JPH0945971A - Method and apparatus of mask alignment for josephson element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To locate the grain boundary junction of a superconductor layer formed on a bi-crystal substrate without putting a mark thereon in the alignment of lithography for Josephson element fabrication process. SOLUTION: Under a state where a light insensitive to a resist layer 15 is passed from a bi-crystal substrate 10 side through the bi-crystal substrate 10, a superconductor layer 14 and the resist layer 15, the image of a junction line 12 of bi-crystal substrate 10 is observed from the resist layer 15 side. Consequently, the production accuracy of bi-crystal substrate 10 and the crystallinity of superconductor layer 14 have not significant effect on the observation. Since the junction line 12 of a bi-crystal substrate 10 can be observed relatively clearly, the grain boundary junction formed along the junction line 12 can be located easily thus facilitating the mask alignment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mask alignment method and apparatus for a Josephson device used for manufacturing a Josephson device.

[0002]

2. Description of the Related Art There is a Josephson element utilizing the Josephson effect in an electronic device to which a high temperature oxide superconductor such as YBaCuO, BiSrCaCuO, TlBaCaCuO is applied. is there. In the bicrystal bonding method, a high-temperature superconductor layer is grown on the polished surface of a bicrystal substrate formed by laminating two crystal plates having different crystal orientations. The grain boundary junction of the high temperature superconductor layer can be formed on a certain bond line.

Although the grain boundary junction on the junction line exhibits the Josephson effect, the portion including the grain boundary junction of the high temperature superconductor layer is used for use as a Josephson device exhibiting desired characteristics.
It is processed into a desired pattern shape by lithography and etching. In lithography, a photosensitive resist layer is formed to cover the superconductor layer, this resist layer is partially exposed using a photomask, and then developed to form an etching resistant resist layer on the superconductor layer. A resist pattern having a desired property is obtained.

Etching is carried out using the resist pattern obtained by this lithography as a protective film, and thereafter,
By removing the remaining portion of the resist layer forming the resist pattern, a Josephson device having desired electric characteristics is formed on the bicrystal substrate. Such a Josephson element is variously applied to, for example, a SQUID (superconducting quantum interference device) or a superconducting transistor.

By the way, since the grain boundary junction portion of the superconductor layer exhibiting the Josephson effect has an extremely narrow width dimension of about several μm, a photomask used prior to the exposure of the resist layer was used. During the mask pattern alignment work, that is, the mask alignment, the work of confirming the position of the grain boundary junction is extremely important. However, even using an optical microscope or the like, it is difficult to directly visually recognize the grain boundary bonding portion of the superconductor layer. This hard-to-see grain boundary junction portion is formed along the junction line of the bicrystal substrate. Therefore, the joining line of the bicrystal substrate is marked in advance, that is, marking is performed, and the mark on the bicrystal substrate that indirectly indicates the position of the grain boundary joining portion is observed, and mask alignment is performed based on this mark. Was there.

[0006]

However, the conventional mask alignment method as described above requires a special marking step for marking, which complicates the lithography step. Therefore, it has been desired to develop a mask alignment method and apparatus that does not complicate the lithography process without adding a special process such as marking for knowing the position of the grain boundary junction.

[0007]

The present invention focuses on the fact that the superconductor layer, the bicrystal substrate, and the resist layer for the Josephson device generally have sufficient translucency to allow the transmission of light. However, instead of observing the grain boundary joint portion that cannot be directly visually recognized, without marking or the like on the joint line of the bicrystal substrate along which this grain boundary joint portion is formed,
By directly observing this joining line, the basic concept is to indirectly know the position of the grain boundary joining portion.

The mask alignment method according to the present invention is
In order to solve the above-mentioned problems, in a lithography for obtaining a desired resist pattern having etching resistance consisting of a resist layer on a superconductor layer formed on a polishing surface where a bonding line of a bicrystal substrate is exposed, a resist layer About the alignment of the mask pattern to the desired position on the bicrystal substrate from the side of the bicrystal substrate, the light not exposed by the resist layer,
With the superconductor layer and the resist layer transmitted, observe the junction line of the bicrystal substrate from the resist layer side, and align the mask pattern with the junction line observed under this transmitted light as a reference. It is characterized by performing.

Further, the mask alignment apparatus according to the present invention provides a Josephson device for a Josephson device on a polishing surface where a bonding line of a bicrystal substrate composed of two crystal plates bonded with different crystal orientations is exposed. A translucent sample stage having a sample surface on which a sample in which a superconductor layer and a photosensitive resist layer are laminated is arranged with the twin crystal substrate facing each other, and the sample on the sample stage is placed from the twin crystal substrate side. It is characterized by including a light source for irradiating, which does not expose the resist layer to light, and an optical means for observing an image of a joining line by the transmitted light of the light source from the resist layer side of the sample on the sample table.

[0010]

In the mask alignment method according to the present invention, the junction line of the bicrystal substrate is observed from the bicrystal substrate side under transmitted light that passes through the bicrystal substrate, the superconductor layer and the resist layer. An image of the joining line by the transmitted light is observed by using an optical means such as an optical microscope. Observing the bonding line under transmitted light means, for example, whether the crystallinity of the bicrystal substrate is good or not, or the manufacturing precision of the bicrystal substrate including the bonding precision of the crystal plates or the crystallinity of the superconductor layer on the bicrystal substrate It is extremely important in that it is possible to observe the joining line relatively clearly without being strongly influenced by the quality.

Instead of observing the bonding line by the transmitted light described above, it is possible to illuminate the light from the resist layer side and observe the image of the bonding line by the reflected light from the front surface or the back surface of the twin crystal substrate or in the vicinity thereof. it can. However, in this case, the manufacturing accuracy of the bicrystal substrate or the quality of the crystallinity of the superconductor layer is strongly influenced, and it is often difficult to observe the bonding line.

On the other hand, according to the method of the present invention, even a joining line which cannot be observed by reflected light can be visually recognized by observation with transmitted light. The cause of this difference can be considered as follows. In the observation with reflected light, the reflected light including the image information of the reflecting surface is
As a result, the contrast of the observed image is significantly weakened because the intensity of the image itself is weak, and the light is scattered while passing through the superconductor layer and the resist layer and further attenuated. On the other hand, in observation with transmitted light, the intensity of the transmitted light transmitted from the twin crystal substrate side is much higher than that of the reflected light, and the transmitted light is scattered by the twin crystal substrate, the superconductor layer and the resist layer. Although it is attenuated, the contrast of the observed image is not significantly weakened as in the case of reflected light.

According to the method of the present invention, in order to obtain a clearer bond line image, when an image of the bond line by transmitted light is observed by using an optical means such as an optical microscope, the back surface of the bicrystal substrate is covered. It is desirable to focus the optical means. This is because the back surface of the bicrystal substrate is generally unpolished and roughened so that the bond line is more clearly shown than on the polished surface and the bond line image is less attenuated by scattering. It is thought that it can be taken.
In reality, a clear bonding line image could be observed when the optical means was focused on the back surface of the bicrystal substrate rather than the polished surface.

In the mask alignment apparatus according to the present invention, since the sample stage for receiving the sample in which the superconductor layer and the photosensitive resist layer are laminated on the polished surface of the bicrystal substrate is transparent, By arranging the light source inside the table or arranging the light source outside the sample table, it is possible to irradiate the sample through the sample table from the side of the twin crystal substrate, which is the back surface of the sample table. It becomes possible to easily carry out the method.

[0015]

The present invention will be described in detail below with reference to the illustrated embodiments. 1 and 2 are a longitudinal sectional view and a plan view, respectively, schematically showing a mask alignment apparatus for carrying out the mask alignment method of the present invention. Prior to the description of the mask alignment method and the apparatus, the present invention will be described. The manufacture of the Josephson device will be described.

FIG. 3 is a perspective view showing an example of a bicrystal substrate used for forming the Josephson device according to the present invention. There is a bicrystal bonding method as a method of obtaining a Josephson junction exhibiting the Josephson effect, and the bicrystal substrate 10 shown in FIG. 3 is used for this bicrystal bonding method. The bicrystal substrate 10 is made of, for example, SrTiO ₃ , Mg
O, CeO ₂ , yttrium-stabilized zirconia (YS
Z), which is a crystal plate having a light-transmitting property, and two rectangular crystal plates 11 a and 1 of the same kind having different crystal orientations.
It can be obtained by, for example, heating in an oxygen atmosphere and integrating them in a state where 1b is bonded.

In the illustrated example, crystal plates 11a and 11b.
Are formed by making the [001] orientations of the Z axis coincide with each other and intersecting the (100) plane at an angle θ around the Z axis. This angle θ is generally set to 20 degrees or more. [001] of both crystal plates 11a and 11b
The azimuth can be offset by an angle φ. In this embodiment, φ is 0 °. The joining line 12 of both crystal plates 11a and 11b is formed on both sides 13a and 13b of the bicrystal substrate 10.
Is exposed to. One surface 13a is mechanically and chemically polished for the formation of the oxide superconductor layer described below, while the other surface 13b is not subjected to any special processing. It is placed on a rough surface.

FIGS. 4A to 4F are explanatory views showing the manufacturing process of the Josephson device using the bicrystal substrate 10. On one polished surface, ie, surface 13a, of the bicrystal substrate 10, for example YBaCuO, BiSrCaCu
By epitaxially growing an oxide high temperature superconductor material such as O or TlBaCaCuO, the high temperature superconductor layer 14 is formed to a thickness of, for example, 100 nm to 400 nm (see FIG. 4A). Due to the growth of the high temperature superconductor layer 14, a grain boundary junction having a width of, for example, 0.2 to 10 μm is formed along the joining line 12 on the surface 13a of the bicrystal substrate 10, and the grain boundary junction causes the Josephson effect. can get. Such a high temperature superconductor layer 14 has a light transmitting property.

In order to obtain a Josephson device exhibiting desired electric characteristics from the high temperature superconductor layer 14, the bicrystal substrate 1
The high temperature superconductor layer 14 on the substrate 0 is subjected to a lithography process and an etching process. In the lithography process, a photoresist material composed of a photosensitive organic polymer solution that is sensitive to ultraviolet light is uniformly applied onto the high temperature superconductor layer 14 using a spin coater or the like. Then, a prebaking process called prebaking evaporates the organic solvent from the applied photoresist material, whereby a photosensitive resist layer 15 having a thickness of, for example, several μm is laminated on the high temperature superconductor layer 14. Formed (see FIG. 4B). This resist layer 15
Shows a translucency like the bicrystal substrate 10 and the high temperature superconductor layer 14.

A desired mask pattern 16 is printed on the resist layer 15 through a photomask (not shown) by, for example, a reflection projection exposure apparatus (see FIG. 4C). An example of a SQUID circuit pattern is shown as the mask pattern 16. In the illustrated example, the mask pattern 16 has a width dimension of several μm and a length dimension of several tens of μm, and each includes a grain boundary junction. A pair of bridge portions 16a arranged as described above are provided. As the light source of the exposure device, ultraviolet light having a wavelength of 350 to 450 nm or
A mercury lamp or the like called Deep UV that emits ultraviolet light having a wavelength of 200 to 300 nm is used.

When the resist layer 15 having the mask pattern 16 printed thereon is developed, if the resist layer is a negative type, the portion except the mask pattern 16 is removed, and the resist layer 15 forms a resist pattern 15A. (See FIG. 4 (d)). Then, if necessary, the developing solution or the rinsing solution is removed and the resist pattern 1
In order to enhance the adhesion between 5A and the high temperature superconductor layer 14,
A baking process called post bake is performed.

The lithography process is completed as described above, and the etching process is subsequently performed. In this etching step, the resist pattern 15A is used as an etching mask, and by dry etching, for example, only the portion corresponding to the resist pattern 15A of the high temperature superconductor layer 14 is left and the other portions are removed (see FIG. 4E). ). After that, the resist pattern 15A is removed, and the Josephson element 14A is obtained by the remaining portion of the high temperature superconductor layer 14 corresponding to the resist pattern 15A (FIG. 4 (f)).
reference).

In the lithography step, which is one of the steps of manufacturing the Josephson element 14A, the mask pattern 16 is formed on the high temperature superconductor prior to the exposure for printing the mask pattern 16 described with reference to FIG. Layer 14
1 for aligning the mask aligner, that is, the mask alignment device 2 shown in FIGS.
0 is used. The mask alignment apparatus 20 is shown in FIG.
As shown in FIG. 2, a sample table 22 on which a bicrystal substrate 10 (see FIG. 4B) in which a high-temperature superconductor layer 14 and a resist layer 15 are formed in a laminated form on a polishing surface 13a is placed as a sample 21 is mounted. And a light source 23 that emits visible light, for example, which does not expose the resist layer 15 to light, and an optical means 24 such as an optical microscope.

The sample table 22 is made of a transparent or semi-transparent synthetic resin material or a light-transmitting material such as glass. In the example shown in FIG. 1 and FIG.
Fixed part 26 having a hemispherical spherical recess 25 formed on the upper surface
And an adjusting portion 28 having a hemispherical lower portion 27 that fits in the spherical recess 25. The adjusting portion 28 has a lower portion 27
Is slidably received in the spherical recess 25 of the fixed portion 26 with an appropriate frictional force. A flat sample surface 29 that receives the sample 21 is formed on the upper surface of the adjustment unit 28. In the sample 21, the other surface, that is, the back surface 13b, which is the rough surface of the bicrystal substrate 10, is made to face the sample surface 29, and
Placed on top. When the adjusting portion 28 is made of a hard material such as glass, it is desirable to cover the sample surface 29 with a transparent synthetic resin material such as vinyl in order to protect the sample 21.

Negative pressure passages 30 and 31 communicating with each other are formed in the fixed portion 26 and the adjusting portion 28, respectively. The negative pressure path 30 that opens to the lower end of the fixed portion 26 is connected to a negative pressure source (not shown) at its open end. Also, the adjusting unit 2
The negative pressure passage 31 which opens to the lower portion 27 of the No. 8 communicates with the upper end of the negative pressure passage 30 via the expansion chamber 32 formed at the open end thereof. The upper end of the negative pressure passage 31 is opened to the sample surface 29, and the negative pressure acting on the negative pressure passages 30 and 31 adsorbs and holds the sample 21 on the sample surface 29. Also, both negative pressure lines 30
And the expansion chamber 32 connecting 31 with
Even if the negative pressure paths 30 and 31 are misaligned due to the adjustment operation, the communication is ensured. Therefore, by forming the expansion chamber 32, the adjustment unit 2
The sample 21 can be reliably sucked and held regardless of the tilting of the sample 8.

The light source 23 irradiates the sample 21 placed on the sample surface 29 of the sample table 22 from the bicrystal substrate 10 side, that is, the back surface 13b side of the bicrystal substrate 10.
It is arranged below the sample table 22 as viewed in FIG. 1, and radiates visible light upward in the drawing. The optical unit 24 transmits the sample 21 and further observes the transmitted light passing through the sample 21.
Is arranged to observe.

Since the light which the resist layer 15 does not sensitize is selected as the light source 23, the resist layer 15 is not sensitized by the transmitted light from the light source 23, and the transmitted light is the side of the bicrystal substrate 10. The sample 21 is irradiated with. A part of the irradiation light from the light source 23 is absorbed or scattered by the sample table 22 and the sample 21, but the remaining part is transmitted through the sample 21. From this, by the optical means 24,
The joining line 12 of the bicrystal substrate 10 due to the transmitted light can be clearly seen, and by knowing the position of the joining line 12, the high temperature superconductor layer 14 formed along the joining line 12 can be seen.
The position of the grain boundary junction can be indirectly known. Thereby, the position of the grain boundary junction corresponding to the joining line 12 can be relatively easily known without marking the joining line 12 of the bicrystal substrate 10 as in the conventional case. Therefore,
When aligning the mask pattern 16, the optical means 2 is used.
4 based on the image of the joining line 12 observed by FIG.
As described along with (c), the mask pattern 16
Can be accurately and easily aligned to a predetermined position where the bridge portion 16a correctly matches the position of the grain boundary junction.

When observing the bonding line 12 by the optical means 24, the focus of the optical means 24 is focused on the polished surface 1 of the bicrystal substrate 10.
A clear bonding line image could be observed when the back surface 13b was aligned with the surface 3a. Further, for example, the light source 23 is arranged on the same side as the optical means 24, and the sample 2
The sample 21 in which the bonding line image cannot be seen when the irradiation is performed by irradiating 1 and observing the bonding line image with the reflected light is also performed by the optical means according to the method of the present invention, as described above. By observing the focal point of 24 under the transmitted light adjusted to the back surface 13b of the bicrystal substrate 10, the image of the bonding line 12 was clearly visible.

FIG. 5 is a drawing showing a photomicrograph of the back surface 13b of the bicrystal substrate 10 by the transmitted light of the present invention.
Is a surface 13 of the bicrystal substrate 10 by reflected light showing a comparative example.
It is drawing which shows the microscope picture of a and the back surface 13b. In each case, the right half is a view of a portion without a superconductor layer. A biphase crystal substrate made of SrTiO ₃ and having an angle θ of the (100) plane of 24 degrees was used as the bicrystal substrate 10, and YBaCuO was used as the high temperature superconductor layer 14. In the micrograph of the transmitted light of the present invention shown in FIG. 5, the portion indicated by the arrow L is a joining line 12 as a black line running in the left-right direction in the figure.
Is clearly visible. On the other hand, in the micrograph of the comparative example shown in FIG. 6A, it is not possible to observe the bonding line image extending to the same portion as the bonding line 12 of FIG. In the photomicrograph of FIG. 6 (a), the focus is on the front surface 13a of the twin crystal substrate 10, but in the case of observation by this reflected light, the reflected light itself is extremely weak even if the back surface 13b is focused, Sufficient contrast cannot be obtained to form an image as a whole (Fig. 6).
(B)).

As described above, according to the present invention, the bonding line 12 can be clearly observed by observing it under transmitted light. The reason for this is that, as described above, the transmission through the bicrystal substrate side is transmitted. The intensity of the light is extremely higher than that of the reflected light, and the light is scattered and attenuated by the bicrystal substrate 10, the superconductor layer 14 and the resist layer 15. Is not significantly weakened as in the case of reflected light, and on the back surface 13b, which is left unpolished and roughened, the bond line 12 is more clearly shown than on the polished surface 13a and is less attenuated by scattering. It is considered that the image of the joining line 12 can be taken in the state. Although the light source 23 can be arranged in the sample table 22 to make the mask alignment apparatus 20 compact, it is preferable to separate the light source 23 from the sample table 22 from the viewpoint of heat dissipation of the light source. .

The present invention is not limited to the above embodiment. We have explained the reflection projection exposure method that transfers the mask pattern onto the resist layer without bringing the photomask into contact with the resist layer.The reduction projection exposure method or the contact exposure method that brings the photomask into close contact with the resist layer on the bicrystal substrate has been described. The invention of the present application can also be applied. Further, although the circuit pattern for the SQUID has been described, the present invention is not limited to this, and can be applied to various Josephson devices using Josephson junctions such as superconducting transistors.

[0032]

According to the method of the present invention described above, by observing the image of the junction line by the transmitted light from the side of the bicrystal substrate, the manufacturing accuracy of the bicrystal substrate or the crystallinity of the superconductor layer can be improved. Since it is possible to observe the bonding line of the bicrystal substrate relatively clearly without being strongly influenced by the quality of the mask, it is possible to perform mask alignment without making the conventional marking for the bonding line on the bicrystal substrate. Can be performed relatively easily, and the Josephson device can be easily produced without complicating the lithography process including the mask alignment. Further, according to the apparatus of the present invention, the sample can be irradiated from the backside of the bicrystal substrate through the sample stage, and the image of the bonding line due to the transmitted light can be easily observed. The method of the invention can be carried out easily and efficiently.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view schematically showing a mask alignment apparatus for carrying out a mask alignment method of the present invention.

FIG. 2 is a plan view of the mask alignment apparatus shown in FIG.

FIG. 3 is a perspective view showing an example of a bicrystal substrate for a Josephson device according to the present invention.

FIG. 4 is an explanatory diagram showing a manufacturing process of the Josephson device.

FIG. 5 is a drawing showing a microscope photograph of the back surface of a bicrystal substrate according to transmitted light of the present invention.

FIG. 6 is a drawing showing a microscope photograph of the surface of a bicrystal substrate by reflected light showing a comparative example.

[Explanation of Codes] 10 Bi-crystal substrate 11a, 11b Crystal plate 12 Bonding line 13a Polished surface 13b Back surface 14 High-temperature superconductor layer 14A Josephson device 15 Resist layer 15A Resist pattern 16 Mask pattern

Claims (3)

[Claims] 1. A photosensitive resist layer is formed to cover a superconductor layer for a Josephson device formed on a polished surface where a bonding line of a bicrystal substrate is exposed, and the resist layer is used as a mask pattern. In the lithography for obtaining a desired resist pattern having etching resistance composed of the resist layer on the superconductor layer after partial exposure along the line, the mask pattern is positioned prior to the exposure of the resist layer. A mask alignment method for aligning, wherein the resist layer side is a state in which light that the resist layer does not sensitize is transmitted from the twin crystal substrate side to the twin crystal substrate, the superconductor layer and the resist layer. And observing the bonding line of the bicrystal substrate from the above, and aligning the mask pattern with the bonding line observed under the transmitted light as a reference. And wherein, mask alignment method for the Josephson element. 2. The Josephson according to claim 1, wherein the observation of the bonding line is performed by an optical microscope, and the focus of the optical microscope is set on the back surface which is the rough surface of the bicrystal substrate. Mask alignment method for device. 3. A superconducting layer for a Josephson device and a photosensitive resist on a polishing surface where a joining line of a bicrystal substrate composed of two crystal plates joined with different crystal orientations is exposed. A translucent sample stage having a sample surface in which a sample in which layers are laminated is arranged with the bicrystal substrate facing each other, and the resist layer for irradiating the sample on the sample stage from the side of the bicrystal substrate. A mask alignment for the Josephson device, including a light source that does not expose the light source and an optical unit that observes an image of the bonding line due to the transmitted light from the light source from the resist layer side of the sample on the sample table. apparatus.