EP0618078A2

EP0618078A2 - An optical scanning device

Info

Publication number: EP0618078A2
Application number: EP94400656A
Authority: EP
Inventors: Masashi C/O Sumitomo 3M Limited Nagashima; Hiroyuki C/O Sumitomo 3M Limited Chiba
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1993-03-29
Filing date: 1994-03-28
Publication date: 1994-10-05
Also published as: JPH06302855A; EP0618078A3

Abstract

An optical scanning device characterized in that a light emitting element array (2) and an auto focussing light source element (3) for adjusting a distance between said light emitting element array and an object (5) to be scanned be said light emitting element (2) array are integrated on a single chip (1) of a semiconductor substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an optical scanning device on which an LED array and an auto focussing element are provided in an integrated structure, particularly an optical scanning device which is a two wavelength LED printer provided with an LED for focal adjustment and a recording array which are integrally formed on the recording LED or laser array chip.
In case of a high resolution printer using a light source, a light from a light source is not focussed on a sensitized paper due to irregularity of rotation of a photosensitive drum or deviation of the sensitized paper, and therefore, the printing quality deteriorates. Accordingly, the optical scanning device such as a printer requires an anti-defocusing mechanism.
Conventionally an auto-focus mechanism for an optical scanning device using an optical element array has not been known.
As an auto-focus mechanism for apparatuses other than the optical scanning device using the optical element array, an auto-focus adjusting technology using a reflected light from a disc in optical disc recording by means of a semiconductor laser has been generally known. This conventional auto-focus adjusting technology for optical disc recording is described below, referring to Fig. 8.
In Fig. 8, 81 is a laser driver, 82 is a semiconductor laser, 83 is a collimator lens, 84 is a beam splitter, 85 is a 1/4 λ plate, 86 is a focal lens, 87 is a lens drive, 88 is a disc and 89 is a photo detector. A reflected light from the disc 88 is guided to the photo detector 89 through the beam splitter 84 and a defocus deviation is detected.
"Astigmatism" is shown in Fig. 9 as a representative detection method. In Fig. 9, a light flux which has passed through the cylindrical lens 91 forms a circular spot on a light receiving surface 92 as shown in (d) when the light is focussed but it forms an oval spot as shown in (c) or (e) when the light is defocused. The signal components of the light flux obtained by the 4- divided photo detector 89 (Fig. 8) are subtracted from each other to be used for alignment of the focal lens 93.
The above prior art shown in Figs. 8 and 9 is an auto focussing technology for optical disc recording and is not an auto focussing technology for the optical scanning device using the optical-element array to which the present invention applies.
Tokyo Kokai (Japanese Patent Application Laid Open) 1990-304515 is known as the auto focussing technology for the optical scanning device. According to the art disclosed in this gazette, auto focussing is carried out independent of recording by making a wavelength of light flux for recording to be different from that of light flux for auto focussing. The region of a wavelength of a light for auto focussing where printing photosensitive material is not sensitive to this light is selected. However, the art disclosed in this gazette is an auto focussing technology for a single semiconductor laserflux and is not the auto focussing technology for the optical scanning device using the light-emitting element array to which the present invention applies.
On the other hand, in an optical scanning device using a light-emitting element array such as an LED array for printing, a fixed type which depends on mechanical accuracy, is conventionally employed.

SUMMARY OF THE INVENTION

If it is necessary to use the above-described prior art shown in Figs. 8 and 9 as auto focussing technique in an optical scanning device using the light-emitting element array such as, for example, an auto focussing technology for printing, the prior art has a problem in that the auto focussing function would be disabled when a laser beam is off or there is no write signal, and has therefore been unapplicable to the auto focussing technology for the optical scanning device using the light-emitting element array.
It can be assumed to use an LED array as a light source for recording and the LEDs which have different wavelengths for auto focussing, by utilizing the art disclosed in the above Tokyo Kokai No. 2-304515. In this case, however, the problem is how to align the LED elements.
The above fixed type focussing dependent on the mechanical accuracy is troublesome because the distance between a surface to be scanned and the scanning light-emitting elements requires readjustment each time the position of the photosensitive drum or the sensitized paper is deviated.
In view of the above problems in the prior art, an object of the present invention is, in the optical scanning device using the light emitting element array, to make it unnecessary to align the elements, and to allow simple and accurate focussing, by integrally forming the auto focussing light source element, by which the distance between the object to be scanned and the light emitting element array for scanning can be automatically adjusted together, with the LED array, together on the same chip.
To achieve the above-described object, the present invention provides an optical scanning device in which a light-emitting element array and an auto focussing light source element for adjusting a distance between the light-emitting element array and an object to be scanned by the light-emitting element array are integrated on a single chip of a semiconductor substrate.
According to a mode of the present invention, the semiconductor substrate is a compound semiconductor substrate, the light-emitting element array is an LED array which is formed by a P-N junction in a semiconductor which is provided on the semiconductor substrate and has a composition different from that of the semiconductor substrate, and the auto focussing light source element is formed by a P-N junction on the compound semiconductor substrate. In addition, it is preferable that the semiconductor substrate is a GaAs substrate, the LED array is an AIGaAs LED array formed on the GaAs substrate, and the auto focussing light source element is a GaAs LED formed on a partly cutaway portion of an AIGaAs LED array formed on the GaAs substrate. The optical scanning device is preferably a printer head for recording information on the object to be scanned. In this case, the wavelength of the light from the LED array is in the region where a photosensitive material used in printing is sensitive and the wavelength of the light from the auto focussing light source element is in the region where the printing photosensitive material is not sensitive.
According to another mode of the present invention, the semiconductor substrate is a compound semiconductor substrate, the light-emitting element array is a laser array formed by a P-N junction in a semiconductor having a composition different from that of the semiconductor substrate and the auto focussing light source element is formed by a P-N junction on the compound semiconductor substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram showing the construction of the LED printer according to an embodiment of the present invention;
Fig. 2 is an explanatory diagram of the wavelength of the focussing LED according to an embodiment of the present invention;
Fig. 3 is a cross sectional view of the LED array chip according to an embodiment of the present invention;
Fig. 4 is a cross sectional view of the LED array chip according to another embodiment of the present invention;
Fig. 5 is a cross sectional view of the LED array chip according to another embodiment of the present invention;
Fig. 6 is a cross sectional view of the LED array chip according to another embodiment of the present invention;
Fig. 7 is a cross sectional view of the LED array chip according to another embodiment of the present invention;
Fig. 8 is an explanatory diagram of the auto focussing technology in conventional optical disc recording; and
Fig. 9 is an explanatory diagram of the conventional astigmatism method.

DETAILED DESCRIPTION OF THE INVENTION

In the optical scanning device using the light-emitting element array, alignment of the LEDs can be made unnecessary, and simple and accurate focussing is enabled by integrally forming the auto focusing light source element, for which the distance between the object to be scanned and the light-emitting element array for scanning can be automatically adjusted, and the LED array is together on the same chip.
The following describes the LED printer as an embodiment but the present invention is not limited to this embodiment and is applicable to an optical scanning device such as a laser printer, scanner, and so fourth.
Fig. 1 is a construction of the LED printer according to the embodiment of the present invention. In Fig. 1, 1 is the LED array chip, 2 is the LED array emitting light having a wavelength lambda 1, 3 is the focussing LED emitting light having a wavelength lambda 2, 4 is the focal lens, 5 is the photosensitive surface of the object to be scanned, 6 is the lens, 7 is the photo detector, and 8 is the position control driver for the focal lens. In the embodiment according to the present invention, the focussing LED 3 is formed on the LED array chip 1 is an integrated structure. Integration of the focussing LED 3 and the LED 2 on the same chip makes it unnecessary to align elements of the LED array 2 and LED 3 so that simple and accurate focussing becomes possible. For reducing the size of the LED array 2 of 480 dpi (Dots per inch) shown in Fig. 1 to 1/5 by means of a high resolution focal lens 4 and printing with the density of 2400 dpi, the focal position should be controlled to be within ± pm from the photosensitive surface since the focal depth of the focal lens is small.
Fig. 2 is a diagram showing the relationship of the LED array 2 for recording, the focussing LED 3 and the photosensitive material with respect to the wavelength. As shown in Fig. 2, the wavelength X2 of a light from the LED 3 is selected to be out of the range of the sensitivity curve of the material of photosensitive surface 5, and therefore, the recording ton the photosensitive surface 5 is not carried out by a light from the photosensitive surface 5 and converged by the lens 6 to reach the photo detector 7, and the position control driver 8 controls the position of the focal lens 4 according to the output of the photo detector 7 to align the focus of the LED array 2 with the photosensitive surface 5. As this auto focussing method, the above described conventional astigmatism method, the method for minimizing the spot size disclosed on Tokyo Kokai 1990-304515, and so forth, can be used.
The LED array 2 according to the embodiment of the present invention is formed on the semiconductor substrate and the semiconductor which epitaxially grows thereon and has a different composition, and the focussing LED 3 is formed at the semiconductor portion of the substrate. The light-emitting part of the existing LED array 2 is formed by m, for example, epitaxially growing GaAsP or AIGaAs, the GaAs substrate and forming a P-N junction. The light emitting wavelength is generally 660-870 nm with AIGaAs and 660-720 nm with GaAs. Accordingly, the auto focussing function can be provided independent of printing by selecting the wavelength λ1 for light emission from the GaAs substrate and recording at a position fully away from the wavelength λ2 for focussing and by selecting a photosensitive material which is not sensitive to the wavelength X2.
Fig. 3 to 6 are respectively cross sectional views of chips each being formed by integrating the recording LED array 2 and the focussing LED 3 according to the embodiment of the present invention.
Commonly in Figs. 3 to 6, 10 and 20 are GaAs substrate, 30 is the insulation layer, and 40 and 50 are the epitaxial layer, the GaAs substrate 20 is an inversion portion of the conductive type of GaAs substrate 10, and the epitaxial layer 50 is an inversion portion of the conductive type of epitaxial layer 40. The boundaries between 10 and 20 and between 40 and 50 are the P-N junction positions respectively. The P-N junction positions are the light-emitting portions from which the light is emitted perpendicularly to the substrate. The wavelength X2 for focussing is obtained from the junction 10-20, and the wavelength AI for recording is obtained from the junction 40-50.
If an n type GaAs is used as the substrate, 10 and 40 are the n type and 20 and 50 are the P type, whereas, if a p type GaAs is used as the substrate, 10 and 40 are the p type and 20 and 50 are the n type.
As a method for forming the LED array part, there is a method for diffusing impurities into the epitaxial layer40 through a mask as shown in Fig. 3, or a method for forming a P-N junction during epitaxial growth and separating elements by etching as shown in Fig. 5. As a method for forming the focussing LED 3, there is a method for exposing the GaAs substrate by partial etching and simultaneously forming the LED 3 in the impurity diffusing process for the recording LED 2 as shown in Fig. 3 or a method for deep diffusion reaching the substrate only for the focussing LED 3 as shown in Fig. 4.
The focussing LED is not limited to one and can be provided at both ends of the LED array chip as shown in Fig. 6. Such provision allows correction of planar position deviation. The thickness of the epitaxial layer is usually a few microns and there is no problem in focussing.
The P-N junction of the epitaxial layer is available as a homo junction, single hereto junction or a double hereto junction, and the double hereto junction is particularly preferable because it prevents leaking emission from the GaAs substrate.
The following describes in further detail the embodiments of the present invention.
As the substrate 10 shown in Fig. 3, an n-type GaAs is used as an example; and as the epitaxial layer40, n-type GaAs is also used as an example. If, for example, A1_xGa_1-x. As is used for the epitaxial layer 40, and if the AI composition of the light emitting part is 20%, then λ1 = 740 nm is obtained and the wavelength difference of 130 nm from the light emission wavelength X2 = 870 nm of GaAs, which is the focussing LED, can be obtained. The p-type layers of the inversion GaAs substrate 20 and the inversion epitaxial layer 50 are formed by thermal diffusion of impurities such as Zn or ion implantation with the insulation layer 30 (Si0₂ etc.) as a mask.
In Fig. 4, two diffusion processes for forming a deep p-type region 20 are required instead of the etching process.
In Fig. 5, separation of elements and etching for exposing GaAs can be simultaneously carried out.
Light emission of 550 to 670 nm is enabled by sung AIGaAsP for the epitaxial layer instead of using AIGaAs or GaAs, and light emission of 470 nm is achieved by using ZnSe. These materials are those which can grow on the GaAs substrate and the present invention can be implemented with these materials.
Fig. 7 is a cross sectional view showing a laserar- ray and a focussing LED which are integrated on the same chip according to another embodiment of the present invention. In Fig. 7, 11 is an n-type GaAs substrate, 21 is a p-type GaAs layer (a layer for which n is inverted to p by diffusing impurities as in the example described above), 31 is an insulation layer, 41 is a AIGaAs layer, 51 is a p-type AIGaAs layer and 61 is an AIGaAs layer, 51 is a p-type A1 GaAs layer and 61 is an A1 GaAs active layer (having a different composition from 41 and 51).
The difference between the laser array shown in Fig. 7 and the LED arrays shown in Figs. 3 to 6 is that, in Fig. 7 since the laser beam is emitted from the end face of the substrate, the focussing LED also uses a light from the end face of the substrate. As in the case of the laser array, the light emission wavelength from the laser active layer 61 can be selected in a region away from the light emission wavelength from the GaAs LED owing to the composition of A1_xGa_1-xAs.
As is apparent from the above description, according to the present invention, in an optical scanning device using an optical element array, by integrating an auto focussing light source element capable of automatically adjusting the distance between the object to be scanned and the scanning optical element on the same chip, alignment of the elements becomes unnecessary, and a simple and accurate focussing becomes possible.

Claims

1. An optical scanning device characterized in that a light emitting element array (2) and an auto focussing light source element (3) for adjusting a distance between said light emitting element array and an object (5) to be scanned be said light emitting element (2) array are integrated on a single chip (1) of a semiconductor substrate.

2. An optical scanning device in accordance with Claim 1, wherein said semiconductor substrate (10) is a compound semiconductor substrate, said light emitting element array is an LED array formed by a P-N junction in a semiconductor which is provided on said semiconductor substrate and has a composition different from that of said semiconductor substrate, and said auto focussing light source element is formed by a P-N junction on said compound semiconductor substrate.

3. An optical scanning device in accordance with Claim 2, wherein said compound semiconductor substrate is a GaAs substrate, said LED array is an GaAsP LED array formed on said GaAs substrate, and said auto focussing light source element is a GaAs LED formed on a partly cutaway portion of the GaAsP LED array formed on said GaAs substrate.

4. An optical scanning device in accordance with Claim 2, wherein said compound semiconductor substrate is a GaAs substrate, said LED array is an AIGalnP LED array formed on said GaAs substrate, and said auto focussing light source element is a GaAs LED formed on a partly cutaway portion of the AIGalnP LED array formed on said GaAs substrate.

5. An optical scanning device in accordance with one of Claims 1 to 4, wherein said optical scanning device is a printer head for recording information on said object to be scanned.

6. An optical scanning device in accordance with claim 5 wherein said light-emitting element array emits light having a wavelength within the range of the sensitivity curve of a photosensitive material, and said auto-focussing light source element emits light having a wavelength outside said range.

7. An optical scanning device in accordance with Claim 1, wherein said semiconductor substrate is a compound semiconductor substrate, said light emitting element array is a laser array which is formed by a P-N junction in a semiconductor, which is provided on said semiconductor substrate and has a composition different from that of said semiconductor substrate, and said auto focussing light source element is formed by a P-Njunction on said compound semiconductor substrate.

8. An optical scanning device in accordance with Claim 7, wherein said semiconductor substrate is a GaAs substrate, said light emitting element array is an AIGaAs laser array formed on said GaAs substrate, and said auto focussing light source element is a GaAs LED formed on a partly cutaway portion of the AlGaAs laser array formed on said GaAs substrate.

9. An optical scanning device in accordance with Claim 7 or 8, wherein said optical scanning device is a head printer for recording information on said object to be scanned.

10. An optical scanning device in accordance with Claim 9, wherein said light-emitting element array is a light-emitting element array for printing, and the wavelength of the light from said auto focussing light source element is outside of the range of the sensitivity curve for said photosensitive material.