High strength carbon fiber composite wafers for microfabrication

Priority is claimed to U.S. Provisional Patent Application Ser. No. 61/689,392, filed on Jun. 6, 2012; which is hereby incorporated herein by reference in its entirety.

This is a continuation-in-part of U.S. patent application Ser. No. 13/453,066, filed on Apr. 23, 2012, which claims priority to U.S. Provisional Patent Application Nos. 61/486,547, filed on May 16, 2011; 61/495,616, filed on Jun. 10, 2011; and 61/511,793, filed on Jul. 26, 2011; all of which are hereby incorporated herein by reference in their entirety.

The present application is related generally to high strength microstructures, such as for example x-ray window support structures.

Carbon fiber composite (CFC) wafers can be used in applications where high strength is desired. Barriers to the development of carbon fiber based structures, especially structures with micrometer-sized features, include difficulties in machining or patterning, and high surface roughness of cured composites. A root mean square surface roughness Rq of typical CFC wafers can be greater than 1 micrometer. Root mean square surface roughness Rq can be defined by the following equation: R q =√{square root over (Σz i 2 )}. In this equation, z represents a height of the surface at different measured locations i.

It has been recognized that it would be advantageous to have a carbon fiber composite wafer having high strength and low surface roughness.

In one embodiment, the present invention is directed to a carbon fiber composite (CFC) wafer that satisfies the needs for high strength and low surface roughness. The CFC wafer comprises at least one sheet of CFC including carbon fibers embedded in a matrix. The wafer can have a thickness of between 10-500 micrometers. The wafer can have a root mean square surface roughness Rq, on at least one side, of less than 300 nm in an area of 100 micrometers by 100 micrometers and less than 500 nm along a line of 2 millimeter length. The wafer can have a yield strength at fracture of greater than 0.5 gigapascals, wherein yield strength is defined as the force, in a direction parallel with a plane of the wafer, per unit area, to cause the wafer to fracture. The wafer can have a strain at fracture of more than 0.01, wherein strain is defined as the change in length caused by a force in a direction parallel with a plane of the wafer divided by original length.

In another embodiment, the present invention is directed to a method of making a CFC wafer that satisfies the needs for high strength and low surface roughness. The method comprises pressing a stack of at least one sheet of CFC between pressure plates with a porous breather layer disposed between at least one side of the stack and at least one of the pressure plates; then heating the stack to a temperature of at least 50° C. to cure the stack into a CFC wafer.

In another embodiment, the present invention is directed to an x-ray window including a high strength support structure. The x-ray window can comprise a support frame defining a perimeter and an aperture with a plurality of ribs extending across the aperture of the support frame and carried by the support frame. Openings exist between the plurality of ribs. The support frame and the plurality of ribs comprise a support structure. A film can be disposed over, can be carried by, and can span the plurality of ribs and can be disposed over and can span the openings. The film can be configured to pass x-ray radiation therethrough. The support structure can comprise a carbon fiber composite material (CFC). The CFC material can comprise carbon fibers embedded in a matrix. A thickness of the support structure can be between 10-500 micrometers. A root mean square surface roughness Rq of the support structure on a side facing the film can be less than 500 nm along a line of 2 millimeter length.

FIG. 1 is a schematic top view of a carbon fiber composite wafer, in accordance with an embodiment of the present invention;

FIGS. 2-3 are schematic cross-sectional side views of a carbon fiber composite wafer, in accordance with an embodiment of the present invention;

FIG. 4 is a schematic cross-sectional side view of portion of a carbon fiber composite wafer, showing measurement of root mean square surface roughness Rq, in accordance with an embodiment of the present invention;

FIG. 5 is a side view of a carbon fiber, in accordance with an embodiment of the present invention;

FIG. 6 is a schematic cross-sectional side view of wafer including multiple carbon fiber composite sheets abutting a polyimide sheet, in accordance with an embodiment of the present invention;

FIG. 7 illustrates a first curing process for manufacture of a carbon fiber composite wafer, in accordance with a method of the present invention;

FIG. 8 illustrates use of o-rings and a vacuum during the first curing process for manufacture of a carbon fiber composite wafer, in accordance with a method of the present invention;

FIG. 9 illustrates a second curing process for manufacture of a carbon fiber composite wafer, in accordance with a method of the present invention;

FIG. 10 is a schematic top view of an x-ray window support structure, in accordance with an embodiment of the present invention;

FIG. 11 is a schematic cross-sectional side view of an x-ray window, in accordance with an embodiment of the present invention;

FIG. 12 is a schematic cross-sectional side view of an x-ray detector, including an x-ray window, in accordance with an embodiment of the present invention.

As used herein, the term “carbon fiber” or “carbon fibers” means solid, substantially cylindrically shaped structures having a mass fraction of at least 85% carbon, a length of at least 5 micrometers and a diameter of at least 1 micrometer.

As used herein, the term “directionally aligned,” in referring to alignment of carbon fibers with support structure members (such as ribs for example), means that the carbon fibers are substantially aligned with a longitudinal axis of the support structure members and does not require the carbon fibers to be exactly aligned with a longitudinal axis of the support structure members.

As used herein, the term “porous” means readily permeable to gas.

Illustrated in FIGS. 1-3 are carbon fiber composite (CFC) wafers 10 and 20 comprising at least one CFC sheet 21 including carbon fibers 12 embedded in a matrix 11 . The matrix can comprise a material that provides sufficient strength and is compatible with the use of the wafer. For example, if the wafer will be used to fabricate an x-ray window support structure, considerations for matrix material may include a low atomic number elements and low outgassing. The matrix can comprise a material selected from the group consisting of polyimide, bismaleimide, epoxy, or combinations thereof. The matrix can comprise a material selected from the group consisting of amorphous carbon, hydrogenated amorphous carbon, nanocrystalline carbon, microcrystalline carbon, hydrogenated nanocrystalline carbon, hydrogenated microcrystalline carbon, or combinations thereof. The matrix can comprise a ceramic material selected from the group consisting of silicon nitride, boron nitride, boron carbide, aluminum nitride, or combinations thereof.

The carbon fibers 12 can be directionally aligned in a single direction A 1 , directionally aligned in multiple directions, or disposed in random directions in the matrix. Three CFC sheets 21 a - c are shown in FIGS. 2-3 . There may be more or less CFC sheets 21 than 3, depending on the desired application. The wafer 20 can have a thickness Th w of between 10-500 micrometers in one aspect, between 20 and 350 micrometers in another aspect, less than or equal to 20 micrometers in another aspect, or greater than or equal to 350 micrometers in another aspect.

CFC wafers per the present invention can have high yield strength. A yield strength at fracture can be greater than 0.1 gigapascals (GPa) in one aspect, greater than 0.5 GPa in another aspect, greater than 2 GPa in another aspect, between 2 GPa and 3.6 GPa in another aspect, or between 0.5 GPa and 6 GPa in another aspect. Yield strength can be defined as a force F in a direction parallel with a plane 33 or 34 of a side 32 a or 32 b of the wafer, per unit area, to cause the wafer to fracture. If fibers are directionally aligned, the force F can be aligned parallel with the fibers.

CFC wafers per the present invention can have high strain. A strain at fracture can be greater than 0.01 in one aspect, greater than 0.03 in another aspect, greater than 0.05 in another aspect, or between 0.01 and 0.080 in another aspect. Strain can be defined as the change in length L caused by a force F in a direction parallel with a plane 33 or 34 of the wafer divided by original length L. If fibers are directionally aligned, the force F can be aligned parallel with the fibers.

The wafer can have two faces or sides 32 a - b and an edge 31 . The sides 32 a - b can have a substantially larger surface area than the edge 31 . The sides 32 a - b can be substantially parallel with each other. One side 32 a can be disposed along, or parallel with, a single plane 33 ; and the other side 32 b can be disposed along, or parallel with, a different single plane 34 .

At least one side 32 a and/or 32 b of the wafer can be smooth, i.e. can have a low surface roughness. A low surface roughness can be beneficial for improving adhesion to other materials, such as to an x-ray window film for example. One measurement of surface roughness is root mean square surface roughness Rq calculated by the equation R q =√{square root over (Σz i 2 )}. The measurement z i can be made along a surface of the wafer by an atomic force microscope. The measurement of z i on a portion of the wafer 40 is shown in FIG. 4 . A distance from a plane 43 , substantially parallel with the wafer, or substantially parallel with the sides 32 a and 32 b of the wafer, can differ by small amounts. These small variations can be recorded, squared, summed, then a square root may be taken of this sum to calculate root mean square surface roughness Rq. A low Rq number can indicate a low surface roughness. The root mean square surface roughness Rq of one or both sides of the wafers of the present invention can be less than 300 nm in one aspect, or between 30 nm and 300 nm in another aspect, in an area of 100 micrometers by 100 micrometers. The root mean square surface roughness Rq of one or both sides of the wafers of the present invention can be less than 500 nm in one aspect, or between 50 nm and 500 nm in another aspect, along a line of 2 millimeter length. The root mean square surface roughness Rq of one or both sides of the wafers of the present invention can be less than 200 nanometers in one aspect, or between 20 nm and 200 nm in another aspect, in an area of 100 micrometers by 100 micrometers.

Shown in FIG. 5 is a side view of a carbon fiber 12 , in accordance with an embodiment of the present invention. At least 50% of the carbon fibers 12 in a wafer can have a diameter D of between 2 and 10 micrometers in one aspect. At least 90% of the carbon fibers 12 in a wafer can have a diameter D of between 2 and 10 micrometers in another aspect. Substantially all of the carbon fibers 12 in a wafer can have a diameter D of between 2 and 10 micrometers in another aspect.

As shown on wafer 60 in FIG. 6 , a polyimide sheet 61 can be cured together with and can abut the sheet(s) 21 of carbon fiber composite. The polyimide sheet can have a thickness Th p , after curing, of between 0.1-100 micrometers.

Also shown on wafer 60 in FIG. 6 are carbon fiber composite sheet thicknesses Th a-c . Each carbon fiber composite sheet 21 a - c in the stack can have a thickness Th a-c of between 20 to 350 micrometers (20 μm<Th a <350 μm, 20 μm<Th b <350 μm, and 20 μm<Th c <350 μm) in one aspect, less than or equal to 20 micrometers in another aspect, or greater than or equal to 350 micrometers in another aspect. There may be more or less than the three carbon fiber composite sheets 21 a - c . These thicknesses are sheet 21 thicknesses after curing.

FIG. 7 illustrates a first curing process 70 for manufacture of a carbon fiber composite wafer, in accordance with a method of the present invention. The method can comprise providing a stack 71 of at least one sheet of CFC 21 a - c , the stack having a first surface 32 a and a second surface 32 b ; pressing P the stack between a first pressure plate 76 a and a second pressure plate 76 b with a porous breather layer 72 disposed between the first surface 32 a of the stack and the first pressure plate 76 a ; and curing by heating the stack 71 to a temperature of at least 50° C. (defining a first curing process). The amount of pressure to be used can depend on the matrix of the carbon composite. Pressure in the range of 50-200 psi has been successfully used. Pressure may be in the range of 25-500 psi.

A solid, polished layer 73 can be disposed between the second surface 32 b of the stack 71 and the second pressure plate 76 b during the first curing process. The polished layer 73 can help create a very smooth surface on the second surface 32 b of the stack 71 . The polished layer 73 can be a highly polished sheet of stainless steel, a silicon wafer, or a glass plate. A fluorine release layer can be used to avoid the stack sticking 71 to the polished layer 73 . For example, a fluorinated alkane monolayer can be deposited on silicon wafers to facilitate release by placing in a vacuum desiccator overnight with 5 mL of Trichloro(1H,1H,2H,2H-perfluorooctyl)silane in a glass vial. The polished layer 73 can have a root mean square surface roughness Rq of less than 300 nm in an area of 100 micrometers by 100 micrometers, on a side facing the stack. Thus, it is not necessary for the polished layer 73 to have a polished surface on both sides.

A polyimide sheet 61 can be cured together with and can abut the CFC sheet(s) 21 . The polyimide sheet 61 can be disposed between the second surface 32 b of the stack 71 and the second pressure plate 76 b . The polyimide sheet 61 can be disposed between the second surface 32 b of the stack 71 and the polished layer 73 (if a polished layer is used). Alternatively, a polyimide sheet 61 can be disposed on both surfaces 32 a and 32 b of the stack 71 . The polyimide sheet(s) 61 can be useful for improving the surface of the final wafer and/or for improving adhesion of the stack 71 to other materials.

The porous layer 72 can allow gas, emitted by the stack, to escape from the press. A multi-layer porous breather layer 72 can be used. For example, the porous breather layer 72 can comprise a porous polymer layer 72 b facing the stack 71 and a nylon mesh 72 a facing the first pressure plate 76 a . A vacuum can aid in removal of the gas. A vacuum pump 75 can be attached by tubing 74 to the press and can draw a vacuum, such as less than 50 torr, between the pressure plates. The vacuum can be maintained through substantially all of the curing process, or through only part of the curing process, such as at least 50% of the curing process.

Shown in FIG. 8 are more details of the press and vacuum. The layers (stack of CFC, porous breather layer 72 , optional polished layer 73 , and optional polyimide layer 61 ) 81 can be in a central portion of the pressure plates 76 a - b . An o-ring 82 can surround the layers 81 . The o-ring 82 can be disposed at least partly in a channel 83 of at least one of the pressure plates 76 a and/or 76 b . The vacuum tube 74 can extend into the central portion of the press, between the layers 81 and the o-ring 82 .

FIG. 9 illustrates a second curing process 90 for manufacture of a carbon fiber composite wafer in accordance with a method of the present invention. After completion of the first curing process 70 , pressure P can be released from the stack and the porous layer 72 can be removed from the stack. A polished layer 73 a and 73 b can be disposed on each side of the stack. Note that if there was a polyimide sheet 61 in the first curing process, this polyimide sheet 61 can remain for the second curing process 90 . The polished layers 73 a and 73 b can have a root mean square surface roughness Rq of less than 300 nm in an area of 100 micrometers by 100 micrometers, on a side facing the stack. The stack 71 (and optional polyimide sheet 61 if one is used) can be pressed between the polished layers by the first and second pressure plates 76 a - b . The stack 71 (and optional polyimide layer 61 ) can be cured by heating the stack to a temperature of at least 50° C.

A benefit of use of the second curing process 90 is that the gas can be removed during the first curing process 70 , then polished layers 73 a and 73 b can be disposed on both sides 32 a and 32 b of the stack 71 , with the result that both sides of the wafer can be highly polished. Thus, both sides of the wafer can have a root mean square surface roughness Rq as specified above.

Shown in FIG. 10 is a support structure 100 for an x-ray window. The support structure 100 can comprise a support frame 101 defining a perimeter 104 and an aperture 105 . A plurality of ribs 102 can extend across the aperture 105 of the support frame 101 and can be carried by the support frame 101 , with openings 103 between the ribs 102 . The support frame 101 and the plurality of ribs 102 can comprise a support structure 100 . The support structure 100 can comprise a carbon fiber composite (CFC) material. The CFC material can comprise carbon fibers embedded in a matrix. Carbon fibers 12 in the composite can be substantially aligned with a direction A 1 of the ribs, with at least one direction of the ribs if the ribs extend in multiple directions, or with all directions of all ribs if the ribs extend in multiple directions.

Carbon fibers in a carbon fiber composite can be graphitic, and thus can be highly resistant to chemical etching. Alternative methods have been found for etching or cutting micro-sized structures in CFC wafers in the present invention. The support structure 100 may be made by cutting a CFC wafer to form ribs 102 and openings 103 . The CFC wafer may be cut by laser milling or laser ablation. A high power laser can use short pulses of laser to ablate the material to form the openings 103 by ultrafast laser ablation. A femtosecond laser may be used. A nanosecond pulsed YAG laser may be used. Ablating wafer material in short pulses of high power laser can be used in order to avoid overheating the CFC material. Alternatively, a non-pulsing laser can be used and the wafer can be cooled by other methods, such as conductive or convective heat removal. The wafer can be cooled by water flow or air across the wafer. The above mentioned cooling methods can also be used with laser pulses, such as a femtosecond laser, if additional cooling is needed.

As shown in FIG. 11 , the support structure 100 can have a thickness Th s of between 10-500 micrometers. Tops of the ribs 102 and support frame 101 can terminate substantially in a single plane 116 . A film 114 can be disposed over, can be carried by, and can span the plurality of ribs 102 and can be disposed over and can span the openings 103 . The film 114 can be configured to pass radiation therethrough, such as by being made of a material and thickness that will allow x-ray radiation to pass through with minimal attenuation of x-rays and/or minimal contamination of the x-ray signal.

As described above regarding FIGS. 6-9 , a polyimide layer 61 can be cured abutting the CFC stack 71 . The polyimide layer 61 can be cut into polyimide ribs 111 and a polyimide support frame 112 , with openings 103 between the ribs 111 , along with the CFC stack 71 . The polyimide ribs 111 and the polyimide support frame 112 can be part of the support structure 100 . The polyimide ribs 111 and the polyimide support frame 112 can be disposed between the CFC stack 71 and the film 114 .

A surface of the support structure 100 facing the film can have low surface roughness. This surface can be CFC 71 or can be polyimide 61 . This surface can have a root mean square surface roughness Rq of less than 300 nm in one aspect, or between 30 nm and 300 nm in another aspect, in an area of 100 micrometers by 100 micrometers. This surface can have a root mean square surface roughness Rq of less than 500 nm in one aspect, or between 50 nm and 500 nm in another aspect, along a line of 2 millimeter length. This surface can have a root mean square surface roughness Rq of less than 200 nanometers in one aspect, or between 20 nm and 200 nm in another aspect, in an area of 100 micrometers by 100 micrometers.

The ribs 102 can have a strain at fracture of greater than 0.01 in one aspect, greater than 0.03 in another aspect, greater than 0.05 in another aspect, or between 0.01 and 0.080 in another aspect. Strain can be defined as a change in length caused by a force in a direction parallel with the ribs divided by original length. If fibers are directionally aligned, the force F can be aligned parallel with the fibers.

The wafers described herein can also be micropatterned by laser ablation and/or water jet to form other structures, such as a flexure mechanical mechanism, a mesoscale mechanical mechanism, a microscale mechanical mechanism, and/or elements in a microelectromechanical system (MEMS).

As shown in FIG. 12 , an x-ray window 125 , including a support structure 100 and a film 114 , can be hermetically sealed to a housing 122 . The housing can contain an x-ray detector 123 . The x-ray detector can be configured to receive x-rays 124 transmitted through the window, and to output a signal based on x-ray energy.