Solid immersion lens structures and methods for producing solid immersion lens structures

[0002] This application is a division of U.S. patent application Ser. No. 09/640,907 filed Aug. 16, 2000, now, U.S. Pat. No. 6,301,055, the entirety of herein incorporated by reference.

[0004] The present invention relates to solid immersion lens (SIL) structures and in particular to techniques for constructing SIL structures, as well as selected applications of such structures.

[0005] Due to the limitations on resolutions obtainable with conventional optical lenses for applications such as microscopy, techniques have been developed to decrease the Rayleigh limit on transverse resolution δ. The Rayleigh limit is given by δ=0.82λ/(NA) where λ is the wavelength and NA is the numerical aperture of the focusing objective (NA=n sin (θ), where n is the refractive index of the medium, and θ is the angle between the outermost rays focusing on the sample and the optical axis).

[0006] Coherent light such as laser light can be used to precisely control the wavelength of illumination λ. One way to decrease the transverse resolution is to increase the index of refraction of the optical medium, such as by use of oil-immersion microscopy or use of a solid immersion lens.

[0007] If an SIL is placed in contact with the sample under examination, illumination can be more readily focused on it, and use of the high NA of the system allows efficient collection of the excitation light with high optical transmission efficiency and observation of the sample with a very high resolution. In most of the cases, the SIL is used primarily for near-field microscopy, where the air gap between the SIL and the sample oblige those who do not want to use evanescent waves to work with a NA smaller than one.

[0008] A problem with the SIL technology is the complexity of its manufacture. For example, a polished glass sphere provided with a sequence of progressively finer alumina powders, requires a polishing time typically of many hours. Furthermore, the result is not perfect, and the polished surface is slightly rounded. Moreover, known lens structures in SIL configurations involve objective lens sets that are self contained and thus are difficult to use in a manner that maintains the lens in immersion contact with the object under observation.

[0009] What is needed is a method for simple, inexpensive and rapid construction of a solid immersion lens and a lens structure which is suited for low-cost, even disposable usage.

[0010] According to the invention, a solid immersion lens structure is formed of an optically clear low temperature moldable material such an elastomer cast to a desired shape and smoothness in a pliant mold which has highly undercut margins. Further according to the invention, a method for construction of a solid immersion lens structure includes providing a pliant mold defining a lens-shaped cavity in which a solid immersion lens is cast, casting a liquid material into the lens cavity, permitting the liquid material to set to form the solid immersion lens portion and removing the solid immersion lens portion from the pliant mold with the highly undercut margins. A specific material for use as the solid immersion lens is a thermally-resilient deformable material such as optically-clear silicone elastomer of a refractive index n greater than 1.2 and preferably greater than 1.4, such as a room temperature vulcanization (RTV) elastomer, specifically General Electric RTV 615. Preferably, the mold itself may be constructed of this material and the SIL structure can be a rigid setting material. The SIL structures according to the invention may be a disposable lens element and/or a light collection element integrated with a specimen carrier of a microfabricated flow cytometer.

[0011] According to a specific embodiment of a method according to the invention, a first liquid elastomer such as RTV is injected into a container and allowed to solidify to a pliant elastomeric solid, then a small nonreactive bead of the shape of the desired lens (a sphere) is placed on the first layer then partially covered with a layer of the liquid elastomer of a controlled thickness less than the diameter of the bead and allowed to solidify to stable pliancy. Thereafter the shaping bead is removed to yield a pliant smooth-walled mold of maximum diameter d with highly undercut margins around an orifice. The mold and adjacent region are then treated with an oxygen plasma to create a nonreactive, nonbinding surface interface. Then a third layer of optically-clear liquid moldable material, such as an RTV elastomer, having a thickness slightly greater than the depth of the mold is injected into the mold and over the region around the orifice and then allowed to solidify. The resultant structure is peeled from the pliant second layer to yield a lens element in the form of a bead embedded on an attached flange, namely a solid immersion lens structure in accordance with the invention. The pliant second layer is reusable as a mold.

[0012] Further according to the invention, a method is provided for imaging an object using a low cost lens element in an SIL configuration. According to this method, an object to be observed, preferably immersed in fluid, is guided along a passage defined by an integrally molded-together body portion and a solid immersion lens portion, where the solid immersion lens portion is optically aligned with a position in the passage. The object is positioned in the passage in alignment with the solid immersion lens portion so that the object is within a field of view extending through the spherical solid immersion lens portion. The object, immersed in a fluid of high index of refraction, is viewed through the spherical solid immersion lens portion of an even higher index of refraction, and the object is imaged onto a viewing surface.

[0013] Further according to the invention, a method is provided for collecting light emissions with high efficiency through a low cost lens element in an SIL configuration. An object to be observed is immersed in fluid and positioned in alignment with the solid immersion lens portion so that the object is within a field of light collection extending through very large numerical aperture spherical solid immersion lens portion. The object, immersed in a fluid of high index of refraction, emits observable optical energy typically by fluorescence in response to excitation, and the emissions at selected wavelengths are collected through the spherical solid immersion lens portion of an even higher index of refraction and directed to a sensor, typically without imaging, so that the emissions can be measured. The structure admits to high collection efficiency.

[0014] Further according to the invention, a solid immersion lens structure comprises a solid immersion lens portion with highly undercut margins interfacing on a flange portion, together with and a body portion in which there is a cavity or passage for carrying an object or sample to be imaged or from which light is to be collected, where at least the solid immersion lens portion is of a molded material formed in a mold of pliant material with highly undercut margins.

[0015] The invention will be better understood by reference to the following detailed description and the accompanying diagrammatic drawings.

[0016] FIG. 1 shows a schematic cross-sectional view of a solid immersion lens structure according to the invention.

[0017] FIG. 2 shows a schematic three-dimensional view of an initial step in a method for producing a solid immersion lens structure in accordance with the invention.

[0018] FIG. 3 shows a cross-sectional view corresponding to FIG. 2 indicating further steps of producing a solid immersion lens structure in accordance with the invention.

[0019] FIG. 4 shows a cross-sectional view corresponding to FIG. 3 showing a subsequent step in a method of producing a solid immersion lens structure in accordance with the invention.

[0020] FIG. 5 shows an imaging system comprising a solid immersion lens structure in accordance with the invention.

[0021] FIG. 6 shows an emission collection system comprising a solid immersion lens structure in accordance with the invention.

[0022] In order to understand the invention, it is helpful to define the terms associated with a solid immersion lens SIL structure 50 as it might be used in a device such as a microscope, spectroscope or cytometer. FIG. 1 illustrates the functioning of a solid immersion lens, with indication of the parameters used to describe the structure and operation. A solid immersion lens portion 51 comprises a sphere of radius r and index of refraction n_s. It is disposed at a highest height h above a surface 27 of a body portion 28 so that a boundary margin 25 is formed which is narrower in diameter than the diameter of the lens portion 51. An observation region 52 is provided at a distance h′ from the surface 27. Samples are placed in the region for observation according to the intended application, such as microscopy, spectroscopy or cytometry. Also shown with the structure 50 is a collection/collimating lens 150. The spherical structure and collection configuration admits to construction of lens systems having a numerical aperture higher than unity, which is particularly useful for ultrasensitive spectroscopy.

[0023] A method for producing a solid immersion lens structure in accordance with the invention is described with reference to FIGS. 2-4. Referring to FIG. 2, a container 10, typically in the form of a shallow dish defining a base wall 12 and four peripheral upstanding sidewalls 14 provides the housing for a mold. To produce the solid immersion lens structure in accordance with the method of the invention, a mold is formed. To form the mold, a first layer 16 of moldable material from which the mold is to be formed is cast into the container 10. The first layer 16 is then permitted to set. Referring now to FIG. 3, once the first layer 16 has set, a mold core 18 is positioned in the container 10 on the first layer 16. In this instance the mold core is a spherical bead of uniformly smooth surface, such as a steel bead of radius r=˜0.8 mm-4.5 mm. A second layer 20 of moldable material is then cast into the container of height h=˜1 mm-5 mm partially to encapsulate the mold core 18, thereby to form a second layer of moldable material 20 immediately adjacent the first layer 16. The second layer 20 defines an upper surface 22 at height h such that an upper portion 18.1 of the generally spherical mold core 18 protrudes through an orifice 23 of diameter d from the upper surface 22 and creates highly undercut margins 25 around the orifice 23. In order for the mold to be reusable where the structure has such undercut margins, the second layer, according to the invention, must be of a pliant material such as a silicone elastomer, such as a room temperature vulcanization (RTV) elastomer, specifically General Electric RTV 615.

[0024] The constraint on the height h is given by the following relation:

[0025] r(1−cos Φ)<h<r+r/n_s

[0026] where

[0027] r is the radius of the sphere,

[0028] h is the height of the layer,

[0029] Φ is the polar angle from the center of the sphere to the edge of the orifice formed by the undercut margins,

[0030] n_sis the index of refraction of the material which forms the lens.

[0031] Thus the geometric details of the mold depend upon the thickness of the second layer 20 relative to the radius of the bead. The RTV an elastomer made by mixing polymers, cross linkers and a catalyst. While it cures at room temperature, it is typically set for two hours at a slightly elevated temperature of 80° C. The preferred RTV comprises a first part of a polydimethylsiloxane bearing vinyl groups and a platinum catalyst and a second part of a cross linker containing silicon hydride (Si—H) groups. Once mixed, the silicon hydride groups form a covalent bond with the vinyl groups.

[0032] Referring to FIG. 4, once the layer 20 has set, the mold core 18 is removed so as to define a lens cavity 24. In this manner, a mold 26 for producing a solid immersion lens structure, in accordance with the invention, is produced. To minimize the chance of bonding between the mold and the lens, the surface is treated with an oxygen plasma to form an anti-adhesive layer 27. For example, oxidized RTV blocks bonding so the molded lens can be removed from the lens cavity 24.

[0033] Still referring to FIG. 4 and to FIG. 1, the solid immersion lens structure 50 is produced by casting a moldable material into the lens cavity 24. The moldable material from which the solid immersion lens portion of the solid immersion lens structure is to be formed is typically cast into the container 10 to fill not only the lens cavity 24, but also to form a layer 28 in the container 10, the layer 28 defining an upper surface 28.1 above (as shown) the lens cavity 24. The thickness h′ above the surface 27 is given by the relation:

[0034] h′=r+r/n_s−h.

[0035] The layer 28 forms a body portion of the solid immersion lens structure 50 when the moldable material of layer 28 has set. In this manner, the body portion of the solid immersion lens structure is integrally molded together with the solid immersion lens portion 51.

[0036] When the layer 28 has set, the solid immersion lens structure in accordance with the invention, which includes a body portion 30 and a solid immersion lens portion 32 is formed. The solid immersion lens structure is then removed from the mold.

[0037] The material from which the SIL structure 50 is made in mold 26 may be of any suitable optically clear material that can be cast as a liquid at a temperature less than the temperature at which the mold 26 is damaged or otherwise undesirably deformed. The SIL structure may cure to a generally rigid solid or a pliant solid. Among the materials considered to be generally suitable are low temperature of formation polymers, room temperature vulcanization elastomers, low temperature of formation epoxies, polyimides, polycarbonates and photoresists. The lens material 50 can be a pliant silicone elastomer. A suitable silicone elastomer is General Electric RTV 615, the same material used to create the mold 26 itself.

[0038] As is clear from FIG. 4, the lens cavity 24 has a transverse dimension greater than the traverse dimension of a orifice 23 of the cavity 24. The moldable material from which the mold 26 is made is deformable so that when the moldable material from which the solid immersion lens structure 50 in accordance with the invention is made, has set, and is removed from the mold 26, a region of the mold 26 adjacent the mouth is permitted to deform thereby to permit the solid immersion lens portion to pass therethrough.

[0039] Referring now to FIG. 1, a solid immersion lens structure, generally indicated by reference numeral 50, is indicated. The structure 50 has been formed in accordance with the method of the invention as described above with reference to FIGS. 2-4 of the drawings. In addition, the structure 50 has been formed to define a sample observation region 52 in a passage extending therethrough. This passage may be formed in any suitable manner, such as by positioning an elongate mold core in the layer 28, with reference to FIG. 4, prior to the moldable material defining layer 28 having set. The elongate mold core from which the passage 52 is formed can be of a material which disintegrates when exposed to a suitable agent. Accordingly, when the structure 50 has been formed, the core can be removed by exposing it to the suitable agent thereby to remove the core from the structure 50 and to yield the hollow region 52. Alternatively, the region 52 can be formed in any one of the ways described in Applicants' co-pending patent application Ser. No. 09/605,520 filed Jun. 27, 2000. For example the region 52 can be formed as part of a two-step construction process whereby the height h′ is precisely defined during the first step and then the passage and a body portion is added as a second step.

[0040] FIG. 5 is illustrative of an imaging system 100 employing an SIL 50 in accordance with the invention. An example is a microscope. The system includes a laser 110 projecting a beam 111, an expansion lens 112, a first collimating lens 114, a partially transmissive mirror 116, a second collimating lens 118, an SIL structure 50, a focusing lens 120, an image detector such as a CCD camera 124 and a control apparatus 123. In operation, the laser 110 projects an illumination beam 111 through expansion lens 112 and collimating lens 114 to produce a broad coherent monochromatic illumination beam 115. The beam 115 is reflected by mirror 116 to second collimating lens 118 through which it is focused through an air medium to the SIL structure 50. Focus adjustment is by means of positioning of the second collimating lens 118 relative to the SIL structure 50. The SIL structure 50 further focuses to a spot in the sample chamber (not shown) within the body portion, in immersion contact with the lens. The sample is positioned by the control apparatus 123. (The control apparatus 123 may both position a platform and supply the object or sample to be viewed.) Light reflected from the object is directed back through the second collimating/focusing lens 118 which focuses to infinity and directs the image 119 through the half silvered mirror 116 to a third focusing/collimating lens 120. The third focusing/collimating lens 120 focuses the image as magnified onto an image sensor 124, such as a CCD array of a CCD camera. The relative positioning of the lens 120 and the image sensor 124 determines focus of the image. Other microscope configurations may be employed as suggested by this configuration. Significantly, the SIL structure 50, although an essential element of the optical system is obtained from a manufacturing process which yields extremely inexpensive optical elements as compared to conventional lenses, so the SIL structure 50, which is integral with the sample carrier, is disposable. This is believed to be a significant advance over conventional SIL technology. This also presents significant practical advantages over methods using oil immersion objectives. It is not necessary to use oil between the lens and sample since they are integrally molded. In addition, an oil immersion objective must be positioned with high accuracy with respect to the sample. However, a solid immersion lens can be fabricated as the appropriate distance away from the sample so that focal precision would be needed to adjust the distance between an oil immersion lens and the sample.

[0041] FIG. 6 is illustrative of a light collection system 200 employing an SIL 50 in accordance with the invention. An example is a cytometer or a highly efficient spectrometer. The system 200 includes a laser 210 projecting a beam 211, an expansion lens 212, a first collimating lens 214, an optional first dichroic filter 217 selected for passing the selected output wavelength of the laser 210, a partially transmissive mirror 216, a second collimating lens 218, an SIL structure 50, a second dichroic filter 219 selected for passing the selected emission of the sample, a collection lens 220, a photon collection device such as a photomultiplier tube 226 and a control apparatus 223. In operation, the laser 210 projects an illumination beam 211 through expansion lens 212 and collimating lens 214 to produce a broad coherent monochromatic illumination beam 215. Its purity is further selected by filter 217 so that the illumination can be used as an excitation probe. The beam 215 is reflected by mirror 216 to second collimating lens 218 through which it is focused through an air medium to the SIL structure 50. Focus adjustment is by means of positioning of the second collimating lens 218 relative to the SIL structure 50. However, as an emission collection apparatus, imaging is not the goal. The SIL structure 50 further concentrates the illumination to in the sample chamber (not shown) within the body portion, in immersion contact with the lens. The sample is excited by the illumination and positioned by the control apparatus 223. (The control apparatus 223 may both position a platform and supply the object or sample to be viewed.) The illumination excites the sample to cause it to emit fluorescent energy which is collected by the high numerical aperture lens and is directed back through the second collimating lens 218 which focuses to infinity and directs the emitted photonic energy through the half silvered mirror 216 to the second filter 219, which blocks any stray excitation, and then through the third collimating lens 220. The third collimating lens 220 concentrates the photonic energy into a collection region of a photon sensor such as a photomultiplier tube (PMT) 226. The relative positioning of the lens 120 and the PMT 226 the collection efficiency. This application is believed to be a new application of an SIL structure. Other configurations may be employed as suggested by this configuration. Significantly, the SIL structure 50, although an essential element of the optical system is obtained from a manufacturing process which yields extremely inexpensive optical elements as compared to conventional lenses, so the SIL structure 50, which is integral with the sample carrier, is disposable. This is believed to be a significant advance over conventional SIL technology.

[0042] The invention has been explained with reference to specific embodiments. Other embodiments will be evident to those of ordinary skill in the relevant art. It is therefore not intended that this invention be limited, except as indicated by the appended claims.