GB2138646A - Correcting for scanning irregularities in a picture recording system - Google Patents

Abstract

In a system for recording, for example, a wiring pattern on a photosensitive recording surface 12, in which a light beam from laser tube 4 is modulated by a picture signal read from memory 1, variations in the rotational speed of polyhedral mirror 7 which scans the light beam across the surface 12 may cause distortion in the recorded picture. To avoid this the picture signals are read out at a rate corresponding to the position of the light beam sensed by a photosensor 16 mounted behind a scale 15 comprising alternately transparent and opaque sections (fig. 3). To ensure synchronization even when no signal is to be recorded on surface 12 the output of laser 4 is modulated between a high (write) level and a level too low to affect the recording surface, but still detectable by photosensor 16. The output signal of the photosensor may be held constant by an amplifier, the gain of which is controlled by the picture signals (fig. 5). Beam splitter 11' allows a single laser to be used for both recording and position sensing and thus reduces the manufacturing cost. <IMAGE>

Description

SPECIFICATION Picture recording method This invention relates to a method for recording a picture such as a wiring pattern or the like by printing the picture on a picture-forming material coated with a film of a photosensitive material, such as board suitable for use in the fabrication of a printed circuit, in accordance with input picture signals without developing dimensional distortion.

Wiring patterns may be formed on insulated boards, which are suitable for use in the fabrication of printed electronic circuits, in accordance with either contact or direct exposure method.

According to the contact exposure method, a board bearing a resist film coated on the surface thereof is exposed to light with a mask film, in which a negative or positive pattern has been formed, kept in contact with the resist film. The thus-exposed board is then developed to fabricate a resist pattern.

The contact exposure method is however accompanied by such drawbacks that it takes first of all several hours to prepare a mask film of a desired picture or pattern for example by means of a coordinate plotter or the like; and dimensional distortion occurs with a wiring pattern formed on an insulated board as such a mask film undergoes shrinkage or expansion and hence develops distortion by variations in temperature and/or humidity. If such a dimensional error arises, some inconvenience would be encountered when drilling the resulting printed circuit in a subsequent step because the positions of at least some holes would be offset from the thus-printed circuit.

Different from the aforementioned conventional contact exposure method, the direct exposure method forms a picture or pattern in the following manner without using any mask film. Namely, two-dimensional picture data are stored as digital picture signals in a memory or the like. The thusstored picture data are thereafter read out as picture signa!s. Then, an insulated board, which is suitable for use in the fabrication of a printed circuit, is scanned by light which has in advance been controlled by the picture signals.

Figure 1 is a block diagram illustrating one example of conventional direct exposure systems.

In a memory 1 there are stored binary picture signals arranged two-dimensionally. The picture signals are controlled by a central processing unit (hereinafter called "CPU" for the sake of brevity) 2 in such a way that they are converted to timeseries scanning signals for recording a picture or pattern, which corresponds to the binary picture signals stored in the memory 1, by the scanning technique.

The resulting scanning signals are fed to an acousto-optic light modulator 3.

A A light beam output from an exposing light beam source, for example, an argon ion laser tube 4 is ON-OFF modulated by the acousto-optic light modulator 3 and is then guided to a polyhedral reflector 7 by way of an expander 9 and fixed mirrors 5, 6. Individual reflecting surfaces of the polyhedral reflector 7 are rotated by a motor 8, whereby reflecting and sweeping the exposing light beam in a direction perpendicular to the sheet of the drawing with a prescribed spread angle (sweep angle).

Then, the exposing light beam travels through a focusing lens 10 arranged very close to the polyhedral reflector 7 and is thereafter reflected by a fixed mirror 11 disposed at a position adjacent to an insulated board 1 2 suitable for use in the fabrication of a printed circuit. The light beam then sweeps and radiates the unexposed insulated board 12 while forming image points thereon.

The insulated board 12 is fixedly mounted on a stage 13, which is movable at a constant speed in a direction perpendicular to the beam-sweeping direction (i.e., in the direction indicated by an arrow 0 in the figure) owing to the provision of a motor 14 so as to form a subscanning feed mechanism.

When the polyhedral reflector 7 is rotated and the stage 13 is moved, the insulated board 12 is thus plane-scanned successively, at the image point of the exposing light beam, all over the surface thereof.

The accuracy of movement of the stage 13 in the subscanning direction, which movement is achieved by mechanically driving the stage 13, may be maintained at a required level without substantial difficulties in the direct exposure system so long as the motor 14 and forcetransmitting mechanism are designed suitably.

However, the accuracy of the beam sweeping speed and width which pertain to the rotation of the polyhedral reflector 7, in other words, the accuracy in the main scanning direction is governed by the design and machining preciseness of the optical system which includes - the polyhedral reflector 7, focusing lens 10, etc. It is not easy to minimise errors, which are caused by designing and/or machining aspects, to satisfactory levels. Furthermore, it is extremely difficult from the technical viewpoint to maintain the linearity of the angle of rotation of the polyhedral reflector 7 and that of image points along the sweeping line of the light beam with sufficient accuracy.

Figure 2 is a block diagram of a conventional exposure system which has purportedly overcome the above-described drawbacks.

In Figure 2, all elements or parts of structure identified by the same reference numerals as those used in Figure 1 serve in the same manner as their corresponding elements or parts depicted in Figure 1. Explanation on such elements or parts is thus obtained.

The system illustrated in Figure 2 is additionally equipped with a narrow grille-like scale 15, photosensor 16 and auxiliary laser tube 17 compared with the system shown in Figure 1.

Furthermore, the fixed mirrors 5, 11 have been replaced by half-mirrors 5', 1 1'.

Figure 3 illustrates a part of the narrow grillelike scale 15 shown in Figure 2.

In Figure 2, the narrow grille-like scale 15 is placed in conjugated relation with the point of exposure of the insulated board 12, i.e., the image point on the recording surface of the insulated board 12 relative to the half-mirror 11'.

Similar to the system depicted in Figure 1, a laser beam output from the argon laser tube 4 is caused to sweep the unexposed insulated board 12 so that the insulated board 12 is exposed to the laser beam. On the other hand, the auxiliary laser tube 1 7 is arranged in such a way that each laser beam, which is to be output from the laser tube 1 7 can follow the same optical axis as laser beams output from the argon laser tube 4.

The laser beam output from the auxiliary laser tube 17 has a wavelength that is outside a color sensitivity range in which a photosensitive material coated on the insulated board 12 is exposed. The laser beam output from the auxiliary laser tube 1 7 sweeps the insulated board 12 and, at the same time, also sweeps the front surface of the narrow grille-like scale 15 in a direction parallel to the length of the scale 15.

The laser beam, which has scanned the narrow grille-like scale 15, is then allowed to pass through the openings (see, Figure 3) of the scale, thereby converted to a laser beam which is repeatedly turned off at a frequency proportional to the sweeping speed. The latter laser beam then enters the photosehsor 16.

The photosensor 16 converts the thus-input laser beam to pulse signals in accordance with the sweeping speed. The resulting pulse signals are then input to the CPU 2, which reads out picture signals from the memory in synchronisation with the pulse signals. Therefore, a distortion-free picture or pattern is exposed on the insulated board 12.

In the conventional system depicted in Figure 2, two types of laser beams having different frequencies are used. Even if a focusing lens minimised in chromatic aberration is used as the focusing lens 10, certain residual chromatic aberration (with respect to magnification) still remains. In addition, further aberration may also occur at the half-mirror 1 1', depending on the incident angle of each laser beam. Accordingly, there is another drawback that the narrow grillelike scale 15, which is illustrated in Figure 3, has to be formed into non-linear configurations so as to correct such errors.

Since there is a rather long distance from the polyhedral reflector 7 to the insulated board 12 when conducting the exposure of the insulated board 12 by sweeping the laser beam at the polyhedral reflector 7, a further drawback may be developed unless the optical axes of the two laser beams are coincided completely. Namely, a difference may be developed between the scanning points of both laser beams on the insulated board 12, depending on the extent of the beam sweeping angle.

With the foregoing in view, the present invention has as its object the provision of a picture recording method which permits to improve the accuracy of each recorded picture or pattern and also to lower the manufacturing cost of a picture recording system, which is suited to practice the picture recording method, compared with the prior art method illustrated in Figure 2 by using only one laser beam.

In one aspect of the invention, there is thus provided a method for recording a desired picture by sweeping a light beam obtained from a light beam source and then scanning, with the thus swept light beam, a recording surface on which a photosensitive material has been coated, characterised in that a portion of the swept light beam is branched out by a half-mirror and then fed to optical sweep position detecting means, and the light beam from the light source is modulated by a picture signal obtained in synchronisation with a position signal produced by the detecting means on the basis of the thus-detected portion of the swept light beam in such a way that the thus modulated light beam is continuously maintained, at least, at such a low light quantity level that is too small to expose the photosensitive material substantially.

In another aspect of this invention, there is also provided a method for recording a desired picture by, sweeping a light beam obtained from a light beam source and then scanning, with the thusswept light beam, a recording surface on which a photosensitive material has been coated, characterised in that a portion of the swept light beam is branched out by a half-mirror and then fed to optical-sweep position detecting means, the light beam from the light beam source is modulated by a picture signal obtained in synchronisation with a position signal produced by the detecting means on the basis of the thus detected portion of the swept light beam in such a way that the thus-modulated light beam is continuously maintained, at least, at such a low light quantity level that is too small to expose the photosensitive material substantially, and the level of the position signal obtained by the detecting means is changed to a predetermined level in accordance with the level of the picture signal which controls the intensity of the light beam obtained from the light beam source.

The above methods of this invention have made it possible to reduce the overall manufacturing costs of picture recording systems, which are respectively suitable for use in the practice of the methods, owing to the use of a single piece of laser tube, although the aforementioned conventional method required an expensive system due to the inevitable dependence on two laser tubes.

Furthermore, the present invention does not require to take chromatic aberration into consideration, thereby permitting to use an inexpensive focusing lens as the focusing lens 10.

It is only necessary to think of aberration of only one colour with respect to the narrow grille-like scale 1 5 the details of which are shown in Figure 3.

The principal feature of this invention resides in the use of a single piece of laser tube only. This has made optical axis alignment work, such as that required when two laser tubes were used, unnecessary. Besides, the above methods have completely solved errors which were caused by variations in scanning angle, which variations were in turn developed due to a difference between the two optical axes.

The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which: Figure 1 is a block diagram illustrating one example of conventional direct exposure systems which have been employed to fabricate printed circuits; Figure 2 is a block diagram of a conventional direct exposure system making additional use of an auxiliary laser beam and adapted to fabricate printed circuits; Figure 3 is a front view of one example of the narrow grille-like scale depicted in Figure 2; Figure 4 is a block diagram of a picture recording system which is useful in the practice of the picture recording method according to the first embodiment of this invention; and Figure 5 is a block diagram of a direct exposure system useful in the practice of the picture recording method according to the second embodiment of this invention.

The present invention has been completed taking a hint from the exposure characteristic, in other words, the relationship between the exposing light quantity and the density of the resulting developed picture or pattern in a photosensitive material coated on an insulated board which is useful to fabricate a printed circuit.

Namely, the present invention uses, generally speaking, a photosensitive material which is useful in the fabrication of printed circuits and has high 6characteristics that it is not exposed at all by light of a certain light quantity or smaller but is certainly exposed by light having a light quantity greater than the certain light quantity.

The first embodiment of this invention will hereinafter be described with reference to Figure 4. The picture recording system of Figure 4 is different from that depicted in Figure 2 in that the auxiliary laser tube 17 has been removed and the half-mirror 5' has been replaced by a fixed mirror 5.

As described with reference to Figure 2, the input laser beam is modulated at the acousto optic light modulator 3 in accordance with binary picture signals fed from the CPU 2 and is then output from the acousto-optic light modulator 3 as laser means having two different light quantity levels, namely, a laser beam having such an intensity that permits to expose the photosensitive material and another laser beam having such an intensity that does not permit to expose the photosensitive material but permits the photosensor 16 to detect the laser beam as pulses.

The bi-level laser beam modulation-controlling technique making use of the acousto-optic light modulator 3 and the laser beam detection technique relying upon the photosensor 16 are by themselves not targets of the present invention.

Any suitable known art may be used for such purposes. Accordingly, such techniques are not described in detail in the present specification.

The present invention permits to omit one of two laser tubes and, in addition, to use a lens which is not specially designed to reduce chromatic aberration. Thus, the present invention has brought about such advantageous effects that such assembly and adjustment work as optical axis alignment has been rendered unnecessary, the manufacturing cost of a colour recording system has been reduced, and the aggravation of errors due to variations in scanning angle, which variations occur by the non-alignment of the optical axes of two laser beams, had been successfully avoided.

In the above-described first embodiment, the narrow grille-like scale 15 and photosensor 16 are used. It may be feasible to employ a onedimensionally disposed photoelectric array sensor instead of such a scale and photosensor.

In the above-described first embodiment, a light beam having such a light quantity that does not substantially expose the picture recording material is allowed to reach the picture recording material whenever a picture-free portion of the picture recording material is scanned. A portion of the picture-recording and sweeping light beam per se is branched out, whereby to detect a position corresponding exactly to a swept point on the picture recording material. On the basis of a resulting position signal, there is produced a feedback signal which is adapted to control the timing of the optical modulation.

In the first embodiment, a portion of a recording light beam is branched out by a half-mirror or the like and two different levels are given to the recording light beam so as to obtain position signals corresponding to sweeping light beams.

Since a photosensor having a single level of sensitivity is used for sweeping Iightbeams both when only a small light quantity that does not substantially expose the picture recording material is allowed to reach the picture recording material and when a picture portion is recorded on the picture recording material, the level of the sweeping light beam having a small light quantity and adapted to scan a picture-free portion may become unduly different from the level of the sweeping light beam having a large light quantity and adapted to scan a picture-bearing portion. The levels of resulting pulse signals may thus be varied, thereby instabilizing the processing of the light beam delivered from the light beam source which processing is supposed to be synchronised with the picture signals.

Figure 5 illustrates a direct exposure system suitable for use in the practice of the picture recording method according to the second embodiment of this invention, which method permits to perform the above-mentioned processing stably.

In summary, the picture recording method according to the second embodiment of this invention comprises, for example, providing an amplifier in a latter stage of the photosensor, changing the amplification factor of the amplifier in accordance with picture signals and feeding back feedback pulse signals, which are adapted to correct the timing of optical modulation by synchronising the timing with picture signals, withsuch signal levels that permit to perform proper digital processing thereon, so that detection pulse signals obtained when scanning a picture-free portion, in other words, while the light quantity of the sweeping beam is at a low level, may always be kept at a prescribed constant level.

In Figure 5, an amplifier 18 has been additionally provided in the feedback circuit of pulse signals P, which feedback circuit extends between the photosensor 16 and CPU 2 in Figure 4. The remaining parts or elements are identical to those shown in Figure 4 and their description is thus omitted herein.

Accordingly, the laser beam which has passed through the narrow grille-like scale 15 is converted to the pulse signals P at the photosensor 16 in accordance with the sweeping speed of the iaser beam. The pulse signals P serve to provide position information indicating correpsonding exposing point of the swept light beam on the insulated circuit 12, for example, by counting the number of pulse signals or a similar method. The pulse signals P are then amplified by the amplifier 18, the amplification factor of which is variable, and are therafter fed back to the CPU 2.

A binary picture signal read out from the CPU 2 has already been input as an amplification factor controlline signal G to the amplifier 18. The amplifier 18 changes over its amplification factor, depending whether the picture signal is at a high level or at a low level. Thus, the amplification factor is changed to a low level when the levels of pulse signals P are high. It is changed to a high level when the levels of pulse signals Pare low.

Owing to the provision of the amplifier 18, it is possible to obtain pulse signals having a prescribed constant level although the photosensor 16 outputs pulse signals having various different levels.

Therefore, in the case of a laser beam modified in accordance with a picture signal corresponding to a picture-free portion and having a small light quantity out of laser beams allowed to pass through the narrow grille-like scale 15, the amplification factor is increased for its corresponding pulse signals so that the S/N ratio of each piece of position information to be fed back to the CPU 2 is improved and the timing control can hence be performed precisely at the CPU 2.

In the case of a laser beam modulated in accordance with a picture signal corresponding to a picture-bearing portion and having a great light quantity, the amplification factor is decreased for its corresponding pulse signals so that pulse signals corresponding to each picture-bearing portion become substantially equal in level to those corresponding to each picture-free portion.

Pulse signals, which have been output from the photosensor 16, are cut at a suitable threshold level by known technique in the above-mentioned manner, thereby feeding back pulse signals Pa having rectangular waves. They are then converted to high frequency pulses by a PLL circuit or the like. A count data on the thusconverted pulses is input to the CPU 2, whereby to control the picture signal reading-out speed from the momory 1 in which picture signals are stored.

Accordingly, it is possible to record a or pattern while achieving exact synchronisation between the revolution speed of the polyhedral reflector 8 and the picture signal reading-out speed.

In the above second embodiment, the laser beam corresponding to a picture-free portion and that corresponding to a picture-bearing portion are both converted to detected sweep position pulse signals P at the photosensor 16 by way of the half-mirror 1 1 t and narrow grille-like reflector 15.

The amplification factors for both types of the pulse signals are respectively changed by picture signals corresponding to the picture-free portion and the picture-bearing portion of the picture or pattern to be recorded. In some instances, it may be better to attenuate only detected sweep portion pulse signals obatined from laser beams corresponding to picture-bearing portions. In some other instances, it may be better to amplify only detected sweep position pulse signals obtained from laser beams corresponding to picture-free positions.

The amplification factor may be changed by changing over the ratio of a feedback resistor and input resistor of an operational amplifier by an analog switch or the like where the amplifier 18 is formed of the operational amplifier or by providing two types of amplifiers, attenuators or the like and changing over their outputs by means of an analog switch or the like, so that sweep position pulse signals of the same level can be output from the amplifier 1 8.

Accordingly, picture signals read out from the momory 1 are converted to tirne-series scanning signals synchronised precisely with the beam sweeping speed, thereby recording a tortion-free picture or pattern on the insulated board 12.

The second embodiment of this invention has been described supposing that an insulated board is exposed to fabricate a printed circuit. Needless to say, the present invention may be equally applied to other picture recording materials (for example, photographic films, heat-sensitive materials suitable for exposure by infrared laser beams, etc.).

In the second embodiment depicted in Figure 5, the amplification or attenuation factor in the feedback loop is changed in accordance with the level of light quantity of the light beam modulated by each picture signal to be recorded so that the resulting sweep position pulse signal can be either amplified or attenuated to a prescribed pulse signal level. Therefore, it is always possible to obtain detected position pulse signals of substantially the same level whether picturebearing or picture-free portions are scanned, whereby allowing to print a prescribed picture or pattern without dimensional distortion.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

Claims (10)

1. A method for recording a desired picture by sweeping a light beam obtained from a light beam source and then scanning, with the thus-swept light beam, a recording surface on which a photosensitive material has been coated, characterised in that a portion of the swept beam is branched out by a half-mirror and then fed to optical sweep position detecting means, and the light beam from the light beam source is modulated by a picture signal obtained in synchronisation with a position signal produced by the detecting means on the basis of the thusdetected portion of the swept light beam in such a way that the thus-modulated light beam is continuously maintained, at least, at such a low light quantity level that is too small to expose the photosensitive material substantially.

2. A method as claimed in Cla#im 1, wherein the optical sweep position detecting means comprises a narrow grille-like scale disposed in conjugated relationship with an image point on the recording surface relative to the half-mirror, at which image point the recording surface is exposed to the swept light beam, and a photosensor.

3. A method as claimed in Claim 1 , wherein the optical sweep position detecting means is a photoelectric array sensor disposed onedimensionally in a plane which is scanned by the portion of the swept light beam.

4. A method for recording a desired picture by sweeping a light beam obtained from a light beam source and then scanning, with the thus-swept light beam, a recording surface on which a photosensitive material has been coated, characterised in that a portion of the swept light beam is branched out by a half-mirror and then fed to optical sweep position detecting means, the light beam from the light beam source is modulated by a picture signal obtained in synchronisation with a position signal produced by the detecting means on the basis of the thusdetected portion of the swept light beam in such a way that the thus-modulated light beam is continuously maintained, at least, at such a low light quantity level that is too small to expose the photosensitive material substantially, and the level of the position signal obtained by the detecting means is changed to a predetermined level in accordance with the level of the picture signal which controls the intensity of the light beam obtained form the light beam source.

5. A method as claimed in Claim 4, wherein the optical sweep position detecting means is provided at the output side thereof with means adapted to change the amplification factor for the position signal depending whether the level of the picture signal is low or high.

6. A method as claimed in Claim 4, wherein the optical sweep position detecting means is provided at the output side thereof with means adapted to amplify the position signal when the level of the picture signal is low but to attenuate the position signal when the level of the picture signal is high.

7. A method as claimed in Claim 1 substantially as herein described with reference to Figure 4 of the accompanying drawings.

8. A method as claimed in Claim 4 substantially as herein described with reference to Figure 5 of the accompanying drawings.

9. A picture recording system for carrying out the method as claimed in any preceding claim.

10. A picture recording system substantially as herein described and shown in the accompanying drawings.