JP2009222754A - Electrostatic latent image measuring device, electrostatic latent image measuring method, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrostatic latent image measuring device and an electrostatic latent image measuring method which can measure size of an electrostatic latent image with high accuracy by measuring the coordinate of a sample, with respect to the change of an observation area by electrification, without damaging the sample, and to provide an image forming apparatus having an image carrier measured by using the device and method. <P>SOLUTION: The device includes a means which irradiates an electron beam and charges a photoconductor sample 30, an exposure optical system which forms the electrostatic latent image, and a scanning means which scans the surface of a sample by the electron beam and measures an electrostatic latent image distribution on the surface of the sample 30 by a detection signal obtained by the scanning. The outside of the electron beam scan area of the sample is irradiated with the luminous flux from a light source by using a semiconductor laser 21 as the light source of the exposure optical system 20. The device includes a shutter 39 for shielding offset light emission by the bias current of the semiconductor laser. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrostatic latent image measuring device, an electrostatic latent image measuring method, and an image forming apparatus. As an electrostatic latent image measuring device in an electrophotographic process, a high-quality image can be obtained. An image forming apparatus.

An object of the present invention is to provide an apparatus and a measuring method for measuring a charge distribution or a potential distribution generated on a surface of a dielectric material with a high resolution on the order of microns, which has been extremely difficult in the prior art. An object of the present invention is to provide an apparatus and method for measuring an electrostatic latent image on a photoconductor exposed by an optical system, and to provide an image forming apparatus capable of obtaining a high-quality image.
Strictly speaking, it is well known that charges are spatially scattered in the sample. For this reason, the surface charge described here refers to a state in which the charge distribution state is largely distributed in the in-plane direction compared to the thickness direction. Moreover, the concept of electric charge includes not only electrons but also ions.
Further, there may be a state in which there is a conductive portion on the surface and a potential distribution is generated on the surface of the sample or its vicinity by applying a voltage to the conductive portion.

Conventionally, as a method for measuring the surface potential of a photoconductor or the like, the sensor head is brought close to a sample having a potential distribution, and the electrostatic attraction or induced current that occurs as an interaction at that time is measured and converted into a potential distribution. There is a method to do. In this method, the resolution is as low as several millimeters in principle, and a resolution of 1 micron cannot be obtained.
Further, as an evaluation of an LSI chip, a method of measuring an electric potential on the order of 1 micron using an electron beam is conventionally known. However, the evaluation is for the conductive part through which the electricity of the LSI flows. The potential is a low potential of about +5 V at the most, and the potential is limited, and the negative voltage of several hundred to several thousand V, which is the measurement object of the present invention. It cannot handle charges.

  As an observation method of an electrostatic latent image using an electron beam, there is a method described in Patent Document 1. In the conventional method for observing an electrostatic latent image, the sample is limited to an LSI chip or a sample that can store and hold an electrostatic latent image. That is, an electrostatic latent image formed on a normal photoconductor that causes dark decay cannot be measured. Since a normal dielectric can hold a charge semipermanently, even if measurement is performed over time after forming a charge distribution, the measurement result is not affected. However, in the case of a photoconductor, since the resistance value is not infinite, the charge cannot be held for a long time, dark decay occurs, and the surface potential decreases with time. The time that the photoconductor can hold the charge is at most several tens of seconds even in the dark room. Therefore, even if an attempt is made to observe in an electron microscope (SEM) after charging and exposure, the electrostatic latent image disappears at the preparation stage.

  Further, in the invention described in Patent Document 2, it is impossible to form a latent image having an arbitrary line pattern, a desired beam diameter, and a beam profile, in addition to the use wavelength being completely different. Cannot achieve the goal.

Therefore, we have devised a method for measuring an electrostatic latent image even for a photoreceptor sample having dark decay (see, for example, Patent Documents 3 to 5).
If there is a charge distribution on the sample surface, an electric field distribution corresponding to the surface charge distribution is formed in the space. For this reason, the secondary electrons generated by the incident electrons are pushed back by this electric field, and the amount reaching the detector is reduced. Therefore, a portion where the electric field intensity is strong is dark and a weak portion is bright and contrasted, and a contrast image corresponding to the surface charge distribution can be detected. Therefore, when exposed, the exposed portion is black and the non-exposed portion is white, and the electrostatic latent image formed thereby can be measured.

By the way, as an exposure light source for forming an electrostatic latent image, a semiconductor laser whose wavelength is in a visible light region to an infrared light region (hereinafter sometimes referred to as “LD”) is used. There is a method of forming an image.
Semiconductor lasers oscillate by providing a drive current that exceeds the reference, but in order to improve the light response, a constant drive current below the reference (this is called the “bias current”) is always applied even when the light is turned off. Supply. When there is a bias current, LED emission occurs. LED light emission means that light is emitted even when the semiconductor laser is used, even when it is turned off. Since the light quantity at this time is weak, it does not affect the electrostatic latent image when the irradiation time is short. However, when the light is irradiated for a long time, the integrated light quantity increases, and when the required exposure amount of the photoreceptor is reached, an electrostatic latent image is formed. As a result, a desired electrostatic latent image cannot be formed.
Therefore, in order to form a desired electrostatic latent image using a semiconductor laser, it is necessary to minimize the irradiation time to a sample that emits light with a bias current at the time of extinction.

By the way, when the sample is charged, the scanning region changes. It was found that the electric field in the device changed due to the charge on the sample surface, and the trajectory of the scanning electrons was bent. For this reason, when data is captured as an image under the same conditions as in the non-charging state, a result slightly different from the actual size is obtained. Therefore, it is necessary to correctly measure the coordinates of the sample from the captured image data.
The amount of change in the scanning region cannot be predicted in advance because various conditions such as the charging potential and the sample height change.

  In a conventional method for measuring an electrostatic latent image, it is known to provide a standard sample whose dimensions are known from the beginning, such as protrusions and holes, and to use this as a reference, and to correct image data based on this reference. However, the standard material is usually a conductor sample and cannot be charged. An insulating sample can be charged, but a desired charging potential cannot be applied to a desired region as a reference, and the charge cannot be removed. There is a photoreceptor sample as a sample from which electric charge can be removed, but it is easily affected by electrostatic fatigue and light fatigue, and the charged state changes and cannot be a standard sample. A method that uses a known protrusion or scratch on a measurement sample as a reference is not appropriate because it damages the sample.

Japanese Patent Laid-Open No. 03-049143 Japanese Patent Laid-Open No. 3-200100 Japanese Patent Application No. 2002-103355 Japanese Patent Application No. 2003-43587 Japanese Patent Application No. 2007-070836

  In view of the problems of the prior art described above, the present invention measures the electrostatic latent image by measuring the coordinates of the sample against the change of the observation area due to charging without damaging the sample. To provide an electrostatic latent image measuring apparatus capable of measuring dimensions with high accuracy, an electrostatic latent image measuring method, and an image forming apparatus having an image carrier measured using these apparatuses and methods. Objective.

  The present invention provides a means for generating a charged charge on a photoconductor sample by irradiating the sample with an electron beam, an exposure optical system for forming an electrostatic latent image, and scanning the sample surface with an electron beam. And an electrostatic latent image measuring device for measuring the electrostatic latent image distribution on the sample surface by a detection signal obtained by the scanning, using a semiconductor laser as the light source of the exposure optical system, The present invention is characterized in that the light beam is irradiated outside the electron beam scanning region of the sample.

  The present invention also provides means for generating a charged charge on a photoreceptor sample by irradiating the sample with an electron beam, an exposure optical system for forming an electrostatic latent image, and scanning the sample surface with an electron beam. And a scanning device that measures the electrostatic latent image distribution on the sample surface based on a detection signal obtained by the scanning, and uses a semiconductor laser as a light source of the exposure optical system, It has a shutter means for shielding offset light emission due to the bias current of the semiconductor laser.

In the above invention, it is preferable to have means for opening the shutter in conjunction with the synchronization signal of the scanning optical system.
There may be provided means for opening the shutter using the synchronization detection signal of the scanning optical system as a trigger signal and closing the shutter after forming the electrostatic latent image.
When Td is the time when the shutter starts to open after the trigger signal is detected, and Tr is the time from when the shutter starts to open until the equivalent of the effective diameter of the laser beam opens, Td + Tr There may be provided means for turning on the semiconductor laser with a time delay.

When the scanning time of one time by the scanning optical system is Tf, the offset light emission due to the bias current when the semiconductor laser is turned off is Poff, and the light quantity when the semiconductor laser is turned on is Pon,
Tr <Tf * Pon / Poff
It is even better to configure so as to satisfy the above condition.
A mechanical shutter may be used as the shutter means.
In order to suppress electromagnetic field fluctuations, it is preferable that the shutter means be disposed outside the vacuum chamber.
There may be provided means for measuring under the condition that there exists a region where the velocity vector in the sample vertical direction of incident charged particles is inverted.

  The present invention also generates a charged charge on the sample by irradiating the sample with charged particles, and forms an electrostatic latent image on the sample on which the charged charge is generated using an exposure optical system using a laser diode light source. In the method of measuring an electrostatic latent image on a photoconductor, the sample surface is scanned with an electron beam, and the electrostatic latent image distribution on the sample surface is measured by a detection signal obtained by the scanning. Before and after the time for forming an image, the light source is shielded so as not to reach the observation region of the sample, thereby suppressing offset light emission due to the bias current of the semiconductor laser.

  According to another aspect of the present invention, there is provided an image forming method in which a latent image is formed by performing optical scanning on a photosensitive surface of a latent image carrier measured using the measuring apparatus or the measuring method, and is developed and visualized. The apparatus has a writing light source wavelength of 780 nm or less, a beam spot diameter in the sub-scanning direction on the surface of the photoconductor is 60 μm or less, and a beam spot diameter in the sub-scanning direction on the photoconductor surface is A. When the latent image diameter in the sub-scanning direction is B, 1.0 <B / A <2.0 is satisfied.

  The semi-invention also provides means for generating a charged charge on a photoconductor sample by irradiating the sample with an electron beam, an exposure optical system for forming an electrostatic latent image, and scanning the sample surface with an electron beam. In the electrostatic latent image measuring device that measures the electrostatic latent image distribution on the sample surface based on the detection signal obtained by this scanning, the dimension on the photosensitive member sample surface is known by irradiating light with a wavelength of 400 to 800 nm. And a means for capturing a latent image obtained as an electrostatic latent image, and a means for measuring the coordinates of the sample.

The apparatus for measuring an electrostatic latent image may include means for projecting and exposing a mask pattern having a known dimension as a means for forming a latent image pattern having a known dimension.
Two or more latent image patterns with known intervals on the sample surface may be formed, and the average magnification may be measured from the captured image data.
Three or more latent image patterns having a known pitch on the sample surface may be formed in a row, and local magnification fluctuations may be measured from the captured image data.
The light beam from the light source is deflected at an equal angular velocity by an optical deflector having a deflecting reflection surface, and the deflected light beam is condensed as a light spot on the surface to be scanned by the scanning imaging optical system, and is applied to the surface of the photosensitive member sample. A latent image pattern with a known interval is formed using an optical scanning device that performs constant-speed optical scanning, and the coordinates of the sample are measured from the data obtained by capturing the latent image obtained as an electrostatic latent image. You may comprise.
It is preferable that the exposure energy density to be irradiated is 0.5 to 10 mJ / m 2 and the size of one latent image is 100 μm or less.
There may be provided means for measuring under the condition that there exists a region where the velocity vector in the sample vertical direction of incident charged particles is inverted.

The present invention also provides means for generating a charged charge on a photoreceptor sample by irradiating the sample with an electron beam, an exposure optical system for forming an electrostatic latent image, and scanning the sample surface with an electron beam. In the electrostatic latent image measurement method for measuring the electrostatic latent image distribution on the sample surface by the detection signal obtained by this scanning, the dimensions are known by irradiating light with a wavelength of 400 to 800 nm on the photosensitive sample surface. The latent image pattern is formed, and the coordinates of the sample are measured from data obtained by capturing the latent image obtained as an electrostatic latent image.
In the above invention, it is more preferable to have means for opening the shutter in conjunction with the pattern lighting of the semiconductor laser.

  The present invention also provides an image formation in which a latent image is formed by performing optical scanning on a photosensitive surface of a latent image carrier measured using the above-described electrostatic latent image measurement apparatus and measurement method, and developed and visualized. The apparatus has a writing light source wavelength of 780 nm or less, a beam spot diameter in the sub-scanning direction on the surface of the photoconductor is 60 μm or less, and a beam spot diameter in the sub-scanning direction on the photoconductor surface is A. When the latent image diameter in the sub-scanning direction is B, 1.0 <B / A <2.0 is satisfied.

  According to the present invention, the semiconductor laser is used as the light source of the exposure optical system and the light beam is irradiated outside the electron beam scanning region of the sample, so that offset light emission due to the bias current of the LD can be shielded. .

According to the device having the shutter means for shielding the offset light emission due to the LD bias current, a desired electrostatic latent image can be formed, and as a result, the electrostatic latent image is measured with a high resolution on the order of microns. It becomes possible.
According to the apparatus having the means for opening the shutter in conjunction with the synchronization signal of the scanning optical system, it can be carried out at the optimum timing of the time when the shutter should be opened. It becomes possible to measure with high resolution.

Since the shutter is opened by using the synchronization detection signal of the scanning optical system as a trigger signal and the shutter is closed after the electrostatic latent image is formed, the shutter opening time can be suppressed to the minimum necessary. An electrostatic latent image can be formed. As a result, the electrostatic latent image can be measured with high resolution on the order of microns.
By optimizing the time from the opening of the shutter to the start of exposure by the LD, a desired electrostatic latent image can be formed. As a result, the electrostatic latent image can be measured with high resolution on the order of microns. it can.
By using a mechanical shutter as the shutter means, offset light emission can be shielded at high speed without deteriorating the transmitted wavefront of the laser light.
By disposing the shutter means outside the vacuum chamber in order to suppress the electromagnetic field fluctuation, it is possible to suppress the trajectory bending of the scanning electron beam due to the fluctuation of the surrounding electromagnetic field.

By having a means to measure under conditions where there is a region where the velocity vector in the sample vertical direction of the incident charged particles inverts, the potential depth can be quantified and the potential distribution can be measured with high accuracy. It becomes possible.
By using an LD as the light source of the exposure optical system and having shutter means for shielding offset light emission due to the bias current of the LD, a desired electrostatic latent image can be formed. As a result, the electrostatic latent image can be formed. Can be measured with high resolution on the order of microns.

By evaluating the electrostatic latent image formed on the photosensitive member using the measuring apparatus and the measuring method described above, it is possible to feed back to the design and the process quality of each process is improved. Accordingly, it is possible to provide a latent image carrier and a scanning optical system that are excellent in image quality, durability, stability, and energy saving. Further, according to the image forming apparatus including the latent image carrier and the scanning optical system, the electrostatic latent image formed on the image carrier is developed and visualized, thereby achieving high density, high image quality, and high durability. An image forming apparatus can be provided.
It is particularly suitable for an image forming apparatus equipped with a multi-beam scanning optical system such as a VCSEL, in which uneven image density is likely to occur.

  According to the present invention, a latent image pattern with a known interval is formed on the sample surface, and this is captured as image data, so that the observation region is not changed due to charging without damaging the sample. By measuring the coordinates of the sample, the dimension of the electrostatic latent image can be measured with high accuracy.

As a means for forming a latent image pattern with a known dimension, a means for projecting and exposing a mask pattern with a known dimension can be used, whereby the dimension of the electrostatic latent image can be measured with high accuracy.
By forming two or more latent image patterns with known intervals on the sample surface and calculating the center position from the captured image data, the average magnification can be measured, and by correcting, the electrostatic latent image Can be measured with high accuracy.
By forming three or more latent image patterns with a known pitch on the sample surface in a row, local magnification fluctuations can be corrected, and local fluctuations due to uneven charging can be corrected well. Can

By using an optical scanning device that performs constant-speed optical scanning on the surface of the photoconductor sample, it is easy to form an arbitrary latent image pattern simply by changing an electrical signal. It is also possible to use the exposure optical system to be evaluated in common.
When the exposure energy density to be irradiated is 0.5 to 10 mJ / m 2 and the size of one latent image is 10 μm or more and 100 μm or less, the calculation of the center position from the latent image pattern can be measured with high accuracy.
By using an LD as the light source of the exposure optical system and having shutter means for shielding offset light emission due to the bias current of the LD, a desired electrostatic latent image can be formed. As a result, the electrostatic latent image can be formed. Can be measured with high resolution on the order of microns.
By measuring the coordinates of the sample with respect to the change in the observation region due to charging without damaging the sample, the dimension of the electrostatic latent image can be measured with high accuracy.

  Embodiments of an electrostatic latent image measuring device, an electrostatic latent image measuring method, and an image forming apparatus according to the present invention will be described below with reference to the drawings.

  FIG. 1 shows an embodiment of the present invention. This embodiment includes a charged particle irradiation unit that irradiates a charged particle beam, an exposure unit, a sample setting unit, a detection unit for primary inverted charged particles, secondary electrons, and the like. Here, the charged particles refer to particles that are affected by an electric field or a magnetic field, such as an electron beam or an ion beam. Hereinafter, an embodiment in which an electron beam is irradiated will be described.

  In FIG. 1, an electron beam irradiation unit 10 includes an electron gun 11 for generating an electron beam, a suppressor electrode 121 for controlling the electron beam, an extraction electrode 122, and an acceleration for controlling the energy of the electron beam. The electrode 123, the condenser lens 13 for focusing the electron beam emitted from the electron gun 11, the movable aperture 15, the eccentric lens for changing the traveling direction of the electron beam, and the electron beam that has passed through the eccentric lens are scanned. And an objective lens 18 for condensing the electron beam that has passed through the scanning lens 17 again. A driving power source (not shown) is connected to each lens.

  A scintillator, a photomultiplier tube, or the like is used as the detector 19 for detecting secondary electrons, primary inversion electrons, or the like. Usually, the scintillator is configured to capture charged particles by applying a high voltage of about 8 to 10 kV. In order to introduce the detected charged particles to the detector 19, an emitted particle generating member that generates emitted particles at the time of charged particle collision is disposed.

The photoreceptor sample 30 is placed on the sample grounding table 16. The back side of the sample 30 is normally used for GND through the sample grounding table 16, but is configured to be able to apply a voltage as necessary.
There are electrons and ions as emitted particles, and it is common to detect and measure electrons. However, it is also possible to detect positive ions by applying a negative pull-in voltage to the detector and observe the contrast image. is there.
Each of the above components is incorporated in a vacuum chamber.

The exposure unit 20 includes a light source 21 such as a semiconductor laser (LD) having a wavelength of visible light to infrared light, a collimating lens 22, an aperture 23, a condensing lens 29, and the like having sensitivity with respect to the photoreceptor sample 30. It is possible to generate a desired beam diameter and beam profile. Further, it is possible to irradiate an appropriate exposure energy with an appropriate exposure time by an LD control means (not shown).
In order to form a line pattern on the sample 30, a scanning mechanism using a galvano mirror or a polygon mirror may be attached to the optical system.

  FIG. 10 shows an example of a scanning unit including the exposure unit 20. As shown in FIG. 10A, the light beam emitted from the light source unit 21 including the semiconductor laser is guided to the polygon mirror 35 through the collimator lens 22 and the cylinder lens 25. The light beam is deflected and scanned when the polygon mirror 35 is rotationally driven, and is imaged on the photosensitive medium as the scanning medium or the image carrier, that is, the sample 30 by the scanning imaging lenses L1 and L2 and the folding mirror 36. . Print data for one line corresponding to each light emission point is stored in a buffer memory in the image processing apparatus that controls the light emission signal at each light emission point. The print data is read for each deflection scan on the deflection reflection surface 1 of the polygon mirror 35, the light beam flashes on the scan line on the sample 30 corresponding to the print data, and an electrostatic latent image is formed according to the scan line. It is formed.

In the configuration example shown in FIG. 10B, a semiconductor laser array in which a plurality of light sources are arranged is arranged in the direction perpendicular to the optical axis of the collimating lens.
FIG. 10C shows a configuration example of a light source unit of an optical scanning device including a surface emitting laser (VCSEL) in which light emitting points are arranged in a plane in the x-axis direction and the y-axis direction. In this configuration example, m (3 in the figure) in the horizontal direction (main scanning direction), n (4 in the figure) in the vertical direction (sub-scanning direction), and a total of m × n (12 in the figure). This is an example using a surface emitting laser having a light emitting point. By applying this configuration example to the optical scanning device shown in FIG. 10A, it is possible to configure to scan m × n scanning lines simultaneously.
Further, in order to form a latent image pattern at a predetermined position, a synchronization detection unit 37 that detects a scanning beam from the light deflection unit may be provided.
The shape of the sample 30 may be a flat surface or a curved surface.

The scanning optical system is preferably arranged outside the vacuum chamber so that the influence of the vibration and electromagnetic field of the polygon mirror and the polygon motor that rotationally drives it does not affect the trajectory of the electron beam. By moving away from the position of the electron beam orbit, the influence of disturbance can be suppressed. The scanning optical system is preferably incident from an optically transparent entrance window.
Between the sample 30 and the light source 21, a shutter 39 capable of temporally blocking the light beam from the semiconductor laser is disposed.

  FIG. 2 is an enlarged cross-sectional view showing a part of the present embodiment. As shown in FIG. 2, an incident window through which the light source can enter from the outside is disposed inside the vacuum chamber at a position of 45 ° with respect to the vertical axis of the vacuum chamber, and a scanning optical system is disposed outside the vacuum chamber. It has a configuration. In FIG. 2, the scanning optical system includes a light source unit, scanning lenses L1 and L2, synchronization detecting means, a polygon mirror 35 as an optical deflector, and the like. The scanning lenses L1 and L2 have fθ characteristics, and the light beam moves at a substantially constant speed with respect to the image plane when the optical polarizer rotates at a constant speed. Further, the beam spot diameter can be scanned substantially uniformly.

  Since the scanning optical system is arranged away from the vacuum chamber, the vibration generated when driving an optical deflector such as a polygon scanner is less affected by being directly propagated to the vacuum chamber. Further, although not shown in FIG. 2, if a damper is inserted between the structure and the vibration isolation table, a more effective vibration isolation effect can be obtained.

Use an LD (laser diode) in the visible to infrared range as an exposure light source to form an electrostatic latent image, scan the laser beam with a deflector such as a polygon mirror, and blink the laser beam Thus, an electrostatic latent image can be formed. In that case, a deflector such as the polygon mirror 35 takes about several seconds to rotate at a constant speed. Further, in order to blink at a desired position, the writing start position is determined by the detection signal from the synchronous detector.
When a latent image pattern is formed by flashing light, it is necessary to increase the light response. For example, when modulation is performed at 1 μs or less, it is necessary to always allow a bias current to flow through the LD even when the image data for improving pattern light pulse reproducibility and droop characteristics is extinguished.

  A semiconductor laser oscillates by applying a drive current that is higher than a reference. However, in order to improve the light response, a constant drive current (bias current) that is lower than the reference is always supplied even when the light is turned off. When there is a bias current, LED emission occurs. That is, when a semiconductor laser is used, it means that light is emitted even when the light is off.

FIG. 4 shows the relationship between the LD drive current IF and the optical output. a is a driving current when the light is turned off, b is a reference current at which laser oscillation starts, and c is a driving current for LD lighting. Pon is the light output when the LD is turned on, and Poff is the light output when the LD is turned off. The relationship of a <b <c is established. When the IF is small, the LED emission is weak, and laser oscillation occurs when the IF reaches the reference current. Usually, the laser oscillation is about 1 to 10 mW. Compared with this, the bias current at the time of extinguishing is several tens of μW, which is about 1/100 or less, and this does not usually cause a problem.
For this reason, the bias current is kept flowing even when the LD is extinguished with emphasis on photoresponsiveness.
However, the integrated light quantity increases when irradiated for a long time, and an electrostatic latent image is formed when the required exposure amount of the photoreceptor is reached. As a result, a desired electrostatic latent image cannot be formed.

Therefore, in the present invention, when forming a desired electrostatic latent image using a semiconductor laser, the luminous flux is out of the electron beam scanning area of the sample in order to minimize the irradiation time to the sample that emits light with a bias current at the time of extinction. The offset light emission due to the LD bias current is shielded.
As a specific means, the shutter 39 may be used between the LD light source and the sample. That is, the offset light emission can be shielded by closing the shutter 39 before the exposure so that the light beam does not pass and opening the shutter 39 so that the light beam can pass during the exposure.

As a result, as shown in FIG. 5A, conventionally, the irradiation time of offset light before exposure is long, and a latent image is formed by offset exposure when the integrated light amount reaches the required exposure energy. As shown in FIG. 5B, the irradiation time of the offset light before exposure can be suppressed as much as possible, and the measurement accuracy can be improved.
Further, after the exposure, if necessary, an exposure end detection signal can be given to close the shutter 39.

  Next, means for forming an electrostatic latent image will be described. First, the photosensitive member sample 30 is irradiated with an electron beam. The acceleration voltage | Vacc | is set to an acceleration voltage higher than the acceleration voltage at which the secondary electron emission ratio is 1, so that the amount of incident electrons exceeds the amount of emitted electrons, so that electrons are accumulated in the sample 30 and charge-up is performed. Wake up. As a result, the sample 30 can be negatively charged uniformly. A desired charging potential can be formed by appropriately controlling the acceleration voltage and the irradiation time.

Next, the photoreceptor sample 30 is exposed by the exposure optical system. The optical system is adjusted to form a desired beam diameter and beam profile. The necessary exposure energy is a factor determined by the photoreceptor characteristics, but is usually about 2 to 6 mJ / m2. For photoreceptors with low sensitivity, ten or more mJ / m 2 may be required. The charging potential and the required exposure energy may be set according to the photoreceptor characteristics and process conditions.
A desired electrostatic latent image such as a latent image pattern as shown in FIG. 11 can be formed by arranging the shutter mechanism described above and cutting off the offset light due to the LD bias current.

  If there is a charge distribution on the sample surface, an electric field distribution corresponding to the surface charge distribution is formed in the space. For this reason, the secondary electrons generated by the incident electrons are pushed back by this electric field, and the amount of electrons reaching the detector 19 is reduced. Accordingly, at the charge leak portion, the exposed portion is black and the non-exposed portion is white, and a contrast image corresponding to the surface charge distribution can be measured.

FIG. 3A illustrates the potential distribution in the space between the charged particle trap 24 and the sample 30 in an explanatory manner with contour lines. The surface of the sample 30 is in a state of being uniformly charged negatively except for the portion where the potential is attenuated due to light attenuation, and the charged particle trap 24 is given a positive potential. In the “potential contour line group”, the “potential becomes higher” as it approaches the charged particle trap 24 from the surface of the sample 30.
Therefore, the secondary electrons el1 and el2 generated at the points Q1 and Q2 in FIG. 3, which are “negatively uniformly charged portions” in the sample 30, are attracted to the positive potential of the charged particle trap 24, and the arrows It is displaced as indicated by G1 or arrow G2, and is captured by the charged particle trap 24.

  On the other hand, in FIG. 3A, the point Q3 is “a portion where the negative potential is attenuated by light irradiation”, and the arrangement of potential contour lines is “as shown by the broken line” in the vicinity of the point Q3. “The closer to Q3 point, the higher the potential”. In other words, an electric force restraining the sample 30 is applied to the secondary electrons el3 generated in the vicinity of the point Q3, as indicated by the arrow G3. For this reason, the secondary electron el3 is captured in the “potential hole” indicated by the broken line potential contour and does not move toward the charged particle trap 24. FIG. 3B schematically shows the “potential hole”.

That is, the intensity of the secondary electrons (number of secondary electrons) detected by the charged particle trap 24 is such that the portion with the high intensity is “the ground portion of the electrostatic latent image (the portion that is uniformly negatively charged: FIG. 3). The portion having a low intensity corresponds to “the image portion of the electrostatic latent image (the portion irradiated with light: the point Q3 in FIG. 3A). Part) ”.
Therefore, if the electrical signal obtained by the secondary electron detection unit 25 is sampled at an appropriate sampling time by the signal processing unit, the surface potential distribution: V (X, Y) using the sampling time: T as a parameter as described above. Can be specified for each “small region corresponding to sampling”. The surface potential distribution (potential contrast image): V (X, Y) is configured as two-dimensional image data by a signal processing unit, and this is output by an output device. Can be obtained as

For example, if the intensity of secondary electrons to be captured is expressed as “brightness or weakness”, the image portion of the electrostatic latent image is dark, the ground portion is bright and contrasted, and a bright and dark image corresponding to the surface charge distribution is obtained. It can be expressed (output). Of course, if the surface potential distribution can be known, the surface charge distribution can also be known.
In this way, a bright and dark contrast image is generated with a large amount of secondary electron detection in the charging portion and a small amount of secondary electron detection in the exposure portion. The dark part can be regarded as a latent image part by exposure. The light / dark boundary can be set as the latent image diameter of the latent image.
In this way, it is possible to measure the electrostatic latent image on the photoreceptor with high resolution on the order of microns.

In order to reduce the influence of offset light emission due to the bias current of the LD, the shutter is open for the exposure time for forming an electrostatic latent image, and the shutter is closed before and after that. Ideally. In order to realize this, it is desirable to link the exposure timing and the shutter opening / closing timing. The timing of exposure is determined by the synchronization signal of the scanning optical system. Therefore, it is desirable to open the shutter in conjunction with the synchronization signal of the scanning optical system.
As means for realizing this, if the synchronization signal of the scanning optical system is a trigger signal for opening the shutter, the timing of writing can be made uniform.

  The above operation in the control system is shown in FIGS. Each step of the flow shown in FIG. 8 is expressed as S1, S2,. 6 and 8, after a control command for measurement is executed (S1), a scanning optical system synchronization signal is detected (S2), and the detection signal is output as a trigger signal for opening the shutter ( S3). Then, in accordance with the timing when the shutter is opened to the effective diameter of the exposure optical system (S4), the semiconductor laser is turned on (S5), and an electrostatic latent image is formed on the sample. Then, after the exposure is completed, the shutter is closed. After exposure, in addition to closing the shutter, the LD bias current may be set to 0 to stop the light emission itself.

  FIG. 7 shows the configuration of the control system of the embodiment described above. This computer is configured so as to control each part around the host computer. Specifically, an electron optical system control signal is sent from the host computer to the electron beam control device, and various data taken from the electron beam control device is input to the computer. The computer also transmits scanning beam condition setting data to the control board, and the control board performs various condition settings. An LD / polygon mirror control signal and a synchronization detection signal are input / output between the control board and the optical unit. The control board also sends a trigger signal to the shutter control controller, and the shutter control controller sends a shutter open / close signal to the shutter based on the trigger signal to control the opening / closing of the shutter.

By the way, in both the electronic shutter and the mechanical shutter, there is a time lag between when the command is given and when the shutter is actually opened.
Now
The time when the shutter starts to open after detecting the trigger signal is Td,
The time from when the shutter starts to open until the equivalent of the effective diameter of the laser beam opens is Tr,
Then, after receiving a synchronization signal as a trigger output, the shutter opens with a delay of Td + Tr. Therefore, the LD must be turned on in consideration of the delay. That is, it is desirable to turn on the LD with a delay of Td + Tr after receiving a synchronization signal serving as a trigger output.

  As a method of delaying the lighting time of the LD, when the single scanning time by the scanning optical system is Tf, the LD is turned on when the natural number n satisfying Td + Tr <n × Tf from the synchronization signal serving as the trigger output. There is a method of forming a latent image. This timing chart is shown in FIG.

According to the embodiment configured as described above, the irradiation time by offset light emission is significantly reduced. However, strictly speaking, offset light emission is irradiated in a time longer than Tr.
The allowable amount of time is Tr <Tf * Pon / Poff
And must be within the range of this equation.

Specifically, when Tf = 250 μs, Pon = 4 mW, and Poff = 20 μW,
Tr <250 μs * 200 = 50 ms
That is, Tr is preferably within 50 ms.
Further, when Tf = 100 μs, Pon = 1 mW, Poff = 50 μW,
Tr <2ms
It becomes. By appropriately selecting conditions and methods that satisfy this condition, it is possible to form an electrostatic latent image with even smaller noise light, and as a result, it is possible to measure the electrostatic latent image with high accuracy. .

As the shutter means, there is a method of changing the optical transmittance like a liquid crystal modulation element. In this case, there is an advantage that no mechanical movable part is required. However, there is a possibility that the light response and the transmitted wavefront will be affected.
A mechanical shutter may be used as the shutter means. The mechanical shutter described here creates a state where there is an object and a state where there is no object with respect to the traveling direction of the optical path. Refers to a means that can. FIG. 9 shows an example. In this example, the slider is configured to slide in a direction orthogonal to the optical path to open and close the optical path.

As described above, the mechanical shutter mechanically controls the opening / closing operation and speed of the shutter using a governor, a spring, or the like. The mechanical shutter includes means for opening and closing from the center to the periphery using a guillotine shutter and a plurality of shutter blades. A guillotine shutter has holes in each of the two plates, and after the plate corresponding to the front curtain travels, the plate corresponding to the rear curtain travels to open and close the hole that is the path of light. is there. The shutter speed can be changed by changing the overlapping state of the holes.
Moreover, although it is mechanical, the structure controllable by an electrical signal may be sufficient. Thereby, it can open and close at a more suitable timing according to the synchronization.
By using a mechanical shutter as the shutter means, offset light emission can be shielded at high speed without deteriorating the transmitted wavefront of the laser light.

  The position of the shutter is more preferably arranged outside the vacuum chamber. When placed inside the vacuum chamber, the surrounding electromagnetic field fluctuates when the mechanical shutter is opened and closed and before and after that, which may bend the trajectory of the scanning electron beam. Can be suppressed.

  By measuring the profile of the surface charge distribution and the surface potential distribution, it is possible to measure the electrostatic latent image with higher accuracy. FIG. 12 is a diagram showing another embodiment of the apparatus for measuring an electrostatic latent image according to the present invention. More specifically, it is an example of a surface potential distribution measuring apparatus. In FIG. 12, the sample mounting table 16 below the sample 30 is connected to a voltage application unit that can apply a voltage ± Vsub. In addition, the upper portion of the sample 30 has a configuration in which a grid mesh 38 is disposed in order to suppress the incident electron beam from being affected by the sample charge.

FIG. 13 shows the relationship between the incident electrons and the sample in the embodiment shown in FIG. 13A shows a case where the acceleration voltage is larger than the surface potential potential, and FIG. 13B shows a case where the acceleration voltage is smaller than the surface potential potential. There is a region where a state where the velocity vector of the incident charged particles in the sample vertical direction is reversed before reaching the sample, and the primary incident charged particles are detected.
The acceleration voltage is generally expressed as positive, but the applied voltage Vacc of the acceleration voltage is negative, and in order to have a physical meaning as a potential potential, it is more preferable to express it as negative. Here, the acceleration voltage is expressed as negative (Vacc <0).
The acceleration potential of the electron beam is Vacc (<0), and the potential potential of the sample is Vp (<0).

A potential is an electrical potential energy possessed by a unit charge. Therefore, the incident electrons move at a speed corresponding to the acceleration voltage Vacc at the potential 0 (V). That is, assuming that the charge amount of electrons is e and the mass of electrons is m, the initial velocity of electrons v0 is
mv02 / 2 = e × | Vacc |
It is represented by In vacuum, due to the law of conservation of energy, it moves at a constant speed in the region where the acceleration voltage does not work, and as it approaches the sample surface, the potential increases, and the velocity decreases due to the influence of Coulomb repulsion of the sample charge.

Therefore, the following phenomenon generally occurs.
In FIG. 13A, since | Vacc | ≧ | Vp |, the electron reaches the sample although the speed is reduced.
In FIG. 13B, in the case of | Vacc | <| Vp |, the velocity of the incident electrons is gradually decelerated by the influence of the potential potential of the sample, and the velocity becomes zero before reaching the sample. And go in the opposite direction.

  In a vacuum without air resistance, the energy conservation law is almost completely established. Therefore, the surface potential can be measured by measuring a condition in which the energy on the sample surface, that is, the landing energy when the energy of the incident electrons is changed, is almost zero. Here, primary inversion charged particles, particularly electrons, are referred to as primary inversion electrons. The secondary electrons generated when the sample reaches the sample and the primary inversion charged particles differ greatly in the amount reaching the detector, so that they can be identified from the boundary of contrast between light and dark.

A scanning electron microscope or the like has a backscattered electron detector. In this case, the backscattered electrons are generally reflected (scattered) on the rear back surface due to the interaction with the material of the sample, and the sample. It refers to the electrons that jump out of the surface. The energy of the reflected electrons is comparable to the energy of the incident electrons. It is said that the intensity of the reflected electrons increases as the atomic number of the sample increases, and this is a detection method for understanding the difference in composition of the sample and unevenness.
On the other hand, primary inversion electrons are electrons that are affected by the potential distribution on the sample surface and reverse before reaching the sample surface, and are completely different phenomena.

  FIG. 14 is a diagram illustrating an example of a latent image depth measurement result. If the difference between the acceleration voltage Vacc and the sample lower applied voltage Vsub is Vth (= Vacc−Vsub) at each scanning position (x, y), Vth (x, y) when the landing energy becomes almost zero. Can be measured to measure the potential distribution V (x, y). Vth (x, y) has a unique correspondence with the potential distribution V (x, y). If Vth (x, y) is a gentle charge distribution, the potential distribution V (x , Y).

  The upper curve in FIG. 14 shows an example of the surface potential distribution generated by the charge distribution on the sample surface. The acceleration voltage of the electron gun for two-dimensional scanning was set to −1800V. The potential at the center (horizontal axis coordinate = 0) is about -600V, the potential increases in the negative direction as it goes from the center to the outside, and the potential in the peripheral region whose radius exceeds 75 μm from the center is about -850V. ing. The oval shape in the middle of FIG. 14 shows an image of the detector output when the back surface of the sample is set to Vsub = −1150V. At this time, Vth = Vacc−Vsub = −650V. The oval shape in the lower part of FIG. 14 shows an image of the detector output obtained under the same conditions as above except that Vsub = −1100V. At this time, Vth is -700V.

  As can be seen from FIG. 14, the surface potential information of the sample can be measured by scanning the surface of the sample with electrons and measuring the Vth distribution while changing the acceleration voltage Vacc or the applied voltage Vsub. By using this method, it is possible to visualize the electrostatic latent image profile on the order of μ, which has been difficult in the past.

  In the method of measuring the latent image profile with primary inverted electrons, the energy of the incident electrons changes drastically, and the trajectory of the incident electrons may shift. As a result, the scanning magnification changes, and distortion aberration is reduced. Will occur. In that case, the electrostatic field environment and the electron trajectory can be calculated in advance, and correction can be performed based on the calculation, thereby making it possible to measure with higher accuracy.

  Although the total exposure energy density given to the photosensitive member is the same, the photosensitive member has a reciprocity failure phenomenon in which the latent image forming state is different when the relationship between the light amount and the exposure time is different. In general, when the exposure energy is constant, the stronger the light quantity, the lower the sensitivity (latent image depth), causing a change in the toner adhesion amount, resulting in a difference in image density. It is considered that when the amount of light is strong, the amount of recombination of carriers increases and the amount of carriers reaching the surface decreases. In the case of a multi-beam scanning optical system such as a VCSEL, image density unevenness appears remarkably.

  By evaluating the electrostatic latent image on the photosensitive member with the electrostatic latent image measuring apparatus of the present embodiment, it is possible to measure with a resolution of 1 μ order. Can be analyzed in detail. As a result, the exposure amount can be optimized, charging and exposure conditions that do not place a burden on the photosensitive member can be known, and energy saving and high durability of an optical scanning device and an image forming apparatus using the photosensitive member can be realized. it can.

  Attempts to reduce the beam spot diameter in the sub-scanning direction to 60 μm or less by optimizing the optical system and shortening the light source wavelength to 780 nm or less in order to improve the image quality of the output image of the image forming apparatus. It has been broken. However, current photoconductors are less sensitive to short-wavelength light, and small-diameter beams are strongly affected by light scattering and charge diffusion within the photoconductor, which increases the latent image diameter and The depth is also shallow, and the final output image has a problem in that the stability of gradation and sharpness cannot be obtained.

FIG. 15 conceptually shows the beam spot diameter and the latent image diameter. The beam spot diameter here is defined as a diameter in a range where the beam spot light quantity distribution is not less than e ^{−2 of the} maximum light quantity. The latent image diameter is defined as the latent image diameter of the latent image at the boundary of the contrast image. It is known that the composition and thickness of the charge transport layer affect the degree of light scattering and charge diffusion, and the composition of the charge generation layer affects the sensitivity, but no clear correlation is known. Therefore, in the electrostatic latent image measurement method implemented using the electrostatic latent image measurement apparatus of the present embodiment, a photoconductor is produced by changing the composition and thickness of the charge transport layer and the composition of the charge generation layer. A latent image is measured by exposing under the same conditions as those used in the apparatus, for example, with a charging potential of 800 V, exposure energy of 4 mJ / m 2 and a light source wavelength of 780 nm or less and a beam spot diameter in the sub-scanning direction of 60 μm or less.

As shown in FIGS. 15A and 15B, when the beam spot diameter in the sub-scanning direction on the photosensitive member surface is A and the latent image diameter in the sub-scanning direction to be formed is B,
1.0 <B / A <2.0
If a photoconductor satisfying the above is selected, an image forming apparatus capable of obtaining a final output image with stable gradation and sharpness can be realized.
Here, the lower limit of 1.0 is a theoretical limit that light scattering and charge diffusion always occur in any photoconductor, and therefore cannot be less than this, and the upper limit of 2.0 is the final output image. This is the limit necessary to ensure the stability of gradation and sharpness.

  Next, a second embodiment of the electrostatic latent image measuring apparatus according to the present invention will be described. FIG. 16 shows a second embodiment. Since most of the configuration is the same as that of the first embodiment shown in FIG. 1, the same components are denoted by the same reference numerals, the description thereof is omitted or simplified, and portions different from the first embodiment are mainly described. To do.

  If there is a charge distribution on the surface of the photoreceptor sample, an electric field distribution corresponding to the surface charge distribution is formed in the space. For this reason, the secondary electrons generated by the incident electrons are pushed back by this electric field, and the amount reaching the detector is reduced. Therefore, a portion where the electric field intensity is strong is dark and a weak portion is bright and contrasted, and a contrast image corresponding to the surface charge distribution can be detected. Therefore, when exposed, the exposed portion is black and the non-exposed portion is white, and the electrostatic latent image formed thereby can be measured.

When the sample is charged, the incident electrons are affected by the charged charge and the trajectory is bent, and the scanning region is changed. FIG. 18 shows the change of the scanning area in charging. FIG. 18 (a) shows the relationship between the uncharged / charged scanning area. When the uncharged scanning area is used as a reference, the incident electron scanning area when the sample is negatively charged is wide. It turns out that it will scan. Distortion may also occur. As a result, the scanning magnification of the observation region in the cross section x is as shown in FIG. It has been found that the magnification depends on the charging potential and is about 5 to 20% smaller in the charging potential range of −500 to −1000 V than in the non-charging state.
Therefore, if the dimensions are measured from the captured image data with the dimensions at the time of non-charging as they are, an error corresponding to the magnification fluctuation occurs. Therefore, it is necessary to accurately measure the magnification during charging.
In order to measure the actual dimension of the captured image data, a pattern having a known dimension is exposed to form an exposure optical system 50 (see FIG. 16) for coordinate correction for forming a latent image pattern having a known dimension. It is one of the features of this embodiment.

  FIG. 2 shows details of the embodiment of the exposure optical system 50 for coordinate correction. In FIG. 2, the exposure optical system 50 includes a light source 51 such as an LD that irradiates light having a wavelength of 400 to 800 nm with sensitivity, a collimating lens 52, an aperture 53, a mask pattern 54, an imaging lens or a collecting lens. An optical lens 56 is provided. In addition, appropriate mirrors 55 and 57 are arranged to bend the optical path.

The optical layout of the exposure optical system 50 for coordinate correction is shown in FIG. If the object distance from the mask pattern 54 to the imaging lens 56 is L1, and the image distance from the imaging lens to the sample surface is L2, the imaging magnification in the direction perpendicular to the plane including the optical axis is β = L2. / L1
It becomes. The mask pattern 54 has a pattern in which the parallel light from the light source 51 is transmitted and the light is blocked. The light beam is transmitted, diffracted or scattered by the mask pattern 54 and travels in the direction of the sample 30. The imaging lens 56 is disposed so that the mask pattern 54 and the surface of the sample 30 are conjugate. Since the imaging magnification β and the pattern size are known in advance, the pattern size and pitch generated on the surface of the sample 30 can be calculated, and a desired latent image pattern can be formed on the sample surface.

  Since the optical element is irradiated obliquely so as not to overlap the electron beam irradiation region, the sample surface (image surface) may be inclined with respect to the optical axis. Further, by tilting the mask pattern 54 according to the tilt of the image plane, the mask pattern can be imaged on the image plane. In the example shown in FIG. 23, the sample 30 is configured to enter obliquely about 45 degrees with respect to the sample 30. In this case, the latent image distribution pattern is √2 times that in the vertical case, but the mask pattern may be designed in consideration of this.

  FIG. 19 shows an example of a mask pattern for exposure. As the exposure pattern, at least one exposure pattern can be used as long as the dimensions and the imaging magnification are known. However, since the latent image is spread on the sample surface as compared with the conjugate image, two or more points are selected. It is preferable to calculate the distance between the centers of one latent image. In order to measure both xy dimensions, it is preferable that there are three or more. FIG. 19A shows an example in which the mask has a total of four latent image formation patterns S1, S2, S3, and S4. The size / pitch of the mask pattern and the imaging magnification of the optical system are known in advance. When the imaging magnification of the optical system is β and the interval between the mask patterns is d / β, S1 and S2 are exposed at the interval d as coordinates on the sample surface to form a latent image. Under this condition, it is captured as latent image data as shown in FIG.

In order to measure the dimension in the direction connecting S1 and S2 with the patterns of S1 and S2, the center position of the captured image data of the latent image pattern by S1 and S2 is P1 (x1, y1), P2 (x2, y1). Then, the interval per pixel in the horizontal image is d / (x2-x1)
Can be obtained at This makes it possible to accurately measure the interval per pixel in the captured image in the P1-P2 direction.

Similarly, by calculating with P1 and P3, if P3 (x1, y3), the interval per pixel in the vertical direction is d / (y3-y1)
Can be obtained at Thereby, it is possible to measure the interval per pixel of the captured image in the P1-P3 direction.

If 1 mm is captured in the horizontal observation region with the number of pixels equivalent to XGA (1024 × 768 pixels), the position can be specified with a resolution of about 1 μm.
There is a method of calculating the center position from the center of gravity of the signal intensity of the latent image. If this is used, the center position can be calculated with one pixel or less, so that the dimension can be measured with high accuracy.

The latent image pattern may have a parallel line shape as shown in FIG. 19C, or three or more latent image patterns as shown in FIG. May be.
Thereby, the average magnification accompanying charging can be favorably corrected.

  FIG. 6 is a flow of the measurement method. Dimensional position measurement electrostatic latent image pattern formation (S11), electrostatic latent image data capture (S12), image processing (S13), latent image pattern extraction (S14), latent image pattern center position calculation (S15), pixel The measurement is performed in the order of dimension measurement per pixel (S16).

  Next, means for forming an electrostatic latent image in the above embodiment will be described. In FIG. 16, first, a photoconductor sample 30 is irradiated with an electron beam from a charged particle optical system. When the acceleration voltage | Vacc | is set to an acceleration voltage higher than the acceleration voltage at which the secondary electron emission ratio is 1, the amount of incident electrons exceeds the amount of emitted electrons, so that electrons are accumulated in the sample and charge up occurs. . As a result, the sample 30 is uniformly charged with negative charges. A desired charging potential can be formed on the sample 30 by appropriately controlling the acceleration voltage and the irradiation time.

  Next, the photoreceptor sample 30 is exposed by the exposure optical system. The exposure optical system has substantially the same configuration as that of the exposure optical system in the first embodiment, and is adjusted so as to form a desired beam diameter and beam profile. If there is a charge distribution on the sample surface, an electric field distribution corresponding to the surface charge distribution is formed in the space. For this reason, the secondary electrons generated by the incident electrons are pushed back by this electric field, and the amount reaching the detector is reduced. Accordingly, at the charge leak portion, the exposed portion is black and the non-exposed portion is white, and a contrast image corresponding to the surface charge distribution can be measured.

By forming a latent image pattern with a known interval on the surface of the sample 30, the average magnification can be measured. However, as described with reference to FIG. 18, strictly speaking, the magnification slightly changes locally. Also, if there is uneven surface charging, it will naturally vary. The amount is not large compared to the variation of the average magnification, but accuracy can be further improved if the local magnification can be corrected.
As this local magnification correction means, it is preferable to form a latent image pattern having three or more lines in a row and a known pitch on the sample surface. More specifically, it is preferable to measure the linearity based on the degree of unequal interval of the latent image pattern by giving an equally spaced pattern.

A conceptual diagram for measuring the local magnification is shown in FIG. It is assumed that three or more (seven in the example of FIG. 21) exposure patterns are formed at equal intervals d on the sample surface. At the time of non-charging, latent image patterns should be formed at equal intervals on the sample surface. However, due to charging, the latent image captured images are at unequal intervals (FIG. 21A).
Assuming that the center positions of adjacent latent image patterns on the captured image data are Pi (xi, y0) and Pj (xj, y0), the interval per image pixel between Pi and Pj is:
d / {P_j (x_i + 1, y0) -Pi (xi, y0)}
Can be expressed as By performing the same calculation for each latent image pattern, a local magnification variation can be calculated. As another calculation method, the calculation may be performed on the basis of one of a plurality of latent image patterns.

  If the center positions of the captured image data of the latent image pattern obtained by S1 and S2 are P1 (x1, y1) and P2 (x2, y1), the results obtained from these are corrected using spline interpolation or approximate curves. Thus, the size of the latent image can be measured more accurately by creating a correction function as shown in FIG.

  A scanning optical system may be used as a latent image pattern forming method for measuring the size of the latent image. The scanning optical system deflects the light beam from the light source at an equiangular velocity by an optical deflector having a deflecting reflection surface, condenses the deflected light beam as a light spot on the surface to be scanned by the scanning imaging optical system, This is an apparatus that performs constant-speed optical scanning on the surface of the photoreceptor sample. Since the sample surface has a constant velocity, it is easy to form an equidistant exposure pattern on the sample surface by subjecting the LD as the light source to modulation at equal intervals as shown in FIG. The pitch can be easily adjusted by changing the frequency.

When the scanning optical system is used, the latent image pattern can be freely changed simply by changing ON / OFF of the electric signal of the LD. For this reason, when changing the enlargement ratio or the like, it is desirable to appropriately select the size and pitch of the pattern according to the change. In this case, in the case of a scanning optical system, it is only necessary to change the lighting condition of the light source.
If the distortion is large, the peripheral pitch may be fine.
In addition, since it is easy to make unequal intervals, latent image patterns are formed at equal intervals in a state where there is distortion due to charging, and magnification fluctuations are measured from ON / OFF conditions of LD electric signals. You can also
Further, the exposure optical system whose dimensions are to be measured and the exposure optical system to be evaluated may be used in common. Since the scanning optical system was originally designed to light up the image pattern at equal intervals, the conditions are compatible with the above two optical systems and not only saves space but also simplifies the system. Can be measured stably.

The dimension measurement may be performed before the measurement and used as correction data, or may be formed simultaneously with the actual latent image evaluation measurement. This example is shown in FIG. As shown in FIG. 24, a latent image pattern for dimensional measurement correction is formed in the periphery having high influence of fluctuation and high sensitivity, and an electrostatic latent image to be evaluated is formed in the vicinity of the center.
By simultaneously forming a latent image pattern for dimension measurement correction and an electrostatic latent image to be evaluated, the influence of magnification fluctuation due to actual charging can be corrected each time, and measurement can be performed with high accuracy. Can do.
By using such a method, it is possible to measure the dimension on the sample surface at the time of charging with a high resolution of 1 μm or less, which has been difficult in the past.

If the exposure energy density is smaller than 0.5 mJ / m 2, the latent image is measured shallowly, so that it is difficult to detect.
If the exposure energy density is greater than 10 mJ / m 2, a latent image is formed, but overexposure causes the latent image to become large and the center position measurement accuracy deteriorates.
When the size of the latent image is 10 μm or less, the size is generally small and the latent image is formed deeply. The beam spot size becomes large, which causes an error.
When the size of the latent image is 100 μm or more, the center position measurement accuracy is deteriorated.

  Therefore, it is desirable that the exposure energy density to be irradiated is 0.5 to 10 mJ / m 2 and the size of one latent image is 10 μm or more and 100 μm or less. Note that the size of the latent image indicates the cross-sectional direction to be measured, and the direction perpendicular thereto may be larger. That is, it may be a line pattern.

  As an exposure light source for forming an electrostatic latent image, an LD (laser diode) in the visible light to infrared light region is used, the laser light is scanned with a deflector such as a polygon mirror, and the laser light blinks. When an electrostatic latent image is formed, a deflector such as a polygon mirror takes about several seconds to rotate at a constant speed. Further, in order to blink the LD at a desired position, the writing start position is determined from the detection signal from the synchronous detector.

When a latent image pattern is formed by blinking light by an LD, it is necessary to increase the light response. For example, when the modulation is performed at 1 μs or less, it is necessary to always allow the bias current to flow even when the image data for improving the pattern light pulse reproducibility and the droop characteristics is turned off. This point was also explained before.
A semiconductor laser oscillates by applying a drive current that is higher than a reference. However, in order to improve the light response, a constant drive current (bias current) that is lower than the reference is always supplied even when the light is turned off. When there is a bias current, LED emission occurs. That is, when a semiconductor laser is used, it means that light is emitted even when the light is off. This point was also explained before.

FIG. 11 shows the relationship between the LD drive current IF and the optical output. a is a driving current when the light is turned off, b is a reference current for laser oscillation, and c is a driving current for c: LD lighting. Pon is the light output when the LD is turned on, and Poff is the light output when the LD is turned off.
The relationship of a <b <c is established. When the IF is small, the LED emission is weak, and laser oscillation occurs when the IF reaches the reference current. Usually, the laser oscillation is about 1 to 10 mW, and the bias current at the time of extinction is about several tens of μW, which is about 1/100 or less, which is not usually a problem.
For this reason, the bias current is kept flowing even when the LD is extinguished with emphasis on photoresponsiveness.
However, when the light is irradiated for a long time, the integrated light quantity increases, and when the required exposure amount of the photoreceptor is reached, an electrostatic latent image is formed. As a result, a desired electrostatic latent image cannot be formed.

Therefore, in the present invention, when a desired electrostatic latent image is formed using a semiconductor laser, the luminous flux is out of the electron beam scanning area of the sample in order to minimize the irradiation time to the sample that emits light with the bias current at the time of extinction. In this configuration, the offset light emission due to the bias current of the LD is shielded.
As an example of specific means, a shutter may be used between the LD light source and the sample. That is, offset light emission can be shielded by closing the shutter before exposure so that the light beam does not pass and opening the shutter so that the light beam passes during exposure.
Then, a desired electrostatic latent image such as a latent image pattern as shown in FIG. 19 can be formed by arranging a shutter mechanism and cutting off the offset light due to the LD bias current.

  Next, examples of the image forming apparatus according to the present invention will be described. This image forming apparatus uses the electrostatic latent image measuring device according to the present invention, and uses a photoreceptor obtained by performing the electrostatic latent image measuring method according to the present invention. FIG. 25 schematically shows an example of a laser printer as an example of an image forming apparatus. The laser printer 100 has a “cylindrical photoconductive photosensitive member” as the image carrier 111. Around the image carrier 111, a charging roller 112 as a charging unit, a developing device 113, a transfer roller 114, and a cleaning device 115 are provided. In this embodiment, a contact-type charging roller 112 that generates less ozone is used as the “charging means”, but a corona charger that uses corona discharge can also be used as the charging means. Further, an optical scanning device 117 is provided to perform “exposure by optical scanning of the laser beam LB” between the charging roller 112 and the developing device 113.

  In FIG. 25, reference numeral 116 denotes a fixing device, reference numeral 118 denotes a cassette, reference numeral 119 denotes a registration roller pair, reference numeral 120 denotes a paper feed roller, reference numeral 121 denotes a conveyance path, reference numeral 122 denotes a discharge roller pair, and reference numeral 123 denotes a tray. Yes. When image formation is performed, the image carrier 111, which is a photoconductive photosensitive member, is rotated at a constant speed clockwise in FIG. 25, and the surface thereof is uniformly charged by the charging roller 112. An electrostatic latent image is formed by exposure by optical writing. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed. This electrostatic latent image is reversely developed by the developing device 113, and a toner image is formed on the image carrier 111.

  The cassette 118 storing the transfer paper is detachable from the main body of the image forming apparatus 100, and the uppermost sheet of the stored transfer paper is fed by the paper feeding roller 120 in the state of being mounted as shown in the figure. The transferred transfer paper is fed to the registration roller pair 119 at the leading end. The registration roller pair 119 feeds the transfer paper to the transfer unit at the same timing as the toner image on the image carrier 111 moves to the transfer position. The transferred transfer paper is superimposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer roller 114. The transfer paper on which the toner image is transferred is fixed on the toner image by the fixing device 116, passes through the conveyance path 21, and is discharged onto the tray 123 by the discharge roller pair 122. After the toner image is transferred, the surface of the image carrier 111 is cleaned by the cleaning device 115 to remove residual toner, paper dust, and the like.

  By carrying out the measuring method according to the present invention using the electrostatic latent image measuring apparatus according to the present invention, a very desirable latent image carrier can be used in the image forming apparatus. Thereby, it is possible to obtain an image forming apparatus having excellent resolution, high definition, high durability, and high reliability.

1 is an optical layout diagram illustrating a first embodiment of a measurement apparatus for an electrostatic latent image according to the present invention. It is a longitudinal cross-sectional view which expands and shows the principal part of the said Example. It is a model figure which shows the principle of the charge distribution and potential distribution detection by a secondary electron. It is a graph which shows the relationship between the drive current and optical output of the semiconductor laser used for the said Example. FIG. 4 shows changes in the amount of light emitted from a light source over time, (a) is a graph according to the conventional method, and (b) is a graph according to the above embodiment of the present invention. It is a timing chart which shows operation | movement of the said Example. It is a block diagram which shows the structure of the control system of the said Example. It is a flowchart which shows the measurement procedure by the said Example. It is sectional drawing which shows the example of the mechanical shutter which can be used for the said Example. It is a perspective view which shows the example of the optical scanning device which can be used for the said Example, and its light source part. It is a model figure which shows the various examples of the latent image pattern formed in the said Example. It is a model figure which shows the modification of the measuring method applicable to this invention. It is a model figure which shows the relationship between the incident electron and sample in the said modification. It is the graph and model figure which show an example of a latent image depth measurement result. It is the graph and model figure which show the relationship between the spot diameter of a laser, and the diameter of a latent image. It is an optical arrangement | positioning figure which shows 2nd Example of the measuring apparatus of the electrostatic latent image concerning this invention. It is an optical arrangement | positioning figure which shows the detail of the exposure optical system in the said 2nd Example. It is a model figure and graph which show change of a scanning field by charging a photo conductor. It is a front view which shows the various examples of the exposure mask pattern which can be used for the said 2nd Example, and the example of the latent image obtained by this mask pattern. It is a flowchart which shows the measurement procedure by the said 2nd Example. It is a model figure and graph which show an example of local magnification fluctuation in a photoconductor. FIG. 5 is a waveform diagram and a model diagram showing a pattern for dimension measurement by a scanning optical system and an operation example of an LD for creating this pattern. It is an optical arrangement | positioning figure which shows the optical layout of the exposure optical system for coordinate correction | amendment in the said 2nd Example. It is a model figure which shows the example of a latent image pattern in the case of forming simultaneously the latent image for dimension measurement applicable to the said 2nd Example, and the latent image for latent image evaluation. 1 is a front view conceptually showing an embodiment of an image forming apparatus according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Electron beam irradiation part 11 Electron gun 19 Detector 20 Exposure part 21 LD light source 30 Photoconductor sample 35 Optical deflector (polygon mirror)
39 Shutter

Claims (21)

Means for irradiating the sample with an electron beam to generate a charged charge on the photoreceptor sample, an exposure optical system for forming an electrostatic latent image, a scanning means for scanning the sample surface with the electron beam, In the electrostatic latent image measuring device for measuring the electrostatic latent image distribution on the sample surface by the detection signal obtained by this scanning,
An apparatus for measuring an electrostatic latent image, characterized in that a semiconductor laser is used as a light source of the exposure optical system, and a light beam of the light source irradiates outside an electron beam scanning region of a sample. Means for irradiating the sample with an electron beam to generate a charged charge on the photoreceptor sample, an exposure optical system for forming an electrostatic latent image, a scanning means for scanning the sample surface with the electron beam, In the electrostatic latent image measuring device for measuring the electrostatic latent image distribution on the sample surface by the detection signal obtained by this scanning,
An apparatus for measuring an electrostatic latent image, comprising: a semiconductor laser as a light source of the exposure optical system, and shutter means for shielding offset light emission due to a bias current of the semiconductor laser.   3. The apparatus for measuring an electrostatic latent image according to claim 1, further comprising means for opening the shutter in conjunction with a synchronizing signal of the scanning optical system.   4. The apparatus for measuring an electrostatic latent image according to claim 3, further comprising means for opening the shutter using the synchronization detection signal of the scanning optical system as a trigger signal and closing the shutter after forming the electrostatic latent image.   When Td is the time when the shutter starts to open after the trigger signal is detected, and Tr is the time from when the shutter starts to open until the equivalent of the effective diameter of the laser beam opens, Td + Tr 5. The apparatus for measuring an electrostatic latent image according to claim 1, further comprising means for turning on the semiconductor laser with a time delay. When the scanning time of one time by the scanning optical system is Tf, the offset light emission due to the bias current when the semiconductor laser is turned off is Poff, and the light quantity when the semiconductor laser is turned on is Pon,
Tr <Tf * Pon / Poff
The electrostatic latent image measuring apparatus according to claim 1, wherein the following condition is satisfied.   The apparatus for measuring an electrostatic latent image according to claim 1, wherein a mechanical shutter is used as the shutter unit.   8. The apparatus for measuring an electrostatic latent image according to claim 1, wherein shutter means is disposed outside the vacuum chamber in order to suppress electromagnetic field fluctuations.   9. The apparatus for measuring an electrostatic latent image on a photosensitive member according to claim 1, further comprising means for performing measurement under a condition in which a region in which a velocity vector in a sample vertical direction of incident charged particles is inverted exists. A charged charge is generated on the sample by irradiating the sample with charged particles, and an electrostatic latent image is formed on the sample on which the charged charge is generated using an exposure optical system using a laser diode light source. In the method for measuring the electrostatic latent image of the photosensitive member, the electrostatic latent image distribution on the sample surface is measured by a detection signal obtained by scanning with an electron beam.
Electrostatic latent image measurement characterized by suppressing offset light emission due to the bias current of the semiconductor laser by shielding light so that the light source does not reach the observation area of the sample before and after the time to form the electrostatic latent image Method. A latent image is formed by performing optical scanning on the photosensitive surface of the latent image carrier measured using the measuring apparatus or measuring method according to claim 1, developed, and visualized. An image forming apparatus characterized in that
The writing light source wavelength is 780 nm or less, the beam spot diameter in the sub-scanning direction on the surface of the photoconductor is 60 μm or less, the beam spot diameter in the sub-scanning direction on the photoconductor surface is A, and the latent image in the sub-scanning direction is formed. An image forming apparatus characterized by satisfying 1.0 <B / A <2.0 when an image diameter is B. By means of irradiating the sample with an electron beam, a means for generating a charged charge on the photosensitive member sample, an exposure optical system for forming an electrostatic latent image, and the sample surface are scanned with the electron beam. In the electrostatic latent image measuring device that measures the electrostatic latent image distribution on the sample surface by the detection signal obtained,
Means for forming a latent image pattern having a known dimension on the surface of the photoreceptor sample by irradiating light having a wavelength of 400 to 800 nm;
Means for capturing a latent image obtained as an electrostatic latent image;
An electrostatic latent image measuring device comprising: means for measuring the coordinates of the sample.   13. The apparatus for measuring an electrostatic latent image according to claim 12, further comprising means for projecting and exposing a mask pattern having a known dimension as means for forming a latent image pattern having a known dimension.   13. The apparatus for measuring an electrostatic latent image according to claim 12, wherein two or more latent image patterns having a known interval on the sample surface are formed, and the average magnification is measured from the captured image data.   15. The apparatus for measuring an electrostatic latent image according to claim 14, wherein three or more latent image patterns having a known pitch on the sample surface are formed in a line, and the local magnification fluctuation is measured from the captured image data. .   The light beam from the light source is deflected at an equal angular velocity by an optical deflector having a deflecting reflection surface, and the deflected light beam is condensed as a light spot on the surface to be scanned by the scanning imaging optical system, and is applied to the surface of the photosensitive member sample. A latent image pattern with a known interval is formed using an optical scanning device that performs constant-speed optical scanning, and the coordinates of the sample are measured from data obtained by capturing the latent image obtained as an electrostatic latent image. The apparatus for measuring an electrostatic latent image according to claim 12.   2. The apparatus for measuring an electrostatic latent image according to claim 1, wherein an exposure energy density to be irradiated is 0.5 to 10 mJ / m 2 and a size of one latent image is 100 μm or less.   The measurement of an electrostatic latent image according to any one of claims 12 to 17, further comprising means for measuring under a condition in which a region where the velocity vector of the incident charged particle in the sample vertical direction is reversed exists. apparatus. A means for generating a charged charge on a photoconductor sample by irradiating the sample with an electron beam, an exposure optical system for forming an electrostatic latent image, and scanning the sample surface with an electron beam are obtained by this scanning. In the method of measuring an electrostatic latent image, wherein the electrostatic latent image distribution on the sample surface is measured by a detection signal obtained,
A latent image pattern having a known size is formed by irradiating light with a wavelength of 400 to 800 nm on the surface of the photoreceptor sample.
A method for measuring an electrostatic latent image, comprising: measuring coordinates of a sample from data obtained by capturing a latent image obtained as an electrostatic latent image.   20. The apparatus for measuring an electrostatic latent image according to claim 12, further comprising means for opening the shutter in conjunction with pattern lighting of the semiconductor laser. 21. Image formation in which a latent image is formed by performing optical scanning on a photosensitive surface of a latent image carrier measured using the measuring apparatus and measuring method according to claim 12, and is developed and visualized. A device,
The writing light source wavelength is 780 nm or less, the beam spot diameter in the sub-scanning direction on the surface of the photoconductor is 60 μm or less, the beam spot diameter in the sub-scanning direction on the photoconductor surface is A, and the latent image in the sub-scanning direction is formed. An image forming apparatus satisfying 1.0 <B / A <2.0, where B is an image diameter

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