WO2002065214A1

WO2002065214A1 - Dose control in response to stepper optical paramaters

Info

Publication number: WO2002065214A1
Application number: PCT/US2002/002018
Authority: WO
Inventors: Anastasia Oshelski Peterson; Richard David Edwards; Anthony J. Toprac; Edward Christopher Stewart
Original assignee: Advanced Micro Devices, Inc.
Priority date: 2001-02-13
Filing date: 2002-01-25
Publication date: 2002-08-22

Abstract

In one illustrative embodiment, a method is provided to controllably expose a layer of photoresist (23) to a light source (40). In particular, the duration of time that the light source (40) is exposed to a layer of photoresist (23) is controlled as a function of optical parameters of the light source (40). In one illustrative embodiment, the method is comprised of energizing a light source (40) and determining an optical parameter of the light source (40). A desired photodose is determined based upon the optical parameters of the light source (40), and the layer of photoresist (23) is exposed to the light source (40) for a duration of time to provide the desired photodose.

Description

DOSE CONTROL, IN RESPONSE TO STEPPER OPTICAL PARAMETERS

TECHNICAL FIELD The present invention is generally related to the field of semiconductor processing, and, more particularly, to a method and apparatus for controlling the amount of light energy delivered to a layer of photoresist on a semiconductor device.

BACKGROUND ART In general, semiconductor devices are manufactured by forming many process layers comprised of various materials above a semiconducting substrate, and, thereafter, removing selected portions of the layers, i.e., patterning the layers. This patterning may be accomplished using known photolithography and etching processes to define the various features of the device, e.g., a gate insulation layer, a gate electrode, metal lines and contacts, etc. This forming and patterning of the process layers is typically performed layer by layer as the individual layers are formed, although multiple layers may be patterned at any given time. Photolithography is a common process used in patterning these various layers. Photolithography typically involves the use of a product known as photoresist. In general terms, photoresist is a product that may be changed from a relatively soluble state to a relatively insoluble state by exposure to a light source. There are positive and negative photoresists currently available on the market.

The photolithography process generally involves forming a layer of photoresist above a previously formed process layer, and exposing selected portions of the layer of photoresist to a light source to form a pattern in the photoresist. The pattern formed in the photoresist is subsequently transferred to the underlying process layer. All of these steps are typically performed in well-known photolithography modules that include a section for depositing the photoresist on the wafer, e.g., a spin-coating station, a device for selectively exposing portions of the photoresist layer to a light source through a reticle or photomask, e.g., a stepper, and a section for rinsing and developing the photoresist layer after it has been selectively exposed to the light source. Thereafter, an etching process, such as a plasma etching process, is performed to remove portions of the underlying process layer that are not covered by the patterned layer of photoresist, le., the patterned layer of photoresist acts as a mask. After the etching process is complete, the patterned photoresist layer is typically removed so that additional process layers may be formed above the now patterned process layer. The purpose of the photoresist application step is to form a thin, uniform, defect-free film of photoresist above the substrate surface. Typically, the photoresist is developed by exposing it to a light source of a preselected intensisty for a preselected duration of time. Overexposure or underexposure may have undesirable effects on the developed layer of photoresist. That is, dimensions of the patterns formed in the photoresist may be affected by other than ideal exposure. This dimensional variation may carry over to the features that are to be formed in the semiconductor device, and, thus, affect the operation of the semiconductor device, or in the worst case render it inoperable.

As is known to those of ordinary skill in the art, the ability of a photomask or reticle to imprint a pattern on a wafer is determined in part by resolution and depth of focus. Simplified equations for resolution and depth of focus are described below: R = ^ (Resolution) (1)

NA²

_£) _ ( .Dep . th of Focus) (2)

2 - NA

The resolution and depth of focus depends mostly on the numerical aperture (NA) of the lens unit and the wavelength (λ) of the incident radiation. The correction factors k] and k₂ depend on the process, material, resist, etc. Other factors, such as chromatic and spherical aberrations in the lens used to project the light on the photomask, also have an effect on the resolution and depth of focus, but their relative contributions are small.

Another factor that theoretically affects the resolution and depth of focus is the partial coherence of the incident radiation. Partial coherence is a relative measure of the degree to which the incident radiation is columnated. For example, light from a laser is typically fully columnated (i.e., very little scattering; perpendicular angle of incidence), and is referred to as fully coherent (i.e., PC = 1). On the other hand, light with a high amount of scattering (i.e., any angle of incidence), such as light that might come from a flashlight, is referred to as incoherent (PC = 0). Light between these extremes is referred to as partially coherent. Typically binary photomasks are used with light having a partial coherence of about 0.65-0.7, and phase-shift masks are used with light having a partial coherence of 0.45-0.6. Due to the nearly vertical walls that define the phase edges of a phase-shift mask, changing the partial coherence of the light has essentially no effect on the resolution or depth of focus.

Steppers commonly include a light source that is normally on, and a shutter positioned between the light source and the semiconductor device. Thus, exposure of the semiconductor device to the light source is controlled by opening and closing the shutter. For a given light intensity, the duration that the shutter needs to be open may be readily calculated or otherwise derived. However, when different semiconductor devices are processed in the stepper, it may be desirable to vary certain optical parameters of the stepper, such as the numerical aperture and partial coherence. Varying these optical parameters may impact the intensity, and, thus, the desired period of exposure. Thus, varying these optical parameters may result in the layer of photoresist being over or underexposed, which may adversely affect feature size, particularly critical dimensions (CD). The present invention is directed to a method of solving or at least reducing some or all of the aforementioned problems.

DISCLOSURE OF INVENTION In one illustrative embodiment, the present invention is directed to a method. The method is comprised of energizing a light source and determining an optical parameter of the light source. A desired photodose is determined based upon the optical parameters of the light source. A device is exposed to the light source for a duration of time to provide the desired photodose.

In another embodiment of the instant invention, the present invention is directed to a system. The system comprises a stepper and a controller. The light source is controUably energizable to provide light to a surface of a semiconductor device. The controller is capable of determining an optical parameter of the light source of the stepper, and controUably energizing the light source for a period of time responsive to the optical parameter. BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference tb the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:

Figure 1 is a cross-sectional view of a process whereby a quantity of photoresist is positioned on a previously formed process layer;

Figure 2 is a cross-sectional view of a layer of photoresist formed by a spin-coating process; Figure 3 depicts one illustrative embodiment of a system that may be employed with the present invention;

Figure 4 illustrates a stylized view of operative components of a stepper of Figure 3; Figure 5 depicts one illustrative embodiment of the present invention in flowchart form; and

Figure 6 depicts one illustrative embodiment of the present invention in flowchart form. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

MODE(S) FOR CARRYING OUT THE INVENTION

Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The present invention will now be described with reference to Figures 1-6. Although the various regions and structures of a semiconductor device are depicted in the drawings as having very precise, sharp configurations and profiles, those skilled in the art recognize that, in reality, these regions and structures are not as precise as indicated in the drawings. Additionally, the relative sizes of the various features depicted in the drawings may be exaggerated or reduced as compared to the size of those features on fabricated devices. Never- theless, the attached drawings are included to describe and explain illustrative examples of the present invention.

In general, the present invention is directed to a method of detecting and/or compensating for variations in optical paramaters of a light source used in a stepper in a semiconductor manufacturing line. As will be readily apparent to those skilled in the art upon a complete reading of the present application, the present method is applicable to a variety of technologies, e.g., NMOS, PMOS, CMOS, etc., and it is readily applicable to a variety of devices, including, but not limited to, logic devices, memory devices, etc. Moreover, it is readily applicable to forming a variety of features on a semiconductor device, such as gate electrodes, conductive metal lines or contacts, etc.

As shown in Figure 1, a wafer or semiconducting substrate 10 having a process layer 18 formed thereabove is positioned on a rotational element, such as a vacuum chuck 12. A vacuum may be applied, as indicated by arrow 14, to secure the substrate 10 to the vacuum chuck 12. The vacuum chuck 12 and the substrate 10 are capable of being rotated in the direction indicated by arrow 26. Photoresist from a source (not shown) is applied on the process layer 18 via a dispenser arm 20. As shown in Figure 1, a puddle of photoresist

21 is formed above the process layer 18. The substrate 10 may or may not be rotating at the time the puddle of photoresist 21 is deposited on the process layer 18. Thereafter, as shown in Figure 2, the substrate 10 is rotated such that the photoresist material is spread across a surface 19 of the process layer 18, forming a layer of photoresist 23 above the surface 19 of the process layer 18.

As will be recognized by those skilled in the art, the process layer 18 is meant to be illustrative only in that it may be comprised of any of a variety of materials, and there may be one or more intervening process layers between the process layer 18 and the substrate 10. For example, the process layer 18 may be comprised of an oxide, an oxynitride, a nitride, silicon dioxide, silicon nitride, a metal, polycrystalline silicon ("polysilicon"), or any other of a variety of materials used in semiconductor processing that may be patterned using photolithographic techniques. Moreover, the photoresist used with the present invention may be either a positive or negative type photoresist.

In the disclosed embodiment, the layer of photoresist 23 is formed by a spin-coating process. In many modern fabrication facilities, a spin-coating process involving a moving dispenser arm 20 is used to form layers of photoresist. In that process, the substrate 10 is rotated at a relatively low speed prior to the deposition of any photoresist material 21 on the process layer 18. As the photoresist material 21 is deposited on the substrate 10, the dispenser arm 20 moves in a more or less radially outward fashion, beginning at the center of the substrate 10 and moving outward. This technique is used to more evenly distribute the photoresist across the surface 19 of the process layer 18.

Of course, as will be apparent to those skilled in the art upon reading the present application, the present invention is not limited to this particular spin-coating technique. For example, the present invention may also be used in processing techniques in which the dispenser arm 20 remains at the approximate center of the substrate 10. In that situation, the substrate 10 is initially rotated at a relatively low speed and photoresist material 21 is dispensed on the approximate center of the process layer 18. At that time, the rotational speed of the substrate is increased to disperse the photoresist. In yet another alternative embodiment, a static-type spin- coating process may be used in which the photoresist material 21 is deposited in the approximate center of a process layer 18 while the process layer 18, i.e., wafer 10, is stationary. Thereafter, the substrate 10 is rotated to disperse the photoresist evenly across the surface 19 of the process layer 18. If desired or required, a separate primer coating process may also be used prior to applying the photoresist above the process layer 18 in any of the above-described spin-coating methods.

Figure 3 depicts one illustrative embodiment of a system 30 that may be used with the present invention. As shown therein, a system 30 for processing wafers 32 is comprised of a photolithography tool 34 used for forming the layer of photoresist 23, an automatic process controller 36, and a stepper 39 for controUably exposing the layer of photoresist 23 to a light source. The controller 36 may take a variety of forms. For example, the controller 36 may be included within the stepper 39, or it may be a separate device electrically coupled to the stepper 39 via a line 35. In the embodiment illustrated herein, the controller 36 takes the form of a computer that is controlled by a variety of software programs. The software programs that directly relate to controlling and or monitoring the light source within the stepper 39 are discussed in greater detail below in conjunction with Figures 5 and 6. Those of ordinary skill in the art will appreciate that the controller 36 need not rely on software for its functionality^', but rather, a hardware controller may be used to provide the functionality described herein and attributed to the controller 36. Further, the controller 36 need not be coupled only to the stepper 39, but rather, could be coupled to and involved in controlling or collecting data from the photolithography tool or other devices involved in the manufacture of semiconductor devices. The stepper 39 may be any of a wide variety of devices used to expose the layer of photoresist 23 to a light source, e.g., a 1500 Stepper manufactured by ASML. Generally, as shown in Figure 4, the stepper 39 includes a plurality of operational components. A light source 40 is positioned above the wafer 10 with a shutter 41 interposed therebetween. Generally, the shutter 41 is operated by the controller 36 between open and closed positions. In the open position, light energy from the light source 40 passes through the shutter 41 and a mask or reticle 42 to impinge upon the layer of photoresist 23. Alternatively, when the shutter 41 is closed, substantially no light energy from the light source 40 reaches the layer of photoresist 23.

A photodose sensor 43, such as a photodiode, is positioned to receive light energy from the light source 40 and provide a signal indicative of the light intensity to the controller 36. The light intensity signal is used by the controller 36 in one embodiment as at least one parameter for controlling the shutter 41. The photodiode 43 may be positioned on either side of the shutter 41 to receive light energy from the light source 40 during at least the time that the shutter 41 is open, and in some instances, at all times.

The photodose sensor 43 may take on any of a variety of forms and may be an array of sensors adapted to detect the presence and/or magnitude of select frequencies of light. That is, an array of sensors may have optic filters (not shown) configured to allow selective frequencies of light to fall upon selected sensors and/or groups of sensors within the array.

The present invention may be employed on a lot-by-lot basis and/or on a wafer-by-wafer basis. In general, the more frequent the measurements, the more accurate will be the light energy delivered to the layer of photoresist 23. That is, the intensity of the light source 40 need not be measured at each exposure, but rather, a previous measurement may be used by the controller 36 to time the opening and closing of the shutter 41. The number of wafers processed between measurements is a matter of design discretion, which depends substantially on the details of the particular embodiment.

Light passing through the shutter 41 ultimately impinges on the layer of photoresist 23 after passing through a photomask or reticle 42. The light causes the layer of photoresist 23 in areas below optically transmissive regions 44 of the photomask 42 to change character or otherwise develop. The nature of this development may be affected by the character of certain optical parameters 45 associated with the light source 40. The optical parameters 45 may be monitored by the controller 36, and in some cases, the controller 36 may be capable of effecting changes to the optical parameters 45. For example, the optical parameters 45 may include a numerical aperture of a lens (not shown) used to focus the light source 40, and the degree of partial coherence of the light source 40. These parameters may be varied when different devices are processed within the stepper 39 or when different layers of a device are processed by the stepper 39. Variations in these optical parameters may affect the time period that the shutter 41 should be held open. For example, a smaller numerical aperture may require a longer exposure, owing to the reduced amount of light passing through the lens (not shown) and onto the layer of photoresist 23. Similarly, increasing partial coherence so that the light emitted from the light source is more highly collimated may require a reduction in the period of time that the shutter 41 should be held open. Accordingly, it would be useful for the automatic process controller 36 to be aware of variations in the optical parameters 45 so that the timing of the shutter 41 may be varied appropriately.

In the illustrated embodiment, the automatic process controller 36 is a computer programmed with software to implement the functions described. However, as will be appreciated by those of ordinary skill in the art, a hardware controller (not shown) designed to implement the particular functions may also be used.

Moreover, the functions of the controller described herein may be performed by one or more processing units that may or may not be geographically dispersed. Portions of the invention and corresponding detailed description are presented in terms of software, or algorithms and symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the ones by which those of ordinary skill in the art effectively convey the substance of their work to others of ordinary skill in the art. An algorithm, as the term is used here, and as it is used generally, is conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of optical, electrical, or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, or as is apparent from the discussion, terms such as "processing" or "computing" or "calculating" or "determining" or "displaying" or the like, refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical, electronic quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. An exemplary software system capable of being adapted to perform the functions of the automatic process controller 36, as described, is the KLA Tencor Catalyst system offered by KLA Tencor, Inc. The KLA Tencor Catalyst system uses Semiconductor Equipment and Materials International (SEMI) Computer Integrated Manufacturing (CIM) Framework compliant system technologies, and is based on the Advanced Process Control (APC) Framework. CIM (SEMI E81-0699 - Provisional Specification for CIM Framework Domain Architecture) and APC (SEMI E93-0999 - Provisional Specification for CIM Framework Advanced Process Control Component) specifications are publicly available from SEMI.

Referring to Figure 5, one illustrative embodiment of a process 500 used to control the stepper 39 is depicted in flowchart form. As shown therein, the present invention comprises the process 500 beginning at block 502 with the controller 36 determining whether the optical parameters, such as the numerical aperture and partial coherence have been changed since the last iteration through the process 500. If the optical parameters have been changed, control proceeds to block 504 where the new optical parameters are determined.

The new optical parameters may be automatically detected by the controller 36 through conventional feedback mechanisms, such as switches, resistors, capacitors, or other types of sensors (not shown). Alternatively, the controller 36 may "know" that the optical parameters have been changed simply because the controller 36 has requested and implemented the changes to the optical parameters. For example, the controller 36 may have a recipe stored therein that controls, inter alia, the numerical aperture and partial coherence of the light source 40. On the other hand, the stepper 39 may control the settings of the optical parameters. In this case, the numerical aperture and partial coherence settings may be communicated from the stepper 39 to the controller 36. In block 506, the process 500 retrieves baselines for the newly set optical parameters. In one embodiment, look up tables have been programmed with photodose values for a variety of numerical aperture and partial coherence settings. Thus, the controller 36 uses the new partial coherence and numerical aperture values to access the look up tables and retrieve the desired photodose that corresponds thereto. Thereafter, in block 508, the retrieved photodose value is set as the desired photodose. In block 510, the layer of photoresist 23 may be exposed to the light source 40 by opening the shutter

41. The photodose sensor 43 detects the intensity of the light source 40 in block 512 and delivers a signal having a magnitude representative thereof to the controller 36. The duration that the shutter 41 is to be held open may then be determined or modified based on the detected intensity and the desired photodose. That is, the process 500 determines how long to hold open the shutter 41 to provide the desired photodose in view of the detected intensity. Finally, in block 516, the exposure process is completed at the conclusion of the time period, and the shutter 41 is closed.

Turning now to Figure 6, an alternative process 500' used to control the stepper 39 is depicted in flowchart form. As shown therein, the process 500' differs from the process 500 principally in the area associated with the baselines for the new optical parameters in boxes 506' and 508.' In some applications, it may be possible to characterize the relationship between the optical parameters and photodose via an equation. In block 506' the new optical properties are used to calculate the desired photodose. For example, in one embodiment the photodose is calculated using the equation:

Photodose = ki - k₂(na) - k₃(PC), where ki, k₂ and k₃ are constants determined by characterization of the process in operation at or near nominal operating conditions.

Thus, the controller 36 uses the new partial coherence and numerical aperture values to calculate the desired photodose that corresponds thereto. Thereafter, in block 508', the retrieved photodose value is set as the desired photodose. Thereafter, the process 500 ' illustrated in Figure 6 is substantially identical to the process 500 of Figure 5. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

1. A method, comprising: energizing a light source; determining an optical parameter of the light source; determining a desired photodose based upon the optical parameters of the light source; and exposing a device to the light source for a duration of time to provide the desired photodose.

2. A method, as set forth in claim 1, wherein determining the optical parameter of the light source further comprises determining a numerical aperture setting of the light source.

3. A method, as set forth in claim 1, wherein determining the optical parameter of the light source further comprises determining a partial coherence setting of the light source.

4. A method, as set forth in claim 1, wherein determining the optical parameter of the light source further comprises determining a partial coherence setting and a numerical aperture setting of the light source.

5. A method, as set forth in claim 1, wherein exposing the device to the light source for the duration of time to provide the desired photodose further comprises: detecting the intensity of the light source; determining a first period of time needed to provide the desired photodose at the detected intensity; and exposing the device to the light source for the first period of time.

6. A system, comprising: a stepper having a light source controUably energizable to provide light to a surface of a semiconductor device; and a controller capable of determining an optical parameter of the light source of the stepper, and controUably energizing the light source for a period of time responsive to the optical parameter.

7. A system, as set forth in claim 6, wherein the controller is capable of determining the numerical aperture of the light source and controUably energizing the light source for a period of time responsive to the numerical aperture.

8. A system, as set forth in claim 6, wherein the controller is capable of determining the partial coherence of the light source and controUably energizing the light source for a period of time responsive to the partial coherence.

9. A method for patterning a layer of photoresist on a semiconductor device, comprising: applying the layer of photoresist to a substrate; providing a light source; interposing a photomask between the light source and the layer of photoresist; energizing the light source; determining an optical parameter of the light source; determining a desired photodose based upon the optical parameters of the light source; and exposing the layer of photoresist to the light source through the photomask for a duration of time to provide the desired photodose.

10. A method, as set forth in claim 9, wherein determining the optical parameter of the light source further comprises determining a numerical aperture setting of the light source.

11. A method, as set forth in claim 9, wherein determining the optical parameter of the light source further comprises determining a partial coherence setting of the light source.

12. A method, as set forth in claim 9, wherein exposing the layer of photresist to the light source for the duration of time to provide the desired photodose further comprises: detecting the intensity of the light source; determining a first period of time needed to provide the desired photodose at the detected intensity; and exposing the device to the light source for the first period of time.