrate to enable wireless or other type of communication of information between the photomask and other elements of mask and wafer manufacturing processes including but not limited to mask making equipment, wafer exposure equipment, design automation tools, simulation tools, process control systems and mask tracking/logistical/shipping systems. The inventive photomask preferably has the ability to detect irradiation used to expose photomasks onto a wafer, such as, for example, 193 nm laser light in a wafer exposure scanner or other appropriate wavelength laser light. As explained in further detail below, such detected information may to be stored and analyzed for improving process capability. Information relevant to the manufacturing of the photomask, such as pattern metrology data, defects, and registration information, as well as information capturing certain characteristics of the device layout written on the photomask, such as frequency composition of the layout, criticality of certain areas of the layout and tolerances required for creating a functioning device may also be stored using the components of the inventive photomask. In addition to storage capabilities, the photomask according to various exemplary embodiments of the present invention may include computational logic needed to draw specific conclusions from stored data and to interact productively with complementary members of the manufacturing process.
The inventive photomask may be used in a closed or open loop manufacturing process to accomplish tasks such as passing of layout specific data to a wafer exposure system in order to optimize the printing of the mask pattern on the wafer, passing of information from the mask to a simulation system to assess the manufacturing attributes of the mask, passing of the manufactured characteristics of the mask into a process control system, monitoring and characterization of the wafer exposure in terms of exposure level, vibration, position, mask illumination characteristics, scattered light etc., monitoring of the physical location of the mask for use in a logistical control system, measurement of the image characteristics in a reflected light mode, and passing of information to a centralized database to allow automated record keeping on mask characteristics, simulation results, exposure information and so on. In general, the inventive photomask may be used to monitor an integrated circuit manufacturing environment, either prior to, during, or after the actual manufacturing process. For the purposes of this disclosure, the term “monitor” is intended to encompass any action that the inventive photomask may take relative to the manufacturing environment, such as, for example, measuring, diagnosing, characterizing, transmitting, receiving, testing, and optimizing.
It should be appreciated that the present invention is applicable to the manufacture of integrated circuits, and the term “integrated circuits” as used herein is intended to cover any devices that include an electric circuit, having semiconductor components or otherwise, including but not limited to display devices, such as liquid crystal displays and plasma displays, microcontrollers, memory devices, processors, sensors, power management circuits and amplifiers.
FIG. 6 shows a system, generally designated by reference number 1, for monitoring an integrated circuit manufacturing process according to an exemplary embodiment of the present invention. The system 1 includes a photomask 10, a sensing station 20, a computer workstation 30, a photomask database 40, an exposure tool, such as microlithographic tool 60, as well as other wafer manufacturing components 50.
FIG. 7 is a detailed plan view of the photomask 10 according to an exemplary embodiment of the present invention. The photomask 10 includes a non-active patterned area 12, an active patterned area 14 and a non-active non-patterned area 14. A detection system, including various detection modules DM1, DM2, DM3 and DM4, is integrated into the photomask 10. The detection system is capable of detecting at least one characteristic and preferably able to sense more than one characteristic of the photomask and/or manufacturing process parameters. In this regard, in order to fully detect and retrieve data from various locations on the photomask, the detection modules DM1-DM4 may be located at different regions within the photomask. For example, FIG. 2 shows the detection module DM1 located in non-active non-patterned region 14, the detection module DM2 located in non-active patterned area 12, and the detection module DM3 located in active patterned area 13. The detection module DM4 may be located in the pellicle plane or at some other standoff distance from the primary mask pattern. The detection modules DM1-DM4 may include circuitry that is capable of detecting and/or monitoring any type of desired information, such as, for example, exposure level, vibration, position, illumination characteristics, scattered light, diffracted light, image characteristics in a reflected light mode, and the physical location of the photomask.
The detection system may also include a photochemical recording medium, such as, for example, a photoresist, that is able to record a signal at the photomask 10 that may be analyzed later using standalone measurement equipment. For example, FIGS. 8A-D shows a photoresist 98 included with the photomask 10, where the photomask 10 includes a first substrate 91, a second substrate 92, one or more layers 94 and a pellicle layer 96. The photoresist 98 may be disposed at the bottom of the photomask 10 (FIG. 8A), at the top of the photomask 10 (FIG. 8B), between the two substrates 91, 92 at the mask plane (FIG. 8C), at the pellicle plane (FIG. 8D) or at any other suitable location on or external to the photomask 10.
The photomask 10 also includes a communication system 16, which preferably includes a wireless transmitter, that sends information gathered by the detections modules DM1-DM4 to the sensing station 20. In this regard, the sensing station 20 may include a wireless receiver for retrieving the information transmitted by the communication system 16. In embodiments of the invention, the photomask 10 may include an interface for connection of hard wiring or a memory storage device such as a memory stick.
The photomask 10 further includes a memory system 18 that uses a storage medium, such as a flash memory. The memory system 18 is preferably configured to store monitored process parameters and photomask manufacturing information. For example, the memory system 18 may store pattern metrology data, a defect profile, registration information, critical dimensions, layout characteristics, image fidelity and materials information. The memory system 18 may also store, either separately or with other information, device layout information, such as, for example, frequency composition of the layout, information relating to criticality of selected areas of the layout, and required tolerances for creating a functioning device.
The photomask 10 may also include a logic processing unit 20 to manipulate data stored in the memory system 18 and/or data retrieved by the detection system to generate information to be transmitted to the sensing station 20. In this regard, software 22 may be integrated into the photomask 10 that generates instructions to be run on the logic processing system 20 to carry out the analysis of data and/or manage the operation of electronics that connect the detection system, the memory system 18 and the logic processing unit 20.
The computer workstation 30 may be accessed by a user to monitor the information retrieved by the sensing station 20 from the photomask 10, and make adjustments to the manufacturing components 50 based on the information transmitted by the photomask 10. Alternatively, or in addition, the computer workstation 30 may include software and or/hardware that automatically adjusts operation of the manufacturing components 50 based on the transmitted information, so that feedback loops may be generated between the photomask 10 and the manufacturing components 50. In this regard, the mask database 40 may be used to store and maintain mask and process specific information. The mask database 40 may be maintained at the computer workstation 30 or at a remote location, and may be integrated into a network for access by other users.
The photomask 10 may also include a test structure 24 that diffracts, directs or otherwise conditions input radiation of a desired frequency or wavelength to render a more meaningful measurement on the electronic or photochemical detectors. The test structures 24 may be placed on the top, bottom or pellicle areas of the photomask. In an exemplary embodiment, the test structure 24 includes a diffraction grating on the top of the mask that works in concert with the detection plane. Such test structures may contain moving parts as in an integrated microelectromechanical systems (MEMS) device or an oscillating device.
The photomask 10 may also include a security system 26 for validating and securing the content of the photomask 10. The security system 26 may incorporate other components of the photomask 10, such as the communication system 16 and the detection system. For example, the communication system 16 may be used to pass an encrypted signal or code to a manufacturing tool to validate the connection of the mask to the tool. Once the connection is validated, the communication system 16 may pass critical information to the tool which is necessary to initialize or continue the manufacturing process. The security system 26 may also incorpotate an optical detector from the detection system that can detect a scattered light thumbprint of the photomask 10 to ensure that the photomask 10 has not been manipulated or replaced in the chip fabrication process.
The security system 26 may also include a thin film or physical blocking aperture that is opened to allow exposure and interrogation of the pattern on the photomask 10 only when certain information is passed to the photomask system. Preferably, the blocking aperture is physically embedded in the photomask itself so that the photomask is not inspectable or able to be exposed while the blocking aperture is enabled. However, the blocking aperture may also be external to the photomask. As an example, as shown in FIGS. 9A and 9B, a blocking aperture made of an electrochromic film 15 may be placed under an absorbing layer 17 of the photomask 10. As known in the art, electrochromic films can be made transparent through the introduction of an electric charge. Other blocking apertures are possible based on mechanical and/or optical means. The blocking aperture may be disabled through a validation step with the wireless detector or with other methods consistent with secure applications.
According an exemplary embodiment of the invention, as shown in FIGS. 9C and 9D, a blocking aperture made of an electrochromic film 406 may be integrated into a photomask 400. The photomask 400 may also include a substrate 402 and one or more pattern layers 404. The electrochromic film 406 is preferably disposed below the pattern layers 404, and either between the substrate 402 and the pattern layers 404 (see FIG. 9C) or below the substrate 402 (see FIG. 9D). The pattern layers 404 may include any number of layers, including antireflective layers, opaque material layers and partially transparent material layers, depending on the type of photomask. The photomask 400 may further include a charge delivery element 408 that delivers electric signals to the electrochromic film 406 to render it transparent or opaque. The charge delivery element 408 may be, for example, a semiconductor element or an electrode electrically connected to an external control unit. The electrochromic film 406 may be made of any known electrochromic material, such as, for example, tungsten oxide (WO3).
FIG. 10 is a flowchart showing a method, generally designated by reference number 100, for monitoring a photomask according to an exemplary embodiment of the present invention. In step S102, a photomask is manufactured based on customer requirements using processes well known in the art. In step S104, critical information is loaded into a mask database during the manufacturing process. Such critical information may include, for example, critical dimensions, layout characteristics, defect profile, registration, image fidelity, and materials information. In step S106, the status of the photomask is updated and the status information is sent to the customer and production control. In step S108, it is determined whether manufacture of the photomask is complete. If not, the process 100 returns to step S102, where manufacturing continues. Otherwise, the process 100 continues to step S110, where the mask database is loaded into the mask electronics module, which can be located either on the photomask or at a remote location.
In step S112, the photomask is shipped and may be electronically tracked using the communication system within the photomask. In step S114, either prior to or upon arrival at the manufacturing site, the mask database is loaded into various components, such as a simulation system, an exposure system and a fab control system. In step S116, the mask database is updated with wafer exposure characteristics. In step S118, the information within the mask database, including the critical mask information and the wafer exposure characteristics, is used to adjust manufacturing parameters prior to and during manufacture of the wafer using the photomask.
FIG. 11 is a flowchart showing a method, generally designed by reference number 200, for monitoring a wafer manufacturing process using a feedback loop according to an exemplary embodiment of the present invention. In step S202 of the process 200, a photomask according to the present invention is loaded into an exposure tool. In step S204, information gathered by the detecting system on the photomask or stored in the memory on the photomask is transmitted to a computer workstation. For example, information regarding diffracted light from the mask as detected by the detection system, or information regarding across field critical dimension variation or pattern layout stored within the memory of the photomask may be transmitted. In step S206, the information transmitted by the photomask is used to monitor and/or adjust manufacturing parameters, as appropriate. For example, intensity detected in step S204 may be compared with a database image of an expected result to monitor the effectiveness of the mask. In the case of the CD variation being transmitted by the photomask, the operation of the exposure tool may be adjusted to compensate for CD error by introducing a dose variation. In the case of the pattern layout being transmitted by the photomask, the exposure tool source may be optimized based on the layout data.
FIG. 12 is a flowchart showing a method, generally designated by reference number 300, for inspecting a photomask using a feedback loop according to an exemplary embodiment of the present invention. In step S302 of the process 300, the photomask is loaded into an inspection tool. In step S304, critical area information stored within the photomask memory is transmitted to the inspection tool. In step S306, defect scoring is obtained using the critical area information transmitted by the photomask.
Now that the preferred embodiments of the present invention have been shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

## Claims

1. A secure photomask comprising:
a substrate comprising one or more pattern layers formed thereon;
a blocking aperture disposed below the one or more pattern layers that prevents at least one of unauthorized use and copying of the photomask, the blocking aperture being an electrochromic film; and
an electric charge delivery element that delivers electric signals to the electrochromic film to switch the electrochromic film from being substantially opaque to substantially transparent and from being substantially transparent to substantially opaque.

2. The secure photomask of claim 1, wherein the electrochromic film comprises tungsten oxide.

3. The secure photomask of claim 1, wherein the one or more pattern layers comprise at least one antireflective layer.

4. The secure photomask of claim 1, wherein the one or more layers comprise at least one opaque layer.

5. The secure photomask of claim 1, wherein the one or more layers comprise at least one partially transparent layer.

6. The secure photomask of claim 1, wherein the blocking aperture is disposed between the substrate and the pattern layers.

7. The secure photomask of claim 1, wherein the blocking aperture is disposed below the substrate.

8. A secure photomask blank comprising:
a substrate comprising one or more layers formed thereon;
a blocking aperture disposed below the one or more layers that prevents at least one of unauthorized use and copying of a photomask formed from the photomask blank, the blocking aperture being an electrochromic film; and
an electric charge delivery element that delivers electric signals to the electrochromic film to switch the electrochromic film from being substantially opaque to substantially transparent and from being substantially transparent to substantially opaque.

9. The secure photomask blank of claim 8, wherein the electrochromic film comprises tungsten oxide.

10. The secure photomask blank of claim 8, wherein the one or more layers comprise at least one antireflective layer.

11. The secure photomask blank of claim 8, wherein the one or more layers comprise at least one opaque layer.

12. The secure photomask blank of claim 8, wherein the one or more layers comprise at least one partially transparent layer.

13. The secure photomask blank of claim 8, wherein the blocking aperture is disposed between the substrate and the one or more layers.

14. The secure photomask blank of claim 8, wherein the blocking aperture is disposed below the substrate.