WO1994028796A1 - X-ray tube and microelectronics alignment process - Google Patents

Abstract

A method for aligning layers used in the production of microelectronics components, comprises superpositioning a first layer (4), having a small selectively positioned x-ray-transparent area (3), over a second layer having an alignment dot (5) selectively positioned on one of its surfaces, the alignment dot being composed of four different metal elements (ABCD) separately located in four equally spaced adjacent areas of the dot, passing an x-ray beam generated in an end-window x-ray tube through the transparent area, irradiating at least a portion of the dot and generating fluorescent x-rays from at least one of the metals, selectively detecting the fluorescent x-rays generated and adjusting the superposed relation of the first and second layers so that the detected x-rays from all four different metals reach a predetermined level.

Description

X-RAY TUBE AND MICROELECTRONICS ALIGNMENT PROCESS

BACKGROUND OF INVENTION

1. Field of the Invention This invention relates to a relatively compact, even pocket size, microfocused x-ray tube having a protruding bright beam spot. It is especially adaptable for an x-ray stepper useful for dimensional control and alignment in the manufacture of microelectronic devices such as an integrated circuit (IC) or computer chip. It is also useful for medical imaging. The invention also relates to a microelectronics alignment, positioning or dimensional control process and system.

ICs are conventionally produced by a lithographic process which includes passing light through a mask onto the surface of a silicon wafer substrate coated with photosensitive chemicals to produce an etched chip.

Over the years, etched lines have become increasingly fine and circuits more complex, resulting in ICs having a greatly increased number of components equivalent to transistors, enabling the power of the computer chips and the electronic devices utilizing them to be substantially increased.

The etching process requires that the light passed through the mask must form sharp line images. This becomes increasingly difficult for very fine lines which require much higher line resolution. Technology using ordinary light has enabled production of ICs with a chip line width of about 0.7 μm, and having components equivalent to about 1 million transistors. The use of ordinary light is limited to producing a line width or resolution of down to about 0.3 μm. Ordinary light waves at 0.5 μm are too large to give sharp images at smaller line widths.

However, the semiconductor industry is moving toward 0.1 μm line resolution and contemplates use of 20 or more masking levels. These requirements indicate an urgent need for improvements in critical dimension controls of mask-to-wafer alignment during wafer exposure, in absolute positioning during mask writing, and in metrology in general.

2. Description of Related Art

X-rays have been investigated for such needs, due to their very short wave length. The generation of x-rays for IC lithography or dimension and positioning control has been a problem. Ordinary sources such as medical x-ray tubes are not useful because of production of unduly strong x-rays which penetrate the mask and wafer and do not enable production of sharp images without damage to the materials.

Soft (low energy) x-rays have been used for lithography and positioning, but have been produced by bulky equipment which is both expensive and not optimal for working with the relatively small wafers from which computer chips are made. For example, synchrotrons have been used to produce soft x-rays for lithography.

U.S. Patent 3,984,680 issued to Henry I. Smith discloses an alignment system for aligning a mask and substrate to be exposed using soft x-rays. Alignment marks with regions that transmit soft x-rays and regions opaque to soft x-rays are provided on the mask. Geometrically similar marks are provided on the substrate. The marks on the substrate contain materials which fluoresce when struck with x-rays used for alignment, i.e., emit x-rays. The emitted x-rays are detected. Soft x-rays from an x-ray source pass through the transparent regions and are absorbed by the opaque regions of the marks on the mask. The amount of x-rays striking the x-ray emitting marks on the substrate is dependent on the alignment or superposition of the mask marks and the substrate marks. The magnitude of the detected emitted x-rays indicates the degree of superposition, and can be used as a basis for moving the mask or substrate into the desired registration. The system of this patent requires operating in a vacuum.

Another alignment system is described in U.S. Patent 3,742,229 issued to Henry I. Smith et al. In this patent, soft x-rays are used with a first registration means on a mask and a second registration means on a substrate.

U.S. Patent 4,238,685 issued to Peter Tischer describes an alignment system employing displacement of x-ray beam source location with respect to a mask in fixed association with a semiconductor body.

U.S. Patent 3,743,842 issued to Henry I. Smith et al. discloses a lithographic process using soft x- rays.

U.S. Patent 5,044,001 issued to Chia-Gee Wang discloses a method and a apparatus for investigating materials in which x-rays are generated in a thin metal foil inside an evacuated x-ray tube, and a specimen outside the tube is exposed to the generated x-rays. The disclosure of U.S. 5,044,001 is incorporated herein by reference.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method for aligning or positioning layers used in the production of microelectronics components, which comprises placing a first layer, having a small selectively positioned transparent area, in closely spaced superposed relation to a second layer, said second layer having an alignment dot selectively positioned on one of its surfaces, said alignment dot being composed of four different metal elements separately located in 4 substantially equally spaced adjacent areas of said dot, positioning close to said transparent area an end-window x-ray tube having a metal foil coated on the inside of said window, generating a bright microfocused x-ray beam in said metal foil, passing said x-ray beam through said transparent area, a surface area of said first layer surrounding said transparent area being opaque to said x- ray beam, said x-ray beam passed through said transparent area irradiating at least a portion of said dot and generating fluorescent x-rays from at least one of said metals, selectively detecting said fluorescent x-rays generated from each of said metals, and adjusting the superposed relation of said first and second layers so that the detected x-rays from all 4 different metals reach a predetermined level.

Also provided according to the invention is an x-ray tube comprising a compact tubular chamber capable of being evacuated and connectable to an electric current supply, said chamber having at one end means for producing a beam of electrons and means for focusing said beam of electrons onto a metal foil, said chamber extending at an opposite end into a more narrow tubular portion having an end wall, a window transparent to x- rays being located in said end wall, a surface of said window inside said chamber having thereon said metal foil, the thickness of the metal foil being selected in relation to selected energies of said electron beam to generate in said foil a microfocused bright beam of x- rays of preselected energies, said beam of x-rays passing through said window.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic illustration of the microelectronics alignment system of the present invention.

Fig. 2 is a diagrammatic representation of an alignment dot used in the present invention.

Fig. 3 is a diagrammatic representation of another alignment dot used in the present invention.

Fig. 4 is a schematic elevational view partly in cross-section of an x-ray tube according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The production of ICs is heavily dependent on dimensional and positional feedbacks throughout the manufacturing process. Critical dimension (CD) metrology currently makes use of the scanning electron microscope (SEM) , particularly the very low energy SEM that limits radiation damage to the top most surface layer of a wafer. However, an SEM is expensive and requires an evacuation process which slows production. Another CD technique makes use of a confocal microscope whose laser interference pattern provides a line resolution at a fraction of the photon wavelength, but it suffers from extremely shallow focal depth. Proposed x-ray steppers have considerable focal depth, but are limited by spot size and beam brightness, and by the need for an iterative process to reach the optimum alignment position.

The present invention provides an x-ray positioning technique that uses a focused x-ray beam to adjust mask-to-wafer alignment, or mask to anchor alignment in direct writing for mask fabrication, without iteration of the alignment signals or wasted mechanical movement. The positioning signals have very large focal depth, and have an accuracy measured in nanometers.

The present x-ray tube is of the end-window type. It emits intense x-ray photons with a beam size typically of 10 μm, and the beam spot can be focused to 10 nm with a magnetic lens. The energy of the x-ray photons from the tube can be tuned between a low of 4.1

KV to a high of typically 20 or 20-30 KV when the tube is used for microelectronics alignment.

At low energy operation, x-ray photons are concentrated at 4.1 KV, which can excite Auger electrons of calcium in a resist. At high energy operation, x-ray photons can excite transition metals such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn. Four of these metals are used for positioning. At high energy, the x-ray photons are relatively transparent to the resist and silicon wafer. Therefore, an alignment spot or dot, hereinafter called an ABCD dot, can be positioned at the back of the wafer. A mask is used which for example may be a silicone oxide membrane having a pattern such as in gold on the upper surface, i.e. the surface facing away from a silicon wafer which is to be etched. Typically, the superposed mask and silicon wafer are spaced by about 10 μm from each other. The mask can include a transparent area surrounded by a spot or area opaque to the high energy x-ray photons, such as a one micron hole in a 3 μm thick gold spot. Preferably the pattern is on said surface and the spot opaque to x-rays is on the lower surface of the mask which faces the substrate or silicon wafer. A gold pattern mask in general has a pattern thickness of 1 μm. Corresponding to the hole in the gold spot, fluorescent spots are attached to a surface of the wafer for alignment, especially to the back side (the side facing away from the mask) . Each mask can have more than one spot for alignment to prevent rotational misalignment. The alignment holes in the gold can be laser drilled holes e.g. of diameter 10, 5, 2, 1 or 0.5 μm.

The x-ray tube of the invention provides a bright x-ray beam originating from a small focal spot, emits most line-energies, shines on the alignment spot and creates 4 different fluorescent signals from 4 correspondingly different elements A, B, C and D (Figs. 2 and 3) , with each signal indicating a different directional correction. Alignment is accomplished when all 4 signals (counting rates) reach certain predefined levels.

The bright focused x-ray beam is engineered to emerge from the neck of a small end-window design. The x-ray beam spot can be placed right next to the mask aperture (within 1 mm) so that much of the intensity of the x-ray beam can be utilized without need for an x-ray lens (such as x-ray fiber concentrator) .

In the end-window tube design, an e-beam is focused on a small end-window target spot and provides the desired line emission. As electrons enter a metal target and slow down via Columb interaction, they lose energy in multiple steps. When the electron energy is higher than certain absorption edge of the target material, the interaction can be bremsstrahlung (continuum) or fluorescent (lines) . When the electron energy is very near the edge but slightly above, the interaction is dominated by the resonant fluorescent absorption. As soon as the energy falls below the edge, only the continuum continues until no energy is left. This spectrum of mixing continuum with lines is true for all solid targets. Therefore, the electron beam energy for any x-ray tube is generally described by E-max or E- peak. For a thin film end-window target, on the other hand, there is an additional design parameter that can define the x-ray spectrum; it is the thickness of the film. The electron beam energy can be started at just above the edge producing mostly line emissions. The thickness is calculated to terminate the x-ray generation as soon as the energy falls below the edge, thereby eliminating the undesired continuum emissions at energy below the line emissions. In such an end-window design, the ratio of line versus continuum is very different from that of a solid target. Most of the x-ray photons can be the desired emissions using such an end-window design.

The line emissions can be selected by the use of different target elements in order to obtain different character emissions. For a typical x-ray tube, filter materials are typically selected using the same material as that of the target in order to obtain the maximum transparency of the desired line emissions while absorbing most strongly the x-ray energies just above the absorption edge in order to enhance the line-continuum ratio. In the end-window design, the target and the filter are combined into the same thin end-window material. This thin target material can provide the desired x-ray spectrum, but cannot hold the vacuum, nor conduct and dissipate the concentrated heat load. Therefore, the thin film is coated on a an x-ray transparent support such as beryllium (Be) whose low Z provides the highest transparency for x-ray photons.

Focusing of the e-beam of an x-ray tube can be electrostatic, electromagnetic, or a combination of the two. The preferred design of the invention is an electrostatic focusing lens housed in the tube, and a further magnetic coil lens housed outside the tube (Fig. 4).

The fluorescent x-rays from the 4 different elements of A, B, C and D of the alignment dot are detected by an x-ray detector which distinguishes the four metals. To help the detector resolve the K lines of the four metals, the choice of the four metals elements can be selected from every other element on the periodic table, instead of from neighboring elements.

The x-ray detector is energy sensitive (it can distinguish K fluorescent lines of four of the nine elements considered) and can be a high resolution proportional counter, or a silicon diode array with fairly thick depletion region ( < 40 μm ) in order to provide reasonable quantum efficiency for x-ray measurements.

The x-ray detector can be made with a donut shape as shown in Figure 1, so that it will pass all the direct unscattered x-ray beam and receive maximally the fluorescent counts from the ABCD dot.

As illustrated in Fig. 1, the x-ray tube 1 of the invention emits a bright concentrated beam through end window 2 positioned near an alignment aperture 3 in mask spot or region 4. The photon flux passes through the small aperture 3 and impinges on the ABCD dot which is on the surface of a silicon substrate (not shown) facing the mask or on the opposite or backside substrate surface. A, B, C and D represent four different metals which emit fluorescent x-rays, selectively detectable by detector 6. Detector 6 may be a Si (Li) detector.

Note that if the beam spot were to be located several mm away inside a glass tube typically found in most conventional high powered x-ray tubes, then the beam would have to be concentrated outside the tube in order to have enough photon flux to reach and pass through the small alignment aperture in the mask and perform the designated alignment functions. The pocket size microfocused x-ray tube of the present invention with its extremely bright beam spot protruding and ready to be placed next to an aperture on a mask therefore is very efficient.

Two embodiments of an ABCD alignment dot are shown in Figs. 2 and 3. Four different metals which emit fluorescent x-rays are represented by A, B, C and D, with the lines surrounding these letters delineating the area 7 occupied by each metal. The four metal-containing areas meet at a central intersection 7a. The area 7 occupied by an individual metal may extend in a direction away from the central intersection as shown by area 8 between parallel lines, and terminates at a desired distance from the central intersection. The extended areas may be omitted. The dot is a very small microdot. It may be produced by chemical vapor deposition of the metals, by vacuum evaporation or sputtering, or by photo- etching a thin film containing the metals or compounds of such metals.

To accomplish alignment of the mask with the silicon wafer, the need for positional correction is indicated by the level of fluorescent x-rays. If only emission from metal A is detected this means only the A area of the dot is being impinged by the x-rays from the x-ray tube passing through the aperture in the mask. Accordingly either the substrate or the mask is moved so that the point at which the areas A, B, C and D meet together is centered under the mask aperture. An off- center position is indicated by a different level of x- rays or amount of fluorescent photons among the four types detected.

Alignment may then be done using micro- manipulators. Motor driven micro-manipulators are available which move with 0.15 μm steps. The same manipulator can be manually controlled to produce 0.05 μm steps.

An x-ray tube according to the invention is illustrated in Fig. 4. The x-ray tube 9 comprises a tubular chamber 10 which can be evacuated by conventional means, not known. The chamber is connectable to an electrical current supply, not shown, such as is illustrated in the above mentioned U.S. Patent 5,044,001. The current supply may be adjusted so that the energy of x-ray photons from the tube ranges from 4.1 KV to 19 KV for use in alignment of microelectronics, and for medical uses up to 70 KV.

End window 11 has on its inside surface a metal foil target 12. The end window may be mounted in a tubular extension 13 of smaller diameter than tubular stem 15. A typical outside diameter of stem 15, shown by a-a is 5/8 inch. Tubular extension 13 is sealed to stem 15 by an indium seal 16. Tubular extension 13 is a preferred embodiment is surrounded by annular magnetic lens 14.

Chamber 10 typically may have an outside diameter of 1 1/2 inches as shown at b-b.

Contained in chamber 10 is filament 17 connectable to said power supply. The e-beam is focused by electrostatic focusing lens 18 and passes through aperture 19. Further focusing may be accomplished by above-mentioned magnetic lens 14.

The x-ray tube of the invention provides an intense x-ray beam emitted from a narrow stem and even more narrow end window. Resulting from the narrow end window design, the thin metal foil 12 and the microfocused emitted x-ray beam, the x-ray tube can be closely placed in proximity to the material or tissue to be exposed to x-rays.

The provided line-emissions may be narrowly tuned, as above explained, by using a metal foil target of preselected thickness. Thickness of the foil can range as low as 0.1 μm or below, or can be thicker such as typically 1.0 μm or up to 2.0 μm. The thicker metal foils are used in connection with higher emitted x-ray energies of up to 70 KV.

Resulting from the ability to tune or select the emitted energy, the x-ray tube can be designed to emit soft x-rays of about 4 KV, typically 4.1 KV, for use in lithography, and higher such as 10-20 or 10-30. KV for microelectronics alignment. Following alignment, the energy can be reduced to 4.1 KV for lithography without replacing the x-ray tube with different equipment. It should be noted that alignment and lithography can be accomplished without need for a vacuum environment.

The use of higher energies for alignment makes it possible to use transition metals for the ABCD dot thus enabling a wider choice of elements, and makes it possible to place the dot on the back of the silicon wafer since the silicon and resist are transparent to the high energy photons. Furthermore, such higher energy photons are transparent to dust and argon, so that the alignment can be done in air and the operation is far simpler than in vacuum.

For medical use the x-ray tube can be designed for higher x-ray energies such as 13 KV or higher for breast tissue, typically 17 KV; about 33 KV for dental or orthopedic use; and 40 to 70 KV for chest x-rays.

Claims

I CLAIM:

1. A method for aligning or positioning layers used in the production of microelectronics components, which comprises placing a first layer, having a small selectively positioned transparent area, in closely spaced superposed relation to a second layer, said second layer having an alignment dot selectively positioned on one of its surfaces, said alignment dot being composed of four different metal elements separately located in 4 substantially equally spaced adjacent areas of said dot, positioning close to said transparent area an end-window x-ray tube having a metal foil coated on the inside of said window, generating a bright microfocused x-ray beam in said metal foil, passing said x-ray beam through said transparent area, a surface area of said first layer surrounding said transparent area being opaque to said x- ray beam, said x-ray beam passed through said transparent area irradiating at least a portion of said dot and generating fluorescent x-rays from at least one of said metals, selectively detecting said fluorescent x-rays generated from each of said metals, and adjusting the superposed relation of said first and second layers so that the detected x-rays from all 4 different metals reach a predetermined level.

2. A method according to claim 1, wherein the alignment is done in the production of an IC, said first layer is a mask and said layer is a silicon wafer.

3. A method according to claim 2, wherein the energy of the x-ray beam passed through said transparent area is from about 10 to about 20 KV.

4. A method according to claim 3, wherein after aligning said layers, the energy of said x-ray beam is reduced to 4.1 KV.

5. A method according to claim 1, wherein the metal foil has a thickness of from 0.1 to 2 μm.

6. A method according to claim 1 wherein alignment is accomplished when the fluorescent x-rays of the 4 elements reach the same level.

7. A method according to claim 1 wherein the x-ray beam consists of line emission x-rays.

8. A method according to claim 1 wherein the metals are transition metals.

9. An x-ray tube comprising a compact tubular chamber capable of being evacuated and connectable to an electric current supply, said having at one end means for producing a beam of electrons and means for focusing said beam of electrons chamber onto a metal foil, said chamber extending at an opposite end into a more narrow tubular portion having an end wall, a window transparent to x- rays being located in said end wall, a surface of said window inside said chamber having thereon said metal foil, the thickness of the metal foil being selected in relation to selected energies of said electron beam to generate in said foil a microfocused bright beam of x- rays of preselected energies, said beam of x-rays passing through said window.

10. An x-ray tube according to claim 9, wherein said window is in a tubular extension extending from and of smaller diameter than said narrow tubular portion.

11. An x-ray tube according to claim 10, further including an annular magnetic focusing lens surrounding said tubular extension.

12. An x-ray tube according to claim 9 wherein the thickness of said metal foil is from 0.1 to μm.

13. An x-ray tube according to claim 9, wherein said preselected energies are 10 to 30 KV.

14. An x-ray tube according to claim 13 wherein said preselected energies of 10 to 30 KV are changeable to 4.1 KV.

15. An x-ray tube according to claim 9, wherein said preselected energies are 13 to 70 KV.