- Email: [email protected]
Zone plate for thermal neutron focusing: design, fabrication and first experimental tests
Microelectronic Engineering 73–74 (2004) 644–650 www.elsevier.com/locate/mee
Zone plate for thermal neutron focusing: design, fabrication and first experimental tests Matteo Altissimo a,b,*, Caterina Petrillo b, Francesco Sacchetti c, Frederic Ott d, Enzo Di Fabrizio a a
LILIT – NNL (National Nanotechnology Laboratory) – TASC-INFM Nanolithography beamline at Elettra Synchrotron Light Source, Area Science Park, S.S.14 Km 163.5, I-34012 Basovizza, Trieste, Italy b INFM and Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy c INFM and Dipartimento di Fisica, Universita di Perugia,Via A. Pascoli, I-06123 Perugia, Italy d Laboratoires Leon Brillouin CEA/CNRS, CEA Saclay, F-91191 Gif sur Yvette, France Available online 14 March 2004
Abstract The aim of this paper is to present the design, the fabrication and the first experimental tests of neutrons focussing for our fabricated phase zone plates. Two different diffractive optical elements will be presented in the article. One is a 5 mm zone plate with a resolution of 400 nm and the other is a matrix (30 30) different Zone Plates, each one 300 lm in diameter, with an outermost zone width of 1 micron. A 4.8 lm thick natural nickel was used as phase shifter. This translates in High Aspect Ratios of 12 and 5, respectively, for these two devices. A series of test measurements carried out on a monochromatic neutrons beam have successfully demonstrated the ability of zone plates in focussing neutrons, thus opening a wide range of possible applications. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Neutron focussing; High aspect ratio zone plates; Nanofabrication
1. Introduction The high penetration power of neutrons makes them ideal for non-destructive studies of materials and enables in situ measurements of the environment-dependent properties. The requirement of non destructive characterization tools, coupled to the ability of manipulating the neutron beam in *
Corresponding author. E-mail address: [email protected] (M. Altissimo).
the sub-millimetre scale by novel neutron optics, would make neutron diffraction and imaging a powerful technique in the rapidly evolving fields of biophysics and nano-science research. The lack of high-efficiency neutrons optical elements poses a limitation in realising neutron focussing devices. Several techniques such as the use of reflective optics have been proposed in the past [1–3]. Recently innovative applications of neutron radiography and imaging have been presented [4,5]. In 1980, Kearney et al. [6], have first demonstrated the successful use of a phase reversal ZP to focus
0167-9317/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2004.03.007
M. Altissimo et al. / Microelectronic Engineering 73–74 (2004) 644–650
Recent and image a cold neutron beam (k 20 A). progresses achieved in nano-fabrication has made possible the production of high-efficiency highresolution ZPs for high-energy X-rays [7], thus opening the way to fabricate neutron ZPs. It is a well-known fact that neutrons, as subatomic particles, can be seen also as waves. In our approach, we devised the neutron optics in the framework of low interacting radiation. The complex refractive index of materials in these conditions is written as n ¼ 1 d þ ib;
ð1Þ
where the real part is related to refraction and the imaginary part to absorption. The deviation from unity of the real part is given by d ¼ ð1=2pÞqk2 bc 105 –106
ð2Þ
with q being the density of scattering centres per unit volume in the sample, k the neutron wavelength, and bc the neutron coherent scattering length. The b parameter for neutron is related to the capture cross section, and is approximately 0 for our purposes [8]. This formal equality can be translated in the design of a zone plate (ZP) suitable for neutrons focussing, as we will discuss in the following sections (see [9] for a complete review of ZPs geometrical and optical properties). There are two main advantages in using a ZP for neutron focussing. First a ZP can theoretically deliver a focal spot of approximately the same size as that of the outermost zone width, coupled, secondly, with high-focussing efficiency. If, for a given wavelength, the phase shifter thickness is optimised in order to shift the phase of the incoming neutron beam of a quantity p, then since b ¼ 0, the ZP theoretical efficiency limit (40 %) can be reached. However, a thermal neutron beam, as delivered by conventional nuclear reactors, is much broader in lateral dimensions and much more inhomogeneous in particle distribution with respect to X-ray beams delivered by synchrotrons. This will degrade the performance of the ZP in terms of ultimate focal spot size. The design of two different ZPs is presented in Section 2. The fabrication process will be described in Section 3. Section 4 will present the first
645
experimental tests carried out in a neutron beamline. In the last section we will discuss the results obtained. 2. Design In order to design an efficient ZP for thermal neutrons focussing (wavelengths ranging from 1.8 two factors have to be taken into account. to 14 A) Firstly, it is necessary to choose a material that will shift the phase of incident neutrons by a factor of p, and secondly, the thickness of material should be small enough to ease the fabrication. Table 1 give a selection of the bc values for some elements in the periodic table. It is worth to note that titanium has a negative coherent scattering length. This means that the real part of the complex refractive index is greater than one, thus allowing a phase advance, rather than a phase delay as it is the case for X-rays. The phase shifter thickness t can be computed by a rearranged phase retardation equation, written as follows: t ¼ k=2d
ð3Þ
in which d is defined in Eq. (2). Therefore, the criterion of minimum thickness is easily satisfied by the use of natural nickel (which is a mixture of all isotopes found in nature), which has the greater coherent scattering length. Choosing nickel has the further advantage that a controlled deposition is performed using a standard electrochemical growth process, commonly available in micro and nanofabrication facilities. The geometry of a ZP is defined by the following relation: f k ¼ drN D;
ð4Þ
Table 1 The table shows the values of the coherent scattering lengths for some selected elements in the periodic table Element
bc (fm)
Ag Gd Ni Ti
5.922 9.5 10.3 )3.438
The values reported are for natural elements, that is, a mixture of all kinds of isotopes found in nature.
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where f is the focal length, k the incoming particle wavelength, drN the outermost zone width and D the ZP diameter. Our goal is to intercept a large portion of the neutron beam, in order to maximize the detector countings, on one side. This means that a large diameter ZP has to be fabricated. On the other side, the fabrication imposes main constraints on drN as will be discussed later. We chose to fabricate two different kinds of ZPs. The first was a large aperture phase reversal ZP, with 5 mm diameter and 400 nm outermost zone width. The second was a square (30 30) matrix, on a 1 1 cm2 surface: The 900 ZPs are identical, each having 300 lm in diameter and an outermost zone width of 1 lm. For both diffractive optical elements the phase shifter thickness was computed to be around 5 lm, to get the maxi thermal neutron mum efficiency around a 7 A wavelength.
3. Fabrication process Considering the dimensions of the ZPs in terms of outermost zone width, and structure height, we adopted a fabrication protocol comprising Electron Beam Lithography (EBL) to make X-ray masks with ZP features, which were then transferred into resist using an X-ray lithography (XRL) process, followed by nickel electroplating. 3.1. EBL Masks fabrication Since the lateral dimensions of the two ZPs optics are bigger than the available writing field on any EBL tool, we were forced to employ a multifield writing strategy. The EBL exposures were all performed using a Leica EBMF 10.5, with 50 kV acceleration voltage and typically 1 nA beam current. In the case of the 5 mm ZP, the full computer aided design (CAD) file was first divided into four quarters. Each of them was located in a known position in four different writing fields, 3.2678 mm wide. Careful field calibration is mandatory, in order to minimize the stitching between the four different fields. After a single field is exposed, the EBL tool interferometric stage is moved in order
to match the following quarter of the ZP with the previous one. The exposures were performed on a silicon nitride (2 lm thick) coated silicon wafer. A chromium/gold layer (10/20 nm thick) layer was deposited on silicon nitride as a plating base. The resist used was a 950 kDalton average molecular weight poly(methyl-metacrylate) (PMMA), spun on the substrate with a thickness of 600 nm. The resist was soft baked at 170 °C on a hot plate for 5 min, then annealed down to room temperature for 10 min. The exposure dose was 500 lC/cm2 . After the exposures, the samples were developed in a methyl-isobutyl-ketone (MIBK) solution, diluted with isopropylalcohol (IPA), (volume ratio: MIBK:IPA ¼ 1:3) for about 1 min at room temperature and rinsed in pure IPA. The X-ray absorber (gold, 400 nm) was deposited by a standard electrochemical process. The XRL mask for the ZP matrix was exposed using a step-and-repeat writing strategy. In each step, a single ZP is exposed at the center of the 0.32678 mm field. The EBL machine interferometric stage is then moved to the next position, and another ZP is exposed. The substrate was the same as in the previous case. For the ZP matrix XRL mask fabrication SAL-601, a chemically amplified negative tone resist, was used. The resist was spun on the substrate with a thickness of 600 nm and then soft baked at 105 °C for 1 min on a hot plate. The exposure dose was 15 lC/cm2 . After the exposure, the sample was post baked at 105 °C on a hot plate for 1 min, then developed at room temperature for 3 min in a commercial developer (Shipley MF312) diluted in DeIonised (DI) water in a 1:1 volume ratio and rinsed in DI water. Again, the gold absorber was deposited in a commercial electrochemical solution, with a thickness of about 400 nm. For both the masks, after the gold deposition, the resist was stripped in hot acetone, and a window was opened throughout the wafer, with standard combined dry/wet etching techniques, in order to obtain an X-ray transparent membrane. 3.2. XRL ZP fabrication The XRL exposures were all performed at LILIT beam line [10], Elettra Synchrotron, in Trieste
M. Altissimo et al. / Microelectronic Engineering 73–74 (2004) 644–650
(Italy) using a JMAR/SAL XRS 200 X-ray stepper. The typical electron current inside Elettras storage ring was 250 mA, with an energy of 2 GeV. Given the high aspect ratio of the structures (around 12 for the 5 mm ZP and around 5 for the ZP matrix), we adopted an advanced XRL fabrication protocol, described in detail in [11]. This protocol involves the use of an adhesion promoter, a soft bake followed by a slow annealing, and a low surface tension rinsing solution. The process was as follows: spinning of the adhesion promoter on the base plated 300 lm thick silicon wafer with a thickness of about 20 nm, followed by a soft baking on hot plate. The resist chosen for the exposures was a 950 kDalton average molecular weight PMMA, spun on the sample with a thickness of about 6 lm, then soft baked in oven at 170 °C for 30 min and annealed down to room temperature for 3 h. The samples were then exposed to the X photons beam. The dose delivered was 4 J/cm2 for both the 5 mm ZP and the ZP matrix. All the samples were
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developed in MIBK:IPA ¼ 1:3 solution for 2 min, then rinsed in a solution of tert-butanol and DI water (volume ratio: 1:1). A reactive ion etching (RIE) cleaning in a oxygen/CF4 plasma was then performed. The RIE is necessary in order to remove the adhesion promoter layer, so as to ensure the electrical contact between the plating base and the electrochemical solution. Both the samples were grown in nickel, in a commercial electrochemical apparatus for about 5 min. Finally the resist was stripped. The results of the exposures are shown in Figs. 1 and 2. Since silicon is transparent to a thermal neutron beam, it is not necessary to etch the wafer, in order to get a transparent membrane, as it is in the case of X-rays ZPs.
4. Experimental tests of zone plate performance The tests were carried out at TPA (Tres Petit angles, very small angles) spectrometer, located in
Fig. 1. The figure shows several pictures of the 5 mm ZP. (a) Optical micrograph of the grown device. (b) Scanning electron microscope (SEM) image of the central zone. Note that the central zone is not round. This is due to the fact that the XRL mask was fabricated using a multifield writing strategy. The squared shape is therefore a measure of the field-stitching error of the EBL writing process. (c) SEM picture of the outermost zones.
Fig. 2. The figure shows several pictures of the ZP matrix. (a) optical micrograph of a region of the matrix. (b) SEM picture, showing the centre of a single ZP, and (c) another SEM picture of the outermost zones of a ZP in the matrix. The small wires that are seen in the three figures are made of nickel, that have grown because of a PMMA slight over-development.
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the Laboratoire Leon Brillouin, Gif Sur Yvette, France [12]. The thermal neutron beam is produced in a nuclear reactor, and it is delivered to the experimental station through suitable neutron guides. The neutron wavelength in TPA spectrometer can be selected through a velocity selector, which provides a passing band 7% around the nominal selected wavelength. From the zone plate optics point of view, this means that the focus has the shape of lengthened ‘‘cigar’’ in the beam direction, while it is also enlarged in the transversal direction. The detector used is a commercial solid state image plate (pixel size of 150 lm). The typical opening time of the neutron shutter was ranging between 5 min and several hours. The performances of the 5 mm ZP were mea which corresponds to a focal sured at k ¼ 6:85 A, length of 3 m and a theoretical efficiency of 40%. A 1 mm pinhole was inserted at 3 m upstream the ZP, so that a parallel beam, 5 mm in diameter, was expected at the detector 4 m downstream the lens. The difference between the intensities collected with and without insertion of the ZP is shown in Fig. 3. The focusing effect is clearly visible and we estimate the efficiency in excess of 20%. The ZP matrix characterisation procedure has been performed in two steps. The first series of tests has consisted in measuring the effect of an individual zone plate of the matrix. Since no highresolution neutron detectors are available, the tests were performed in an indirect geometry: the zone plate matrix was tested at a wavelength k ¼ 13:7 with a 0.8 mm pinhole placed at the image A,
Fig. 4. Experimental setup for the test of the single ZP in the matrix. From the left-side: the neutrons beam coming out from the velocity selector with a nominal wavelength of 13.7 A, passes through a 1 mm source pinhole (not shown in the figure) and impinges the ZP matrix. A 0.8 mm collimation pinhole is inserted at the focal plane, 23.6 cm downstream of the matrix. Because of the ZP dimensions and spacing in the matrix, only the neutrons which have passed through a single ZP can reach the image plate detector, 4 m downstream.
position, 23.6 cm from the lens (see Fig. 4). In this experimental configuration, without the matrix, the geometrical shadow of the pinhole is expected, whereas a broad spot is formed at the detector position when the matrix is in place. Fig. 5 shows the intensity map measured on the image plate detector with (Fig. 5(a)) and without (Fig. 5(b)) a single matrix element in the beam. The difference of the two collected images is shown in Fig. 5(c). The second series of test has consisted in characterizing the whole array. The neutron detector was moved at the focal plane, 23.6 cm away from the zone plate matrix, as in Fig. 4 (with the image plate at the 0.8 mm pinhole position). In this case, there were two experimental limitations: the incoming beam is not fully homogeneous across the
Fig. 3. The figure shows the setup used in testing the 5 mm ZP. The intensity distribution is obtained by subtracting the obtained images with and without insertion of the ZP into the beam. Note that the spot size is close to 5 mm, as expected from the experimental geometry.
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Fig. 5. Intensities of neutron beam (a) with the matrix in the optical path, (b) without the matrix, (c) difference in intensities with and without the ZP matrix. (b) Collimation pinhole ‘‘shadow’’ at the detector position. It is worth to note that in this measurement, what we were seeing was the de-magnified image of the source pinhole. Calculations show that the spot is around 2.3 mm in diameter, close to the value that can be extrapolated from experimental data.
Fig. 6. Results for the whole ZP matrix test. (a) Shows the neutrons distribution on a 1 cm2 area. (b) The effect of the matrix insertion into the beam path. The intensity redistribution is clearly evident. It was not possible to measure the focal dimension, estimated to be around 50 lm, because of the low spatial resolution of the image plate detector. The image plate in this experimental configuration was moved to the matrix focal position, 23.6 cm downstream the matrix.
whole section of the ZP matrix (see Fig. 6(a); the second limitation is given by the Image Plate resolution (about 150 lm), where one expects individual focussing spots of about 50 lm in the focal plane. Nevertheless, it has been possible to image the individual focussing spots even though the detector resolution was limited. These spots appears as a strong modulation of the original neutron intensity across the detector (see Fig. 6(b).
5. Discussion The present results show that a ZP is an excellent choice to focus neutrons at sub-millimetre
scale. The potential applications that would benefit from the use of such optics range from high-resolution neutron imaging to protein crystallography of tiny samples (50–100 lm). Use of single lenses or stacks of them could also be exploited in small angle scattering applications. Several experimental problems prevent us from the full characterization of focus lateral dimensions, namely: finite size of the pinhole source, non-homogeneous spatial distribution of the incoming neutron beam, non perfect monochromaticity of the beam itself and low spatial resolution of the image plate. Nevertheless, we believe that our experiment is a successful demonstration of thermal neutron focussing properties of ZPs. With respect to other optics used in the neutron field, ZPs have the advantage of high-efficiency, theoretically slightly more than 40%, coupled with a relatively simple fabrication process. This theoretical limit can be reached, by a more aimed fabrication process, since layers of low atomic weight transition metals, with a thickness of some lm, do not efficiently absorb thermal neutrons. A drawback is that highly collimated neutron beams have to be delivered to the ZP, in order to approach the focussing diffraction limit. In the neutron field, this means that only a small percentage of neutrons coming out from the reactor are used, translating in a low number of detected particles. Anyway in our experiments, even for short exposure times of the image plate detector (few minutes), the number of neutron collected per pixel by the detector was
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high enough (around 2000 counts) to establish that the detected signal was higher than the noise level (around 10/20 counts in dark areas).
References [1] M.R. Eskildsen, P.L. Gammel, E.D. Isaacs, C. Detlefs, K. Mortensen, D.J. Bishop, Nature 391 (1998) 563–566. [2] M.A. Kumakhov, V.A. Sharov, Nature 357 (1992) 390– 391. [3] H. Chen, R.G. Downing, D.F.R. Mildner, W.M. Gibson, M.A. Kumakhov, I.Yu. Ponomarev, M.V. Gubarev, Nature 357 (1992) 391–393. [4] B.E. Aliman, P.J. McMahon, K.A. Nugent, D. Paganin, D.L. Jacobson, M. Arif, S.A. Werner, Nature 408 (2000) 158–159.
[5] B. Masschaele, S. Baechler, P. Cauwels, M. Dierick, J. Jolie, W. Mondelaers, Rad. Phys. Chem. 61 (2001) 623– 624. [6] P.D. Kearney, A.G. Klein, G.I. Opat, R. G€ahler, Nature 287 (1980) 313–314. [7] E. Di Fabrizio, F. Romanato, M. Gentili, S. Cabrini, B. Kaulich, J. Susini, R. Barrett, Nature 401 (1999) 895–898. [8] V.F Sears, Neutron Optics, Oxford Series on Neutron Scattering in Condensed Matter, Oxford University Press, New York, 1989, p. 71. [9] A.G. Michette, Optical Systems for Soft X-rays, Plenum Press, New York/London, 1986, pp. 165–207. [10] F. Romanato, S. Cabrini, E. Di Fabrizio, L. Vaccari, M. Altissimo, D. Cojoc, L. Businaro, MicroElectron. Eng. (2000) 101–107. [11] M. Altissimo, F. Romanato, L. Vaccari, L. Businaro, D. Cojoc, B. Kaulich, S. Cabrini, E. Di Fabrizio, MicroElectron. Eng. 61–62 (2002) 173–177. [12] www-llb.cea.fr/index_e.html.
Zone plate for thermal neutron focusing: design, fabrication and first experimental tests Matteo Altissimo a,b,*, Caterina Petrillo b, Francesco Sacchetti c, Frederic Ott d, Enzo Di Fabrizio a a
LILIT – NNL (National Nanotechnology Laboratory) – TASC-INFM Nanolithography beamline at Elettra Synchrotron Light Source, Area Science Park, S.S.14 Km 163.5, I-34012 Basovizza, Trieste, Italy b INFM and Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy c INFM and Dipartimento di Fisica, Universita di Perugia,Via A. Pascoli, I-06123 Perugia, Italy d Laboratoires Leon Brillouin CEA/CNRS, CEA Saclay, F-91191 Gif sur Yvette, France Available online 14 March 2004
Abstract The aim of this paper is to present the design, the fabrication and the first experimental tests of neutrons focussing for our fabricated phase zone plates. Two different diffractive optical elements will be presented in the article. One is a 5 mm zone plate with a resolution of 400 nm and the other is a matrix (30 30) different Zone Plates, each one 300 lm in diameter, with an outermost zone width of 1 micron. A 4.8 lm thick natural nickel was used as phase shifter. This translates in High Aspect Ratios of 12 and 5, respectively, for these two devices. A series of test measurements carried out on a monochromatic neutrons beam have successfully demonstrated the ability of zone plates in focussing neutrons, thus opening a wide range of possible applications. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Neutron focussing; High aspect ratio zone plates; Nanofabrication
1. Introduction The high penetration power of neutrons makes them ideal for non-destructive studies of materials and enables in situ measurements of the environment-dependent properties. The requirement of non destructive characterization tools, coupled to the ability of manipulating the neutron beam in *
Corresponding author. E-mail address: [email protected] (M. Altissimo).
the sub-millimetre scale by novel neutron optics, would make neutron diffraction and imaging a powerful technique in the rapidly evolving fields of biophysics and nano-science research. The lack of high-efficiency neutrons optical elements poses a limitation in realising neutron focussing devices. Several techniques such as the use of reflective optics have been proposed in the past [1–3]. Recently innovative applications of neutron radiography and imaging have been presented [4,5]. In 1980, Kearney et al. [6], have first demonstrated the successful use of a phase reversal ZP to focus
0167-9317/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2004.03.007
M. Altissimo et al. / Microelectronic Engineering 73–74 (2004) 644–650
Recent and image a cold neutron beam (k 20 A). progresses achieved in nano-fabrication has made possible the production of high-efficiency highresolution ZPs for high-energy X-rays [7], thus opening the way to fabricate neutron ZPs. It is a well-known fact that neutrons, as subatomic particles, can be seen also as waves. In our approach, we devised the neutron optics in the framework of low interacting radiation. The complex refractive index of materials in these conditions is written as n ¼ 1 d þ ib;
ð1Þ
where the real part is related to refraction and the imaginary part to absorption. The deviation from unity of the real part is given by d ¼ ð1=2pÞqk2 bc 105 –106
ð2Þ
with q being the density of scattering centres per unit volume in the sample, k the neutron wavelength, and bc the neutron coherent scattering length. The b parameter for neutron is related to the capture cross section, and is approximately 0 for our purposes [8]. This formal equality can be translated in the design of a zone plate (ZP) suitable for neutrons focussing, as we will discuss in the following sections (see [9] for a complete review of ZPs geometrical and optical properties). There are two main advantages in using a ZP for neutron focussing. First a ZP can theoretically deliver a focal spot of approximately the same size as that of the outermost zone width, coupled, secondly, with high-focussing efficiency. If, for a given wavelength, the phase shifter thickness is optimised in order to shift the phase of the incoming neutron beam of a quantity p, then since b ¼ 0, the ZP theoretical efficiency limit (40 %) can be reached. However, a thermal neutron beam, as delivered by conventional nuclear reactors, is much broader in lateral dimensions and much more inhomogeneous in particle distribution with respect to X-ray beams delivered by synchrotrons. This will degrade the performance of the ZP in terms of ultimate focal spot size. The design of two different ZPs is presented in Section 2. The fabrication process will be described in Section 3. Section 4 will present the first
645
experimental tests carried out in a neutron beamline. In the last section we will discuss the results obtained. 2. Design In order to design an efficient ZP for thermal neutrons focussing (wavelengths ranging from 1.8 two factors have to be taken into account. to 14 A) Firstly, it is necessary to choose a material that will shift the phase of incident neutrons by a factor of p, and secondly, the thickness of material should be small enough to ease the fabrication. Table 1 give a selection of the bc values for some elements in the periodic table. It is worth to note that titanium has a negative coherent scattering length. This means that the real part of the complex refractive index is greater than one, thus allowing a phase advance, rather than a phase delay as it is the case for X-rays. The phase shifter thickness t can be computed by a rearranged phase retardation equation, written as follows: t ¼ k=2d
ð3Þ
in which d is defined in Eq. (2). Therefore, the criterion of minimum thickness is easily satisfied by the use of natural nickel (which is a mixture of all isotopes found in nature), which has the greater coherent scattering length. Choosing nickel has the further advantage that a controlled deposition is performed using a standard electrochemical growth process, commonly available in micro and nanofabrication facilities. The geometry of a ZP is defined by the following relation: f k ¼ drN D;
ð4Þ
Table 1 The table shows the values of the coherent scattering lengths for some selected elements in the periodic table Element
bc (fm)
Ag Gd Ni Ti
5.922 9.5 10.3 )3.438
The values reported are for natural elements, that is, a mixture of all kinds of isotopes found in nature.
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M. Altissimo et al. / Microelectronic Engineering 73–74 (2004) 644–650
where f is the focal length, k the incoming particle wavelength, drN the outermost zone width and D the ZP diameter. Our goal is to intercept a large portion of the neutron beam, in order to maximize the detector countings, on one side. This means that a large diameter ZP has to be fabricated. On the other side, the fabrication imposes main constraints on drN as will be discussed later. We chose to fabricate two different kinds of ZPs. The first was a large aperture phase reversal ZP, with 5 mm diameter and 400 nm outermost zone width. The second was a square (30 30) matrix, on a 1 1 cm2 surface: The 900 ZPs are identical, each having 300 lm in diameter and an outermost zone width of 1 lm. For both diffractive optical elements the phase shifter thickness was computed to be around 5 lm, to get the maxi thermal neutron mum efficiency around a 7 A wavelength.
3. Fabrication process Considering the dimensions of the ZPs in terms of outermost zone width, and structure height, we adopted a fabrication protocol comprising Electron Beam Lithography (EBL) to make X-ray masks with ZP features, which were then transferred into resist using an X-ray lithography (XRL) process, followed by nickel electroplating. 3.1. EBL Masks fabrication Since the lateral dimensions of the two ZPs optics are bigger than the available writing field on any EBL tool, we were forced to employ a multifield writing strategy. The EBL exposures were all performed using a Leica EBMF 10.5, with 50 kV acceleration voltage and typically 1 nA beam current. In the case of the 5 mm ZP, the full computer aided design (CAD) file was first divided into four quarters. Each of them was located in a known position in four different writing fields, 3.2678 mm wide. Careful field calibration is mandatory, in order to minimize the stitching between the four different fields. After a single field is exposed, the EBL tool interferometric stage is moved in order
to match the following quarter of the ZP with the previous one. The exposures were performed on a silicon nitride (2 lm thick) coated silicon wafer. A chromium/gold layer (10/20 nm thick) layer was deposited on silicon nitride as a plating base. The resist used was a 950 kDalton average molecular weight poly(methyl-metacrylate) (PMMA), spun on the substrate with a thickness of 600 nm. The resist was soft baked at 170 °C on a hot plate for 5 min, then annealed down to room temperature for 10 min. The exposure dose was 500 lC/cm2 . After the exposures, the samples were developed in a methyl-isobutyl-ketone (MIBK) solution, diluted with isopropylalcohol (IPA), (volume ratio: MIBK:IPA ¼ 1:3) for about 1 min at room temperature and rinsed in pure IPA. The X-ray absorber (gold, 400 nm) was deposited by a standard electrochemical process. The XRL mask for the ZP matrix was exposed using a step-and-repeat writing strategy. In each step, a single ZP is exposed at the center of the 0.32678 mm field. The EBL machine interferometric stage is then moved to the next position, and another ZP is exposed. The substrate was the same as in the previous case. For the ZP matrix XRL mask fabrication SAL-601, a chemically amplified negative tone resist, was used. The resist was spun on the substrate with a thickness of 600 nm and then soft baked at 105 °C for 1 min on a hot plate. The exposure dose was 15 lC/cm2 . After the exposure, the sample was post baked at 105 °C on a hot plate for 1 min, then developed at room temperature for 3 min in a commercial developer (Shipley MF312) diluted in DeIonised (DI) water in a 1:1 volume ratio and rinsed in DI water. Again, the gold absorber was deposited in a commercial electrochemical solution, with a thickness of about 400 nm. For both the masks, after the gold deposition, the resist was stripped in hot acetone, and a window was opened throughout the wafer, with standard combined dry/wet etching techniques, in order to obtain an X-ray transparent membrane. 3.2. XRL ZP fabrication The XRL exposures were all performed at LILIT beam line [10], Elettra Synchrotron, in Trieste
M. Altissimo et al. / Microelectronic Engineering 73–74 (2004) 644–650
(Italy) using a JMAR/SAL XRS 200 X-ray stepper. The typical electron current inside Elettras storage ring was 250 mA, with an energy of 2 GeV. Given the high aspect ratio of the structures (around 12 for the 5 mm ZP and around 5 for the ZP matrix), we adopted an advanced XRL fabrication protocol, described in detail in [11]. This protocol involves the use of an adhesion promoter, a soft bake followed by a slow annealing, and a low surface tension rinsing solution. The process was as follows: spinning of the adhesion promoter on the base plated 300 lm thick silicon wafer with a thickness of about 20 nm, followed by a soft baking on hot plate. The resist chosen for the exposures was a 950 kDalton average molecular weight PMMA, spun on the sample with a thickness of about 6 lm, then soft baked in oven at 170 °C for 30 min and annealed down to room temperature for 3 h. The samples were then exposed to the X photons beam. The dose delivered was 4 J/cm2 for both the 5 mm ZP and the ZP matrix. All the samples were
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developed in MIBK:IPA ¼ 1:3 solution for 2 min, then rinsed in a solution of tert-butanol and DI water (volume ratio: 1:1). A reactive ion etching (RIE) cleaning in a oxygen/CF4 plasma was then performed. The RIE is necessary in order to remove the adhesion promoter layer, so as to ensure the electrical contact between the plating base and the electrochemical solution. Both the samples were grown in nickel, in a commercial electrochemical apparatus for about 5 min. Finally the resist was stripped. The results of the exposures are shown in Figs. 1 and 2. Since silicon is transparent to a thermal neutron beam, it is not necessary to etch the wafer, in order to get a transparent membrane, as it is in the case of X-rays ZPs.
4. Experimental tests of zone plate performance The tests were carried out at TPA (Tres Petit angles, very small angles) spectrometer, located in
Fig. 1. The figure shows several pictures of the 5 mm ZP. (a) Optical micrograph of the grown device. (b) Scanning electron microscope (SEM) image of the central zone. Note that the central zone is not round. This is due to the fact that the XRL mask was fabricated using a multifield writing strategy. The squared shape is therefore a measure of the field-stitching error of the EBL writing process. (c) SEM picture of the outermost zones.
Fig. 2. The figure shows several pictures of the ZP matrix. (a) optical micrograph of a region of the matrix. (b) SEM picture, showing the centre of a single ZP, and (c) another SEM picture of the outermost zones of a ZP in the matrix. The small wires that are seen in the three figures are made of nickel, that have grown because of a PMMA slight over-development.
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the Laboratoire Leon Brillouin, Gif Sur Yvette, France [12]. The thermal neutron beam is produced in a nuclear reactor, and it is delivered to the experimental station through suitable neutron guides. The neutron wavelength in TPA spectrometer can be selected through a velocity selector, which provides a passing band 7% around the nominal selected wavelength. From the zone plate optics point of view, this means that the focus has the shape of lengthened ‘‘cigar’’ in the beam direction, while it is also enlarged in the transversal direction. The detector used is a commercial solid state image plate (pixel size of 150 lm). The typical opening time of the neutron shutter was ranging between 5 min and several hours. The performances of the 5 mm ZP were mea which corresponds to a focal sured at k ¼ 6:85 A, length of 3 m and a theoretical efficiency of 40%. A 1 mm pinhole was inserted at 3 m upstream the ZP, so that a parallel beam, 5 mm in diameter, was expected at the detector 4 m downstream the lens. The difference between the intensities collected with and without insertion of the ZP is shown in Fig. 3. The focusing effect is clearly visible and we estimate the efficiency in excess of 20%. The ZP matrix characterisation procedure has been performed in two steps. The first series of tests has consisted in measuring the effect of an individual zone plate of the matrix. Since no highresolution neutron detectors are available, the tests were performed in an indirect geometry: the zone plate matrix was tested at a wavelength k ¼ 13:7 with a 0.8 mm pinhole placed at the image A,
Fig. 4. Experimental setup for the test of the single ZP in the matrix. From the left-side: the neutrons beam coming out from the velocity selector with a nominal wavelength of 13.7 A, passes through a 1 mm source pinhole (not shown in the figure) and impinges the ZP matrix. A 0.8 mm collimation pinhole is inserted at the focal plane, 23.6 cm downstream of the matrix. Because of the ZP dimensions and spacing in the matrix, only the neutrons which have passed through a single ZP can reach the image plate detector, 4 m downstream.
position, 23.6 cm from the lens (see Fig. 4). In this experimental configuration, without the matrix, the geometrical shadow of the pinhole is expected, whereas a broad spot is formed at the detector position when the matrix is in place. Fig. 5 shows the intensity map measured on the image plate detector with (Fig. 5(a)) and without (Fig. 5(b)) a single matrix element in the beam. The difference of the two collected images is shown in Fig. 5(c). The second series of test has consisted in characterizing the whole array. The neutron detector was moved at the focal plane, 23.6 cm away from the zone plate matrix, as in Fig. 4 (with the image plate at the 0.8 mm pinhole position). In this case, there were two experimental limitations: the incoming beam is not fully homogeneous across the
Fig. 3. The figure shows the setup used in testing the 5 mm ZP. The intensity distribution is obtained by subtracting the obtained images with and without insertion of the ZP into the beam. Note that the spot size is close to 5 mm, as expected from the experimental geometry.
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Fig. 5. Intensities of neutron beam (a) with the matrix in the optical path, (b) without the matrix, (c) difference in intensities with and without the ZP matrix. (b) Collimation pinhole ‘‘shadow’’ at the detector position. It is worth to note that in this measurement, what we were seeing was the de-magnified image of the source pinhole. Calculations show that the spot is around 2.3 mm in diameter, close to the value that can be extrapolated from experimental data.
Fig. 6. Results for the whole ZP matrix test. (a) Shows the neutrons distribution on a 1 cm2 area. (b) The effect of the matrix insertion into the beam path. The intensity redistribution is clearly evident. It was not possible to measure the focal dimension, estimated to be around 50 lm, because of the low spatial resolution of the image plate detector. The image plate in this experimental configuration was moved to the matrix focal position, 23.6 cm downstream the matrix.
whole section of the ZP matrix (see Fig. 6(a); the second limitation is given by the Image Plate resolution (about 150 lm), where one expects individual focussing spots of about 50 lm in the focal plane. Nevertheless, it has been possible to image the individual focussing spots even though the detector resolution was limited. These spots appears as a strong modulation of the original neutron intensity across the detector (see Fig. 6(b).
5. Discussion The present results show that a ZP is an excellent choice to focus neutrons at sub-millimetre
scale. The potential applications that would benefit from the use of such optics range from high-resolution neutron imaging to protein crystallography of tiny samples (50–100 lm). Use of single lenses or stacks of them could also be exploited in small angle scattering applications. Several experimental problems prevent us from the full characterization of focus lateral dimensions, namely: finite size of the pinhole source, non-homogeneous spatial distribution of the incoming neutron beam, non perfect monochromaticity of the beam itself and low spatial resolution of the image plate. Nevertheless, we believe that our experiment is a successful demonstration of thermal neutron focussing properties of ZPs. With respect to other optics used in the neutron field, ZPs have the advantage of high-efficiency, theoretically slightly more than 40%, coupled with a relatively simple fabrication process. This theoretical limit can be reached, by a more aimed fabrication process, since layers of low atomic weight transition metals, with a thickness of some lm, do not efficiently absorb thermal neutrons. A drawback is that highly collimated neutron beams have to be delivered to the ZP, in order to approach the focussing diffraction limit. In the neutron field, this means that only a small percentage of neutrons coming out from the reactor are used, translating in a low number of detected particles. Anyway in our experiments, even for short exposure times of the image plate detector (few minutes), the number of neutron collected per pixel by the detector was
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high enough (around 2000 counts) to establish that the detected signal was higher than the noise level (around 10/20 counts in dark areas).
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