DE10321680A1 - Photolithographic film quality control procedure images mask structures using higher order beams from diffraction grating creating intensity profile for comparison with reference - Google Patents

Abstract

A photolithographic film (24) quality control procedure images (28, 30) mask structures on a substrate (38) using the diffraction grating (16) in the mask (14) with asymmetric illumination to create high order beams (32, 34) that are focussed (28, 30) overlaid on the substrate plane (38) to create an overlaid intensity profile (40) for comparison with reference profiles to determine imaging errors.

Description

The The invention relates to a method for determining the quality of a Pellicles attached to a frame arranged on a mask Protection of a structure arranged on the mask against contamination is attached with microscopic particles. The invention relates in particular methods for quality determination by Pellicles, which are used in 157 nm lithography.

Surfaces on Masks in which active structures for transfer to semiconductor wafers are generally formed with the help of transparent Protective materials against external influences, such as for example contaminating particles or mechanical influences, protected. These protective devices, also called pellicles, have so far been used mostly from thin Polymer films are formed, which are mounted on a frame, which is attached to the mask, the frame being the active, structures to be imaged on a wafer on the mask. contaminating Particles can not store in areas of active structures, but only collect on the elastic, due to the arrangement pellicle membranes spaced from the active structures of the frame. You can thus contaminating particles during the exposure of a semiconductor wafer in the beam path of the exposure system between the mask and objective lenses, but they are precisely because of the distance from the active structures in a focus position and therefore have a negligible Influence on the image.

With the transition structure sizes are becoming smaller and smaller in the area of manufacturing Circuits also reduce the exposure of semiconductor wafers wavelength used associated. The increasing energy with each technology generation of radiation transmitting the polymer compound of the pellicle disadvantageous especially from an exposure wavelength of 157 nm and less to a severe degradation or blackening of the Polymer membrane after exposure from, for example, 3 to 5 lots of 25 wafers each. This blackening causes inhomogeneities on one side Wafer-to-wafer basis in relation to the radiation doses received from semiconductor wafers on the other hand, it can also lead to superficial intensity gradients come between the substructures on a wafer.

you therefore goes to radiation stable materials for Find and deploy pellicles. For exposure to light the wavelength 157 nm are no longer called soft pellicles Polymer films are used, but, for example, 800 μm thick fluorine-doped Quartz plate, so-called hard pellicles. These ensure adequate resistance as well as one over many Constant transmission across exposures. The named thickness of 800 μm guaranteed also sufficient rigidity of the quartz plate, so that damage through external influences can be avoided.

On Problem, which is due in particular to these hard pellicles the big Thick and comparatively high refractive indices of n = 1.5 ... 1.6 arise can is that that Hard pellicle as an optical element in the beam path of the imaging system can act and, for example, through spherical aberration to distortions of the structures to be imaged on the wafer. However, it is possible, the effects due to the spherical Correct aberration by adjusting the lens system, for example by moving lens elements in the beam path.

In particular dynamic effects when exposing a wafer with a a pellicle Can mask but can hardly be compensated. It can be, for example in order to excite internal vibrations of the act hard pellicles attached to the frame, or the hard pellicle is due to the gravity and / or thickness variations of the pellicle bent out of its ideal position on the mask.

Around hence incorrectly placed pellicles on the trench on the mask to be able to identify are after applying the pellicle or before using the mask in an exposure device metrological Investigations to determine the quality of the Pellicles or the pellicle assembly. As a rule interferometric measurements were carried out, which involved a great deal of effort it is determined how, for example, the pellicle layer in parallel, i.e. the polymer film in conventional Pellicles or the quartz plate in hard pellicles to the chrome layer on the surface of the Mask are arranged. Such measurements are typically immediate after attaching the pellicle to the frame of the mask, for example made at the mask manufacturer. Further investigations concern the control of fluctuations in the thickness of the polymer film or the quartz plate. These measurements are usually the responsibility area of the pellicle manufacturer.

The measurements of the so-called "post-mount metrology" are mostly carried out interferometrically on useful structures. It measures how large the offset or the resolution at different positions on the mask. The quality of the image and thus of the pellicle mounting can be determined from this. Line-column patterns or contact hole arrangements are used as useful structures, for example.

The interferometric measurements are carried out ex situ. This leads to the disadvantage that information regarding goodness of a pellicle neither at the time of pellicle mounting nor at the time of exposure. On the one hand, therefore neither immediately applying a new pellicle to the mask can be repeated, still suitable Adjustments to the lens system can be made.

It is therefore the object of the present invention, the quality determination of pellicles or pellicle mounting to improve the quality of the exposure of a wafer by means of a mask equipped with a pellicle, and to increase the throughput of wafer exposures.

The Task is solved by a method with the features according to claim 1. Advantageous refinements are the dependent Claims too remove.

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

Preferably so-called phase diffraction gratings are used, such as are described in US 2003/0020901 A1. Alone embodiments and examples described in said publication Diffraction gratings are also advantageous for the present invention applicable. It can be used only with these diffraction gratings Measure the structure of the mask with the hard or soft pellicle to be examined but it is also possible Masks with active product structures in one or more peripheral Areas, such as that after the projection on the wafer as a saw frame designated area to be provided with diffraction gratings.

The Diffraction gratings have the consequence that one in the exposure device, such as a scanner or stepper, incident on the mask and the diffraction grating Light beam into a first light beam representing the 0th diffraction order and representing exactly one diffraction order of a pair of higher diffraction orders second light beam and possibly not at this point further interested light rays decomposed even higher diffraction orders becomes. Due to the different directions of diffraction, different ones Areas of the pellicle spaced from the active mask structures radiated by the light rays. Depending on the orientation of the diffraction grating on the mask and depending on the grating constant of the diffraction grating, the second light beam can be turned into a desired one Area of the pellicle bent, i.e. relative to the direction of incidence of the still undeflected beam are deflected onto the mask.

In the image plane, i.e. for example one with a photosensitive Layer of coated wafer or one arranged in the wafer plane System of photo detectors etc., become the first and the second Partial beam superimposed again by focusing, so that the resulting Pattern represents a wave aberration caused by the pellicle. The representing the 0th diffraction order The first light beam serves as a reference beam, in relation to which is the local aberration properties of the pellicle for everyone Angle and distance from a reference point on the mask or the numerical aperture can be determined with the help of the second light beam.

Becomes performed the procedure on the same exposure device with the same mask without that this Hard or soft pellicle has been assembled beforehand, so the influence of aberration properties of the lens system in a comparison of the measurement results with a mask subtract with assembled pellicle. In the event that the aberration properties the pellicle exceeds a predetermined tolerance value, it is intended to either remove the pellicle from the frame and reassemble, or to install a completely new pellicle. With the help of the invention is therefore an accurate characterization of goodness of pellicles or pellicle assembly. In particular, it can also the influence of Pellicle frame on the mask on the aberration properties of the Hard or soft pellicles can be determined. This is particularly so important because of the vibration properties of the pellicle during the Moving the mask when scanning in an exposure device from the Tension or torsion properties due to the pellicle frame depend.

Another advantageous aspect of the inven is that with a pellicle which is mounted on a frame on a mask with known aberration properties, the dynamic properties of the scanning mechanism of an exposure device can be examined.

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

With the method according to the invention there is the particular advantage that the examination of hard or soft pellicles in the exposure devices themselves - in situ - in relation are carried out on their goodness can. It is therefore possible directly as a result of an observed aberration behavior, for example the lens system to the individual, current situation in one Adjust exposure device. A time-consuming 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 using a mask can be increased can. Waiting times for determining the measurement results are thus avoided.

The The invention is now to be illustrated using an exemplary embodiment with the aid of a Drawing closer explained become. In it show:

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

2 a flow diagram of an embodiment of a method according to the invention.

In 1a is a schematic representation of the beam path of an exposure device, in which the inventive method is to be carried out. The exposure device is, for example, a scanner. Using a radiation source 10 becomes a ray of light 12 the wavelength of 157 nm. The radiation source 10 includes, for example, an F ₂ laser. The beam of light 12 will put on a mask 14 which includes a transparent quartz support plate on which a phase diffraction grating 16 is formed. The phase diffraction grating 16 has three periodically repeating transparent areas 18 . 19 . 20 between which a relative phase difference of 90 ° (first area 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 the next grid section is 180 °.

The phase difference is determined, for example, by an etched depression 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 area 19 twice the width 2W has. Such a phase diffraction grating is described for example in the mentioned publication US 2003/0020901 A1.

On the mask 14 is a framework 22 attached. On the frame 22 is a hard pellicle 24 mounted that in 1a is exaggeratedly shown in a bent state representing a vibration. This dynamic state (acceleration, deceleration, excitation of natural vibrations of the pellicle) is caused, for example, by a movement 26 the mask 14 induced in the scanner during the scanning process.

Through the phase diffraction grating 16 becomes the incident light 12 into a number of diffraction orders 32 . 34 decomposed, each represented by partial beams. In 1a is a first partial beam representing the 0th diffraction order 32 and a second partial beam representing the 1st diffraction order 34 shown which are diffracted by the diffraction grating in different directions. The diffraction grating described above 16 is configured such that of the pair of first diffraction orders only the partial beam 34 the plus first diffraction order, but not another sub-beam, for example the minus first diffraction order. This diffraction grating enables the generation of further partial beams, which represent even higher diffraction orders 16 not excluded, but they are at such a large angle by the diffraction grating 16 distracted that it was the aperture 36 of the lighting system cannot happen.

Through the lens system 28 . 30 the two partial beams 32 . 34 in the image plane 38 focused and superimposed, resulting in a sinusoidal intensity profile in the specific case of the embodiment. This is due to the diffraction grating 16 resulting spectrum of diffraction orders is in 1b shown, with the dominant light contribution to the intensity profile 40 in the image plane 38 through the plus first diffraction order 34 is contributed. In the schematic representation of the 1b is through the diffraction grating 16 obliterated minus first diffraction order 34 ' marked with dashed lines.

The partial beams 32 . 34 transmit the pellicle locally in different areas due to the different diffraction directions, see that the local aberration properties of the pellicle 24 based on that in 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 depending on the degree of transmission properties, the overall quality of the pellicle used 24 are determined.

To the right of 1a is the point of passage of the respective partial beams in plan view 32 respectively. 34 through the aperture 36 shown. The partial beam 32 according to the 0th diffraction order can be viewed as a reference beam while using the first sub-beam 34 corresponding to the plus first order of diffraction by suitable choice of the angular orientation of the diffraction grating on the mask and the grating constant at a desired position within the aperture opening 36 can be brought. A large number of diffraction gratings is advantageously used for this purpose 16 arranged on the mask with different orientations and lattice constants in order to be able to characterize the individual aberration contributions of the pellicle. This is analogous to determining the distribution of the transmittance of the pellicle 24 ,

In 2 A flowchart is shown, which shows an embodiment of the method according to the invention, which is based on an arrangement 1 can be applied. First, the mask described 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 performed, wherein the light is diffracted on the phase diffraction gratings, so that the pellicle 24 is irradiated with partial beams diffracted in different directions.

In the image plane 38 is a wafer provided with a photosensitive layer, on which the partial beams are focused and superimposed again, so that the in 1a shown sinusoidal profile 40 is formed in the lacquer. The exposed and developed varnish for measuring the intensity profile is examined in a dark field microscope. Alternatively, it is also possible to do this directly in the image plane, for example with sensors attached to the wafer stage 38 to scan the resulting intensity profile by moving the wafer days. According to the latter aspect, there is the advantage that the wafer substrate also 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 library of reference profiles created in advance. Each of the reference profiles is assigned a contribution of aberrations of different orders, such as choma, defocus, three-wave (three-leaf clover), etc. The aberration of the pellicle can be precisely determined by identifying the reference profile that best matches the measured intensity profile. Through a multitude of measured intensity profiles 40 , each one on the mask 14 Diffraction gratings arranged with different orientations and grating constants 16 are assigned, it is possible to have an aberration map of the pellicle 24 to create.

Without the mask or wafer having to be removed from the exposure system, a detailed lens adjustment can now be made in the scanner in order to determine the exposure properties for a further mask that has a frame 22 and a pellicle 24 of the type currently being examined.

Instead of comparing intensity profiles with reference profiles or deriving the aberration directly from the intensity profile, the focus of the exposure system can also be varied according to a further exemplary embodiment, the focus setting for which the intensity profile provides a maximum contrast being identified for each diffraction grating. In this way they deliver through each diffraction grating 16 intensity profiles generated a defocus characterization of the pellicle 24 , which can also be used to infer the aberration properties.

In order to be able to subtract the influence of the lens aberration from the current measurement results, it is advantageous to 2 steps of the method according to the invention, 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 determined in this way are then subtracted from the combined aberration properties of the pellicle and lens which are determined later, so that 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 areas of the Beu

supply gitters

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 (11)

Procedure for determining the quality of a pellicle ( 24 ) on a mask ( 14 ) arranged frame ( 22 ) to protect one on the mask ( 14 ) arranged structure is secured against contamination with microscopic particles, comprising the steps: - providing an exposure device comprising a lens system ( 28 . 30 ) for imaging structures formed on masks in a substrate plane ( 38 ) - Provision of the mask ( 14 ) with the structure, the structure comprising a number of diffraction gratings each rotated relative to one another by an angle in their alignment on the mask ( 16 ) includes, - irradiation of the diffraction gratings ( 16 ) on the mask ( 14 ), so that in each case at least one first partial beam representing the zeroth diffraction order ( 32 ) and exactly one second partial beam ( 34 ), which represents a pair of higher diffraction orders, by asymmetrical diffraction of the incident light beam ( 12 ) is generated, - radiation through the pellicle ( 24 ) with the diffraction on the diffraction grating ( 16 ) generated first and second partial beams ( 32 . 34 ), - focusing and superimposing the first and second partial beams ( 32 . 34 ) in the substrate plane ( 38 ) by means of the lens system ( 28 . 30 ) to form a superimposed intensity profile ( 40 ) in a substrate plane ( 38 ), - measuring the respective intensity profile ( 40 ), - determining one by the pellicle ( 24 ) caused optical imaging error as a function of the measured intensity profile ( 40 ). 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 of which represents an optical aberration. A method according to claim 1, characterized in that - the steps radiating through the pellicle ( 24 ) and focusing the partial beams ( 32 . 34 ) for different focus settings of the lens system ( 28 . 30 ) are repeated, - for each of the diffraction gratings rotated by the angle ( 16 ) a focus setting for which a maximum amplitude in the superimposed intensity profile ( 40 ) is achieved, is determined, - a curve is plotted with the respectively determined focus setting against the angle of the alignment on the mask, - the plotted curve is compared with at least one reference curve, which each represents an optical imaging error. Method according to one of Claims 1 to 3, characterized in that settings of the lens system ( 28 . 30 ), in particular an intended focus setting, for carrying out an exposure of a substrate with that with the pellicle ( 24 ) provided mask ( 14 ) be adjusted. Method according to one of the preceding claims, characterized in that - in additional steps, the optical imaging properties of the lens system ( 28 . 30 ) using the mask ( 14 ) with which the diffraction gratings ( 16 ) comprehensive structure, but without a on a frame ( 22 ) the mask ( 14 ) attached pellicle ( 24 ) are determined, - the additionally determined optical imaging errors of the lens system ( 28 . 30 ) of those for the pellicle ( 24 ) and the lens system ( 28 . 30 ) subtracted optical aberrations determined together. Method according to one of the preceding claims, characterized in that the pellicle ( 24 ) comprises a solid, transparent, essentially inelastic material, preferably quartz. A method according to claim 6, characterized in that the pellicle ( 24 ) has a thickness of at least 800 μm. A method according to claim 6 or 7, characterized in that the material has a temperature and light resistance to incident light ( 12 ) with a wavelength of 157 nm or less. Method according to one of claims 1 to 5, characterized in that the pellicle ( 24 ) comprises an elastic polymer film. Method according to one of the preceding claims, characterized in that settings of the lens system ( 28 . 30 ) due to the determination of the aberration to compensate for the optical aberration can be adjusted. Use of the method according to one of claims 1-9 for determining the suitability of a material for the frame ( 22 ) with which the pellicle ( 24 ) on the mask ( 14 ) is attached.