DE10321680B4 - Method for determining the quality of a pellicle - Google Patents

Abstract

A method of determining the quality of a pellicle (24) attached to a frame (22) disposed on a mask (14) for protecting a structure disposed on the mask (14) from microscopic particle contamination comprising the steps of:
Providing an exposure apparatus comprising a lens system (28, 30) for imaging structures formed on masks into a substrate plane (38),
Providing the mask (14) with the structure, the structure comprising a number of diffraction gratings (16) twisted against each other at an angle in their orientation on the mask,
- Irradiating the diffraction gratings (16) on the mask (14), so that in each case at least one zeroth diffraction order representing the first partial beam (32) and each exactly a second partial beam (34), which represents a pair of higher diffraction orders, by asymmetric diffraction of incident light beam (12) is generated,
- Radiation of the pellicle (24) with the generated by diffraction at the diffraction grating (16) first ...

Description

The The invention relates to a method for determining the quality of a Pellicles standing on a frame arranged on a mask for Protection of a structure arranged on the mask from contamination attached with microscopic particles. The invention relates in particular method for determination of quality of pellicles used in 157 nm lithography.

Surfaces on Masks in which active structures for transfer to semiconductor wafers are formed, in general, with the help of transparent Protective materials against external influences, such as for example, contaminating particles or mechanical effects, protected. These also called pellicle protections have been so far mostly thin Formed polymer films which are mounted on a frame, which is mounted on the mask, where the frame is the active, on a wafer to be imaged structures on the mask surrounds. contaminating Particles can become so not in areas of active structures store, but accumulate only on the elastic, due to the arrangement the frame spaced from the active structures pellicle membranes. Although you can thus contaminating particles during the exposure of a semiconductor wafer in the beam path of the exposure system between the mask and the objective lenses, but they are just because of the distance from the active structures not in a focus position and therefore have a negligible Influence on the illustration.

With the transition to ever smaller structural sizes is in the field of manufacturing integrated Circuits also reduce the exposure of the semiconductor wafer used wavelength associated. The resulting increase in energy with each technology generation the polymer compound of the pellicle transmits radiation disadvantageous way, especially from an exposure wavelength of 157 nm and less to a strong degradation or blackening of the Polymer membrane after the exposure of, for example, already 3 to 5 lots of 25 wafers each. This blackening on the one hand, inhomogeneities on one Wafer-to-wafer basis with respect to the radiation doses received by semiconductor wafers On the other hand, it can also cause superficial intensity gradients come between the substructures on a wafer.

you therefore, Radiation-stable materials for To search for and use pellicles. For exposure to light the wavelength 157 nm are no longer called the soft pellicles Polymer films used, but for example 800 microns thick fluorine-doped Quartz plate, so-called Hartpellicles used. These ensure a sufficient resistance as well as one over many Exposures constant transmission. The said thickness of 800 microns guaranteed also a sufficient rigidity of the quartz plate, so that damage by external influences can be avoided.

One Problem, but especially with these Hartpellicles due the big Thickness and comparatively high refractive indices of n = 1.5 ... 1.6 arise can, is that that Hartpellicle as an optical element in the beam path of the imaging system can act and distortions, for example, by spherical aberration can lead the structures to be imaged on the wafer. It is, however possible, the effects due to the spherical Aberration correct by adjusting the lens system, for example by moving lens elements in the beam path.

Especially dynamic effects when exposing a wafer through a with equipped with a pellicle Mask can however, they are hardly compensated. This can be for example to be excited by a scan process, internal vibrations of the Hartpellicles attached to the frame act, or the Hartpellicle is due to its own gravity and / or thickness variations of the pellicles bent out of its ideal position on the mask.

Around therefore incorrectly attached to the trench on the mask pellicles to be able to identify are after applying the pellicle or before using the mask in an exposure device metrological Investigations on the determination of the quality of the Pellicles or pellicle assembly. As a rule, be this interferometric measurements carried out in which at great expense is determined, such as parallel, for example, the pellicle layer, d. H. the polymer film in conventional Pellicles or the quartz plate in Hartpellicles to the chromium layer on the surface of the Mask are arranged. Such measurements typically become immediate after attaching the pellicle to the frame of the mask, for example made at the mask manufacturer. Further investigations concern the control of thickness variations of the polymer film or the quartz plate. These measurements are usually in the area of responsibility of the pellicle manufacturer.

The measurements of the so-called "post-mount metrology" are mostly carried out interferometrically on user structures. It is measured how large the offset or the resolution at different positions of the mask. From this, the quality of the picture and thus of the pellic le-mountings are determined. As utility structures, for example, line-column patterns or contact hole arrangements are used.

The interferometric measurements are performed ex-situ. This leads to the disadvantage that information concerning the goodness of a pellicle neither at the time of pellicle mounting nor at the time of exposure.

On the one hand therefore, can not immediately apply a new pellicle the mask can be repeated, nor can appropriate adjustments of the Lensensystems performed become.

US 2003/0020901 A1 discloses a method of detecting optical aberrations of a lens system using a reticle having a phase diffraction grating that causes asymmetric diffraction of radiation. A pellicle is not provided on the reticle. US 2002/0086224 A1 discloses a reticle mask with a pellicle. JP 04130711 A discloses a method of producing an optical image using a reticle mask having two pellicles on opposite sides, correcting aberrations caused by the reticle mask and its pellicles in terms of focusing and magnification.

It the object of the present invention is the determination of quality of pellicles or pellicle mounting, the quality of the exposure a wafer by a fitted with a pellicle mask, as well as to increase the throughput of wafer exposures.

The Task is solved by a method having the features according to claim 1 and by a Use of this method according to claim 10. Advantageous embodiments are the dependent claims remove.

The inventive method can be used on elastic soft pellicles as well as on radiation stable Hartpellicles be applied. The pellicle to be examined is on a mask, which is provided with a Pelliclerahmen mounted. The mask includes special diffraction gratings, with which incident light rays preferably can be bent asymmetrically. Asymmetric here means that of a couple higher Diffraction orders, for example a +1. and a -1. Diffraction order, only exactly one diffraction order through the diffraction grating is generated while the light contributions through the individual grating components at the respective complementary diffraction order extinguish.

Preferably, so-called phase diffraction gratings are used, as described for example in the publication US 2003/0020901 A1 are described. All embodiments and examples of diffraction gratings described in the cited document are advantageously also applicable to the present invention. It is possible to measure a mask with the hard or soft pellicle to be examined, which is provided specifically only with these diffraction gratings as structures, but it is also possible to use masks with active product structures in one or more peripheral areas, for example after the projection onto the wafer As sawing frame designated area to be provided with diffraction gratings.

The Diffraction gratings have the consequence that a in the exposure device, such as a scanner or stepper, incident on the mask and the diffraction grating Light beam in a 0th diffraction order representing the first light beam and representing exactly one diffraction order of a pair of higher diffraction orders second light beam and optionally not at this point further interested light beams even higher diffraction orders decomposed becomes. Due to the different diffraction directions are different Regions of pellicles spaced from the active mask structures respectively irradiated by the light beams. Depending on the orientation the diffraction grating on the mask and depending on the lattice constant of the diffraction grating, the second light beam in a desired Area of the pellicle diffracted, d. H. relatively from the direction of incidence of the still undeflected beam are deflected onto the mask.

In the picture plane, d. H. for example, one with a photosensitive Layer of coated wafers or arranged in the wafer plane System of photodetectors, etc., become the first and the second Sub-beam by focusing again Überla siege, so that the resulting Pattern represents a wave aberration caused by the pellicle. Of the the 0th diffraction order representing first light beam serves as a reference beam, in relation to what the local aberration properties of pellicles for each Angle and distance from a reference point on the mask or the numerical aperture opening be determined with the help of the second light beam.

According to the invention, the method is additionally carried out on the same exposure apparatus with the same mask, without the hard or soft pellicle was previously mounted. As a result, the influence of the aberration properties of the lens system can be subtracted in a comparison of the measurement results in a mask with mounted pellicle. In the event that the aberration properties of the pellicle exceed a pre-specified tolerance value, it is intended to either remove the pellicle from the frame and he neut to mount, or to install a completely new pellicle. With the aid of the invention, therefore, a precise characterization of the quality of pellicles or the pellicle assembly is made possible. In particular, it is also possible to determine the influence of the pellicle frame on the mask on the aberration properties of the hard or soft pellicle. This is particularly important because the vibration characteristics of the pellicle during scanning of the mask during scanning in an exposure apparatus depend on the tensioning or torsion properties due to the pellicle frame.

One Another advantageous aspect of the invention is that in a Pellicle, with known aberration properties on a frame mounted on a mask, the dynamic characteristics of the Scanning mechanism of an exposure device can be examined.

At long last can by the invention also for Pelliclerahmen used materials such as metals, metal foams As well as quartz coatings are checked for their suitability.

With the method of the invention has the particular advantage that the study of hard or soft pellets in the exposure devices themselves - ie in situ - in relation be performed on their goodness can. It is therefore possible directly as a result of a detected aberration behavior, for example the lens system to the individual, current situation in one Adjust exposure device. An expensive removal of the mask from the exposure device, the transport to a metrology device and the investigation in the metrology device can be omitted, so that the throughput of Products that are made with the help of a mask can be increased can. Waiting times for determining the measurement results are thus avoided.

The Invention will now be described with reference to an embodiment with the aid of a Drawing closer explained become. Show:

1 a schematic representation of an optical path with mask and pellicle for carrying out the method according to the invention,

2 a flowchart of an embodiment of a method according to the invention.

In 1a is a schematic representation of the beam path of an exposure device shown, in which the inventive method is to be performed. The exposure device is, for example, a scanner. By means of a radiation source 10 becomes a ray of light 12 the wavelength 157 nm generated. The radiation source 10 includes, for example, an F ₂ laser. The light beam 12 is on a mask 14 which comprises a transparent support plate made of quartz, on which a phase diffraction grating 16 is formed. The phase diffraction grating 16 Each has three periodically repeating transparent areas 18 . 19 . 20 between which a relative phase difference of 90 ° (first range 18 to the second area 19 and second area 19 to the third area 20 ) consists. The phase difference from the third area 20 to the first area 18 of the next grating section is 180 °.

The phase difference, for example, by an etched recess in the quartz substrate of the mask 14 allows. The first area 18 and the third area 20 have the same width W, while the second range 19 twice the width 2W has. Such a phase diffraction grating is for example in the cited document US 2003/0020901 A1 described.

On the mask 14 is a frame 22 attached. On the frame 22 is a Hartpellicle 24 mounted that in 1a is exaggerated in a bent state representing a vibration. This dynamic state (acceleration, deceleration, excitation of natural oscillations of the pellicle) is for example due to a movement 26 the mask 14 during the scan process induced in the scanner.

Through the phase diffraction grating 16 becomes the incident light 12 in a number of diffraction orders 32 . 34 decomposed, which are each represented by partial beams. In 1a is a first partial beam representing the 0th order of diffraction 32 and a second partial beam representing the 1st diffraction order 34 represented diffracted in different directions from the diffraction grating. The diffraction grating described above 16 is configured such that of the pair of first diffraction orders, only the sub-beam 34 the plus first diffraction order, but not a further partial beam, for example, the minus first diffraction order is generated. The generation of further partial beams, which represent even higher diffraction orders, is through this diffraction grating 16 Although not excluded, these are under such a large angle through the diffraction grating 16 deflected that it the aperture opening 36 of the lighting system can not pass.

Through the lens system 28 . 30 become the two partial beams 32 . 34 in the picture plane 38 focused and superimposed, wherein in the specific case of the embodiment, a sinusoidal intensity profile results. That is due to the diffraction grating 16 resulting spectrum of diffraction orders is in 1b represented, wherein the dominant light contribution to the intensity profile 40 in the picture plane 38 through the plus first diffraction order 34 is contributed. In the schematic representation of 1b is the one through the diffraction grating 16 extinguished minus first diffraction order 34 ' dotted marked.

The partial beams 32 . 34 Due to the different diffraction directions, the pellicle transmits locally in different areas, so that the local aberration properties of the pellicle 24 on the basis of the image plane 38 superimposed intensity profile 40 can be evaluated. The amplitude, the phase position and the frequency of the sinusoidal profile 40 depend in particular on the different light paths of the partial beams 32 and 34 but also on the degree of transmission properties, the overall on the quality of the pellicles used 24 are determined.

On the right side of 1a is in plan view the passage point of the respective partial beams 32 respectively. 34 through the aperture opening 36 shown. The partial beam 32 according to the 0th diffraction order can be considered as a reference beam, while using the first partial beam 34 according to the plus first order of diffraction by suitable choice of the angular orientation of the diffraction grating on the mask and the lattice constant to a desired position within the aperture opening 36 can be brought. Advantageously, this is a plurality of diffraction gratings 16 arranged with different orientations and lattice constants on the mask in order to characterize the individual Aberrationsbeiträge of Pellicles can. This is analogous to the determination of the distribution of the transmissivity of the pellicle 24 ,

In 2 a flowchart is shown which shows an embodiment of the inventive method, which is based on an arrangement according to 1 can be applied. First, the described mask is provided with the number of phase diffraction gratings. Next, a hard pellicle, ie, a quartz plate with a thickness of 800 μm, is placed on the pellicle frame 22 on the mask 14 assembled. As described, the exposure with a wavelength of 157 nm in the in 1a shown scanner, wherein the light is diffracted at the phase diffraction gratings, so that the pellicle 24 is irradiated with diffracted in different directions partial beams.

In the picture plane 38 a wafer provided with a photosensitive layer is arranged, on which the partial beams are focused and superimposed again, so that the in 1a shown sinusoidal profile 40 in the paint is formed. In a dark field microscope, the exposed and developed lacquer for measuring the intensity profile is examined. Alternatively, it is also possible to use sensors mounted directly on the wafer stage, for example, in the image plane 38 To sample the resulting intensity profile by wafer-wafer processes. According to the latter aspect, there is the advantage that also the wafer substrate does not have to be removed from the exposure chamber.

The intensity profile characterized or measured on the basis of the measurement can now be compared with a previously prepared library of reference profiles. Each of the reference profiles is assigned a contribution of aberrations of various orders such as choma, defocus, three-wave clover, etc. By identifying the reference profile that best matches the measured intensity profile, the aberration of the pellicle can be accurately determined. Through a variety of measured intensity profiles 40 , each one on the mask 14 diffraction gratings arranged with different orientation and lattice constant 16 It is possible to have an aberration map of the pellicles 24 to create.

Without having to remove the mask or wafer from the exposure system, a detailed lens adjustment can now be made in the scanner to provide the exposure characteristics for another mask associated with a frame 22 and a pellicle 24 of the currently examined type is equipped to perform.

Instead of comparing intensity profiles with reference profiles or deriving the aberration directly from the intensity profile, according to a further embodiment, the focus of the exposure system can be varied, wherein for each diffraction grating that focus setting is identified for which the intensity profile provides a maximum contrast. In this way, they deliver through each diffraction grating 16 intensity profiles produced a defocus characterization of the pellicle 24 , which can also be closed on the Aberrationseigenschaften.

In order to be able to subtract the influence of the aberration of the lens from the current measurement results, it is provided according to the invention, in addition to those in 2 illustrated steps of the inventive method, a conventional aberration measurement using the mask with the phase diffraction gratings 16 perform without a pellicle 24 on the mask 14 is mounted. The aberration properties thus determined are then subtracted from the later determined combined aberration properties of pellicle and lens hiert, so that a characterization of only the pellicle is possible.

10

radiation source

12

incident beam of light

14

mask

16

Phase grating

18 19, 20

transparent out of phase regions of the diffraction grating

22

Pelliclerahmen

24

pellicle

26

dynamic Movement of the mask, scanning process

28 30

lens system

32

first Partial beam (0th diffraction order)

34

second Partial beam (+1 diffraction order)

34 '

second Partial beam (-1. Diffraction order, extinguished by diffraction grating arrangement)

36

aperture

38

image plane

40

intensity profile

Claims (10)

Method for determining the quality of a pellicle ( 24 ) on a mask ( 14 ) ( 22 ) to protect one on the mask ( 14 ) is attached prior to contamination with microscopic particles, comprising the steps: - providing an exposure apparatus comprising a lens system ( 28 . 30 ) for imaging structures formed on masks into a substrate plane ( 38 ), - providing the mask ( 14 ) having the structure, the structure having a number of diffraction gratings (each rotated by an angle in their orientation on the mask) ( 16 ), - irradiating the diffraction gratings ( 16 ) on the mask ( 14 ), so that in each case at least one zeroth diffraction order representing the first partial beam ( 32 ) and in each case exactly one second partial beam ( 34 ), which represents a pair of higher orders of diffraction, by asymmetric diffraction of the incident light beam (US Pat. 12 ), - irradiation of the pellicle ( 24 ) with the by diffraction at the diffraction grating ( 16 ) generated first and second partial beams ( 32 . 34 ), - focusing and superimposing the respective first and second partial beams ( 32 . 34 ) in the substrate plane ( 38 ) by means of the lens system ( 28 . 30 ) for forming in each case a superimposed intensity profile ( 40 ) in a substrate plane ( 38 ), - measuring the respective intensity profile ( 40 ), - determining one by the pellicle ( 24 ) caused optical aberration depending on the measured intensity profile ( 40 ), wherein - additionally the optical imaging properties of the lens system ( 28 . 30 ) using the mask ( 14 ) with which the diffraction gratings ( 16 ) comprehensive structure, but without one on a frame ( 22 ) of the mask ( 14 ) attached pellicle ( 24 ) and wherein - the additionally determined optical aberrations of the lens system ( 28 . 30 ) of the pellicle ( 24 ) and the lens system ( 28 . 30 ) are subtracted together. Method according to claim 1, characterized in that a respectively measured intensity profile ( 40 ) is compared with a further number of provided reference profiles, each representing an optical aberration. Method according to claim 1, characterized in that - the steps of irradiating the pellicle ( 24 ) and focusing the partial beams ( 32 . 34 ) for different focus settings of the lens system ( 28 . 30 ), for each of the angular gratings ( 16 ) each have a focus setting for which a maximum amplitude in the superimposed intensity profile ( 40 ), a curve with the respective determined focus adjustment is plotted against the angle of the alignment on the mask, the plotted curve is compared with at least one reference curve, which represents an optical aberration in each case. Method according to one of claims 1 to 3, characterized in that depending on the determined optical aberration settings of the lens system ( 28 . 30 ), for carrying out an exposure of a substrate to the one with the pellicle ( 24 provided mask ( 14 ) be adjusted. Method according to one of the preceding claims, characterized in that the pellicle ( 24 ) comprises a solid, transparent, substantially inelastic material. Method according to claim 5, characterized in that the pellicle ( 24 ) has a thickness of at least 800 μm. A method according to claim 5 or 6, characterized in that the material is a temperature and light resistance to irradiated light ( 12 ) having a wavelength of 157 nm or less. Method according to one of claims 1 to 4, characterized in that the pellicle ( 24 ) comprises an elastic polymer film. Method according to one of the preceding claims, characterized in that adjustments of the lens system ( 28 . 30 ) are adjusted as a result of determining the aberration to compensate for the optical aberration. Use of the method according to one of claims 1 to 9 for determining the suitability of a material for the frame ( 22 ), with which the pellicle ( 24 ) on the mask ( 14 ) is attached.