FR2943456A1 - ELECTRONIC LITHOGRAPHY METHOD FOR IMAGING CATHODOLUMINESCENCE. - Google Patents

Abstract

An electronic lithography method for producing devices from objects of micrometric or nanometric dimensions deposited or integrated on a substrate, comprising: a) depositing a layer of resin above a surface of said substrate; b) exposing the resin by scanning an electron beam to selectively release predefined regions of the resin; c) developing the insolated resin to selectively release predefined regions from said surface; and d) the manufacture of the device by etching and / or deposition of material through the resin mask thus obtained; characterized in that it also comprises, after deposition of the resin but prior to its insolation, the acquisition of an image of said object obtained by scanning the surface of the substrate by an electron beam at a dose insufficient to insoluate the resin, and by detecting the cathodoluminescence thus induced; said image being used for controlling the electron beam during said insolation step.

Description

The invention relates to an electronic lithography method for producing devices (making electrical contacts, making photonic crystals, etc.) around objects of micrometric or nanometric dimensions, whether they be on the surface of a substrate (nanowires, nanotubes, nanoparticles ...), buried within a few micrometers (quantum boxes ...), or integrated in objects (quantum well or quantum dot in a nanowire for example).

Nano-objects or nanostructures such as nanowires, nanotubes, nanoparticles or quantum dots are the subject of intense research activity both fundamental and applied. Indeed, these elements are considered as being the basic elements of new electronic or optoelectronic components to exceed the performance of traditional components made from macroscopic crystals of semiconductor materials. Nano-objects or objects of nanometric dimensions are understood to mean objects having at least one dimension - and preferably at least two dimensions - less than one micrometer. For example, a nanowire or a nanotube may have a diameter of a few tens or hundreds of nanometers and a length of several micrometers. Similarly, object of micrometric dimensions means an object having at least one dimension - and preferably at least two dimensions - less than one millimeter.

In order to be able to characterize and / or exploit the various physical properties of these objects, it is generally necessary to produce more or less complex devices on and / or around said objects (planar electrical tracks made on a nanowire placed on an insulating substrate, photonic crystal or nano-turn centered on a buried quantum box, etc.). For said objects, whose size and position are not known deterministically, this operation is complex. Indeed, nanowires, nanotubes or nanoparticles are most often manufactured in large numbers with a certain dispersion in size from substrates incompatible with their characterization or use, or even in a liquid medium. It is therefore necessary to deposit them on a different substrate, for example silicon with possibly a SiO2 layer. Quantum boxes, which are usually buried, are made by techniques that do not allow to know their spatial distribution. For example, the manufacture of electrical contacts by electronic lithography on a nanowire is difficult, because the lithographic pattern must be aligned very precisely on said nanowire. However, the latter is not visible just before the lithography operation itself, because it is covered with a thick layer of electron-sensitive resin. Under these conditions, the observation of the nanowire by conventional electron microscopy is not possible without insolating the resin, while the usual optical microscopy techniques have insufficient resolution.

Conventionally, the procedure is as follows. Registration pins are deposited on the substrate before spreading the resin, and an electron microscopy image is acquired so as to precisely determine the position of the nanowire relative to said pads. After spreading a layer of resin, the pads remain observable by electron microscopy because of their thickness and / or their chemical contrast. It is therefore possible to acquire a second image of said pads at a very low dose, without causing insolation. Then, the two images (without resin and resin) are realigned, which allows to drive the electron beam blind.

Processes for producing electrical contacts on nanoobjects based on the principle explained above are described by the following articles: E. Stern et al. Methods for fabricating Ohmic contacts to nanowires and nanotubes, J. Vac. Sci. Technol. B 24 (1), 2006, pp. 231-236; Th. Weimann et al. Electrical and Structural Characterization of Single ZnO nanorods, Microelectronic Engineering 85 (2008), pp. 1248-1252; - R. M. Langford et al. Comparison of different methods to contact nanowires, J. Vac. Sci. Technol. B 24 (5), 2006, pp. 2306 to 2311; and - Y.F. Hsiou et al. The Chinese Journal of Physics, Vol. 43, No. 1-II, pp. 293-298 (2005). These methods have the disadvantage of being complex and expensive, requiring additional technological steps for depositing the registration pads. In addition, the accuracy of the alignment is adversely affected by its indirect nature. Similar problems also arise in the case where the object to be contacted is not deposited on the substrate, but integrated in the latter (which is the case of quantum boxes which are generally buried a few hundred nanometers below the surface ) or when two contacts must be taken as close as possible to a quantum well or a quantum dot present in a nanowire but whose position is not precisely known. The invention aims to solve the aforementioned drawbacks of the prior art.

The basic idea of the invention is to observe the object by cathodoluminescence after deposition of the resin, just before the lithography step itself. The electronic beam of insolation can thus be driven directly on the basis of a digital image of the object, without the need for tracking pads.

Cathodoluminescence is a well-known physical effect that consists of the emission of infrared, visible or ultraviolet light by a sample irradiated by an electron beam. It is known to exploit this effect to obtain high resolution optical images. To do this, a cathodoluminescent sample is placed in an electron microscope, and scanned by an electron beam. The light emitted by cathodoluminescence is picked up by an optical system provided for this purpose, and used to form a point-by-point image of said sample. The spatial resolution of said image depends only on the diameter of the electron beam and the scanning pitch, and is not limited by the wavelength of the light: it can therefore reach 10 nm. This technique is described, for example, in the High Spatial Resolution Cathodoluminescence technical note, Simon A. Galloway, 2002, available on the Gatan website (http://www.gatan.com/files/PDF/products/ High_Spatial_note. Pdf). See also the book Scanning Electron Microscopy and Microanalysis, Publication of the National Group of Scanning Electron Microscopy and MicroAnalyses, published by François Brisset, EDP Sciences 2008, ISBN 978-2-7598-0082-7, and in particular its chapter XXX, An introduction to the cathodoluminescence of semiconductors by one of the inventors (F. Donatini). The inventors have realized that a cathodoluminescence image having a sufficient resolution can be obtained by using a low electronic dose, insufficient to insolate the resin, which is not possible in conventional electron microscopy. To do this, it is sufficient to choose a substrate having, at least in a spectral range of observation, a cathodoluminescence substantially lower or stronger than that of the object, and a sufficiently transparent resin in said spectral range. However, experience shows that this condition is most often satisfied, especially since it is possible to freely choose the spectral range of observation. The inventors have also realized that it is advantageous to operate at low temperature, for example between 250 K (cooling with liquid nitrogen) and 3 K (cooling with liquid helium). In fact, a lowering of the temperature increases the cathodoluminescence, which makes it possible to increase the contrast of the electronic dose image equal to and decreases the sensitivity of the resin, which makes it possible to increase the dose, and therefore the contrast of the image, without causing sunstroke.

An object of the invention is therefore an electronic lithography method for producing devices from objects of micrometric or nanometric dimensions deposited or integrated on a substrate, comprising: a) the deposition of a layer of resin above a surface of said substrate; b) exposing the resin by scanning an electron beam to selectively release predefined regions of the resin; c) developing the insolated resin to selectively release predefined regions from said surface; and d) the manufacture of the device by etching and / or deposition of material through the resin mask thus obtained; characterized in that it also comprises, after deposition of the resin but prior to its insolation, the acquisition of an image of said object obtained by scanning the surface of the substrate by an electron beam at a dose insufficient to insoluate the resin, and by detecting the cathodoluminescence thus induced; said image being used for controlling the electron beam during said insolation step.

According to particular embodiments of the invention: said image acquisition and insolation steps can be performed by cooling the substrate to a temperature of between 3K and 250K. Said step of acquiring an image can be performed in a limited spectral range by means of a monochromator. The substrate may be chosen so as to have, in an observation spectral region, a cathodoluminescence emissivity which is at least an order of magnitude smaller than that of said object. Alternatively, the substrate may be chosen so as to have, in an observation spectral region, a cathodoluminescence emissivity which is at least an order of magnitude greater than that of said object, so as to allow observation of a shadow; projected by the latter. Said resin may have a transmission greater than 50%, and preferably greater than or equal to 70%, in an observation spectral region of between 115 and 1700 nm. - No registration structure for controlling the electron beam during said insolation step should be performed near said object. Other characteristics, details and advantages of the invention will emerge on reading the description made with reference to the appended drawings given by way of example and which represent, respectively: FIG. 1, a simplified diagram of an apparatus for electronic lithography adapted for the implementation of the invention; FIG. 2, an image obtained by detecting secondary electrons of a ZnO nanowire deposited on an Si / SiO 2 substrate before deposition of the lithography resin; FIG. 3, a secondary electron image of the same nanowire obtained after spreading the resin, at the maximum allowed observation dose; FIG. 4, a panchromatic cathodoluminescence image of the same nanowire, produced with the same electronic dose as in the case of FIG. 3 but at a temperature of 5 K; FIGS. 5 and 6, two secondary electron images of the same nanowire, showing the electrical contacts produced by electronic lithography using the method of the invention. The cathodoluminescence imaging electronic lithography device of FIG. 1 essentially comprises: an electron beam emitter 10 emitting an electron beam 11 that can be used both for acquiring an image or for irradiating an electronic lithography resin; a parabolic mirror 20, having an orifice 21 for the passage of the beam 11 and for collecting and collimating the light emitted by photoluminescence by a sample 50; a monochromator 30 provided with light detectors 31 and 32, receiving the light collected by the mirror 20; a cold plate 40 on which the sample 50 is placed, constituted by a substrate 52 and a nanowire 55 deposited on said substrate; and a computer 60 connected to the monochromator 30 for acquiring a cathodoluminescence image by scanning the sample 50 and for controlling the beam 11 emitted by the electronic gun 10. In a conventional manner, the assembly is kept under vacuum. The electronic cannon 10 is that of a conventional scanning electron microscope; the whole device is actually made from such a microscope. It may be, for example, a Quanta 200 microscope marketed by the company FEI, whose electron source is a tungsten filament. The electron acceleration voltage can be varied between 200 V and 30 kV and the beam current between 5 pA and 50 nA. The steering of the beam can be achieved by a conventional control device, such as a DigiScan II device from Gatan. The parabolic mirror 20 can be made by machining the diamond tool with an aluminum block. The positioning of the mirror relative to the sample and the axis of the electron beam is achieved by means of a positioning system in the x-y plane (perpendicular to the direction of the beam 11, indicated by z); the adjustment in the z direction is done by vertical displacement of the plate 40. The mirror is positioned, to within a few micrometers, so that its focus is located on the surface of the sample, in correspondence of the point through which the l beam axis 11. The opening 21 has a diameter of the order of 2 mm. The light collected by the mirror is focused on the entrance slot of the monochromator by a plano-convex lens.

The monochromator 30 may be for example a HR460 Jobin Yvon 2 networks, 600 and 1800 lines / mm respectively, blazed in the near UV. The associated detectors are a photomultiplier and a CCD matrix of 256 x 1024 pixels cooled to 140 K by liquid nitrogen. The photomultiplier is used for panchromatic or monochromatic cathodoluminescence imaging in a spectral range between 200 and 900 nm, but also for obtaining spectra by rotation of the network; the CCD matrix allows rapid acquisition of spectra and spectrum profiles.

The cold plate is a liquid cooled G3 platinum CF302 with liquid helium to adjust the temperature of the sample between 5 and 300 K. The characteristics of this system have been described in detail, but only as a non-limiting example. . The sample 50 consists of a Si substrate 52 covered with a 500 nm layer of SiO 2, on which is deposited a nanowire 55 made of ZnO having a diameter of the order of 100 nm and a length of 5 μm. Figure 2 shows an image of this sample, acquired by observation of secondary electrons with a dose of 1 μC / cm 2 of accelerated electrons at 30 kV, at a temperature of 300 K (ambient). Figure 3 shows an image taken under the same conditions, but after depositing a layer of lithographic resin (PMMA: polymethylmethacrylate) 400 nm thick above the sample. The observation dose (1 μC / cm 2) is 250 times less than that required for exposure of the 300K resin. Under these conditions, the nanowire 55 is invisible. It is for this reason that, in conventional processes, tracking pads are provided near the nanowire. FIG. 4 shows a panchromatic image of cathodoluminescence (in negative) of the nanowire 55 under the resin, the sample having been cooled to 5K. The spectral range of the image is 200 ± 900 nm; the transmission of PMMA in this range is greater than 70%. This image was also acquired with an electronic dose of 1 pC / cm2, but this time the nanowire is perfectly visible. In addition, thanks to the cooling, the insolation dose of the resin increased from 250 pC / cm 2 (at 300 K) to 550 pC / cm 2: the safety margin on the dose thus more than doubled. It is therefore possible to increase the dose to 550 pC / cm2 (by increasing the current and / or the exposure time) in order to insolate the resin according to the desired pattern, by controlling the electron beam with the aid of the previously recorded cathodoluminescence image. For example, the pattern may consist in making four contacts along the wire and pads for external connection under points connected to said contacts.

At 300 K, the resin is then developed with a standard MIBKIIPA (methyl isobutyl ketone / isopropyl alcohol) mixture, and a metallization bilayer (140 nm Ti - 40 nm Au) is deposited by evaporation. Then, the metal deposited on the non-insolated resin is removed (so-called lift-off stage) with acetone or N-methyl-2-pyrrolidone (NMP). Figures 5 and 6 show two images, at two magnifications and different angles, electrical connections thus obtained. In this case, the cathodoluminescence of the nanowire over the entire spectral range considered was much more intense than that of the substrate, which made it possible to produce panchromatic images. In some cases, a significant difference in emissivity will be observed only in a specific spectral range: it will therefore be necessary to make a narrow-band image using the monochromator 30 (the cathodoluminescence spectra of the nano-object and the substrate may be acquired before spreading the resin).

Finally, in other cases, the cathodoluminescence of the nanowire will be lower than that of the substrate: in this case we can acquire an image of the shadow projected by said nano-object. It is therefore understood that the method can be easily generalized to other objects, other substrates and also other resins of electronic lithography. In particular, the application of the method is not limited to the production of electrical contacts. On the contrary, many other applications are possible, such as the production of photonic crystals by deposition of dielectric materials through the openings made in the resin, the deposition of micro-turns to apply magnetic fields to quantum boxes, etc.

Claims (7)

REVENDICATIONS1. An electronic lithography method for producing devices from objects of micrometric or nanometric dimensions deposited or integrated on a substrate, comprising: a) depositing a layer of resin above a surface of said substrate; b) exposing the resin by scanning an electron beam to selectively release predefined regions of the resin; c) developing the insolated resin to selectively release predefined regions from said surface; and d) the manufacture of the device by etching and / or deposition of material through the resin mask thus obtained; characterized in that it also comprises, after deposition of the resin but prior to its insolation, the acquisition of an image of said object obtained by scanning the surface of the substrate by an electron beam at a dose insufficient to insoluate the resin, and by detecting the cathodoluminescence thus induced; said image being used for controlling the electron beam during said insolation step. The method of claim 1, wherein said image acquisition and insolation steps are performed by cooling the substrate to a temperature of between 3K and 250K. 3. Method according to one of the preceding claims, wherein said step of acquiring an image is performed in a limited spectral range by means of a monochromator (30). 4. Method according to one of the preceding claims, wherein the substrate is chosen to have, in an observation spectral region, a cathodoluminescence emissivity of at least an order of magnitude lower than said object. 5. Method according to one of claims 1 to 3, wherein the substrate is chosen to have, in an observation spectral region, a cathodoluminescence emissivity greater than at least an order of magnitude to that of said object, in order to allow the observation of a shadow projected by the latter. 6. Method according to one of the preceding claims, wherein said resin has a transmission greater than 50%, and preferably greater than or equal to 70%, in a spectral region of observation between 115 and 1700 nm. 7. Method according to one of the preceding claims, wherein no registration structure for controlling the electron beam during said insolation step is performed near said object.