GB2284902A - Integrated electro-optical liquid crystal device and methods of using such a device. - Google Patents

Abstract

An integrated electro-optical device comprising a pixellated liquid crystal modulation layer (6) and means for absorbing polarised light superposed over an electronic detection layer (a - Si3). Modulator pads for the liquid crystal layer (6) may be formed as electrically conductive combs to achieve optical polarisation. As shown a ferroelectric liquid crystal layer incorporating a pleochroic dye also functions as a controlled absorber for polarised light. The device may be used as an optical mask comparator. An image directed through a glass The differences where the first image is dark and the second image is light result in current flow in the detection layer. The process is repeated using inverse images to detect inverse differences. Application to testing integrated circuit chips is described. Further, an integrated electro-optical device, optionally of the above type, has a plurality of pixels (A', B' Fig 6 not shown) of the liquid crystal layer for each pixel (A, B) of the detector array. This allows motion detection by optical shuttering of the detector pixels by electrically modulating the pixellated liquid crystal layer. <IMAGE>

Description

INTEGRATED ELECTRO-OPTICAL LIQUID CRYSTAL DEVICE AND METHOD OF USING SUCH A DEVICE.

This invention relates to an integrated electro-optical device comprising a liquid crystal layer and an electronic detection layer, and to methods of using such a device. Such a device may be used for dynamic optical filtering, optionally with a non-volatile memory, of an optical image.

Liquid crystal layers are well known as optical modulators, controlled by an adjacent electronic backplane. Typically, terroelectric liquid crystal cells are used, and silicon or gallium arsenide backplanes. The parallel input/output capability of such an optical modulator is realised by providing local electronic processing on the same integrated circuit: an image can be read into the electronic backplane through the transparent liquid crystal, thresholded, and then recorded in the liquid crystal by the application of suitable voltages to pixellated modulator pads. The thresholded image can then be read in parallel, i.e. in the operation, by a read beam which is transmitted through the liquid crystal and reflected from the backplane. Such an arrangement is disclosed in "The design of smart SLMs and applications in optical systems", D. Vass et al, Applied Optics and O-E Cony, Nottingham [7 990].

Most liquid crystal materials require an element to analyse the polarisation state of the light in order to achieve optical modulation, and devices of the type described above are unable to read directly onto the electronic backplane the image which has been stored in the liquid crystal.

According to the invention, there is provided a device as defined in the appended Claim 1.

Preferred embodiments of the invention are defined in the other appended claims.

By providing an integrated polarising element between the liquid crystal layer and the electronic detection layer, it is now possible to process the image in the plane of the liquid crystal, whilst maintaining the advantages of stability, size and resolution in an integrated device. For example, the device can be used as a novelty filter, by storing a first image in the liquid crystal layer and using it to modulate a second image transmitted through that layer and detecting the emergent light in the detector plane.

Liquid crystal devices which incorporate in the liquid crystal anisotropic dyes, also called guest-host pleochroic dyes, allow the liquid crystal layer also to function as a polariser. We have discovered that such liquid crystal materials can be used to achieve direct imaging in a similar way, and accordingly the invention also provides an integrated electro-optical device comprising a pixellated liquid crystal layer whose pixels are responsive to electric control signals to vary their absorption of polarised light, and a superposed electronic detector layer.

A further problem with currently-available integrated light-modulation devices is the inflexibility of the optical link between an object and the detector array of the device: the spatial resolution of the device has to be predetermined.

It is possible to provide an integrated electro-optical device comprising a pixellated liquid crystal optical modulation layer over a pixellated optoelectronic detector array, in which there is a plurality of pixels of the liquid crystal layer for each pixel of the detector array. As described by way of example below, this enables the liquid crystal layer effectively to act as a shutter for the detector array, and an arrangement such as this has a wealth of useful applications. The device may be used in a method of detecting the positions of edges in an object, and it may be used in a method of detecting motion of an object. Further applications of a device according to this aspect of the invention are disclosed and claimed in our co-pending patent application no

WM & C reference 230P67616 and M6365; SLE reference 93001SLE; Yamamoto reference S93048/GBNB; "Imaging Apparatus and Method").

The invention will be further described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a cross-sectional view of an integrated electro-optical device constituting a first embodiment of the invention; Figure 2 is a cross-sectional view of a device constituting a second embodiment of the invention; Figure 3 is a cross-sectional view of a device constituting a third embodiment of the invention; Figure 4 represents the wiring pattern of an integrated circuit under test; Figure 4b represents the wiring diagram of a perfectly-formed integrated circuit corresponding to that of Figure 4a; Figure 4c represents the use of a device embodying the invention to compare the perfect and test integrated circuits; Figure 4d represents the difference image produced by the device embodying the invention during the process represented in Figure 4c; Figure 5 represents one modulator pad/pixel of a combined detector and polariser array of another embodiment of the invention; and Figure 6 represents the optical arrangement for detecting the position of edges in an object using a further embodiment of the invention.

The integrated electro-optical device shown in Figure 1 comprises a glass substrate 1 on which is formed a transparent plane electrode 2, for instance of indium tin oxide (ITO). A layer 3 of amorphous silicon is formed on the electrode 2 and constitutes an unpixellated detector layer.

A further glass substrate 4 has formed thereon another transparent plane electrode 5 of ITO. A combined liquid crystal and polariser 6 is enclosed between the electrode 5 and the amorphous silicon layer 3 and comprises a ferroelectric liquid crystal (FLC) incorporating a pleochroic dye. The anisotropic dyes dissolved in the liquid crystal ("guest-host liquid crystal layer") have polarisation dependent absorptions and act as an integrated analysing element. Such FLC devices can typically achieve an on-off contrast ratio to polarised light of 10:1.

The device shown in Figure 1 may be used as an optical novelty filter.

Light spatially modulated with an image to be stored in the FLC is directed through the glass substrate 1 and the electrode 2 onto the amorphous silicon layer 3. The amorphous silicon creates a charge carrier pattern corresponding to the image and, by applying a suitable potential difference across the electrodes 2 and 5, charges are injected into the FLC so that a non-volatile image corresponding to the image to be stored is stored in the FLC.

Light spatially modulated with an image to be filtered is directed through the substrate 4 and the electrode 5. For correct operation, the light is required to be suitably polarised and this may be achieved by passing the spatially modulated light through a polariser (not shown) located above the substrate 4. The image to be filtered is filtered by the image stored in the FLC and differences between the images may be detected quantitatively by monitoring the current flowing across the amorphous silicon layer 3 when a suitable potential difference is applied across the electrodes 2 and 5 with a polarity such as not to alter the image stored in the FLC.

The image stored in the FLC is the optical inverse of a filtering image with respect to which novelty is to be detected. Thus, light or transparent regions of the stored image correspond to dark or opaque regions of the filtering image. The amorphous silicon layer 3 thus receives light only in those regions where the image to be filtered is light and the filtering image is dark. In order to detect changes from light to dark, both images may be inverted and the process repeated. The device therefore detects differences or changes between the images.

The position of single changes can be detected by providing separate "X" and "Y" electrodes attached to the amorphous silicon layer 3, where the ratio of the current received by the two electrodes indicates the position of the change. Position detection of multiple changes may be achieved by providing a degree of pixellation of the amorphous silicon layer 3.

The device shown in Figure 2 differs from that shown in Figure 1 in that, instead of using a pleochroic dye in the FLC 6 as a polariser, a separate metal polariser 7 is provided between the FLC and the amorphous silicon layer 3. The metal polariser may comprise elongate fingers as described hereinafter.

In order to provide pixellated detection, the devices shown in Figures 1 and 2 may be modified by replacing the amorphous silicon layer 3 with a detector array. Such an arrangement has the advantage that the detector array can provide a direct indication of the position of the or each change, while permitting parallel read-in of the image to be stored.

Figure 3 shows a device which combines the use of an amorphous silicon layer 3 with a detector array 8. The device differs from that shown in Figure 1 in that the glass substrate 1 is replaced by a semiconductor substrate 9 on which is formed the array of detectors 8.

The upper surface of the substrate 9 is polarised and the electrode 5 is formed thereon.

In use, the image to be stored is read-in in parallel by means of the amorphous silicon layer 3. The array of detectors 8 then detects changes between the image to be filtered and the filtering image. The detectors 8 are "pixellated" and thus allow the positions of detected changes to be indicated directly.

Instead of using the amorphous silicon layer to read-in the image in parallel, other geometries are possible where charge is injected directly from each detecting pixel. For instance, each detector pixel may be associated with a modulating pad whose voltage is varied according to the state of the associated detector pixel. Alternatively, the image may be read-in serially after detection by reading the image data from the detector pixels into a memory and then reading the image into the liquid crystal using a standard electronic matrix addressing scheme. As another alternative, the amorphous silicon layer may be disposed between the liquid crystal/polariser and the detector array. The quantity of change in an image may then be monitored by operating the amorphous silicon in detection mode. Any subsequent read-out of the difference image recorded on the detector array may then be made contingent on the size of the quantity of change, which thus acts as a "trigger" signal.

An application of the use of the "difference image" will now be described with reference to Figures 4a-4d. Integrated circuits need to be tested for faults, for instance by comparison with a perfect integrated circuit or chip. This example of chip testing is a specific case of the more general application of the comparison process, where faults or changes in a well-defined scene are detected. Rapid visual inspection of the complex metal pattern on a particular layer of a VLSI chip is required, with high spatial resolution, for example to 50 microns.

A small region of a chip to be tested is shown in Figure 4a, in which dark areas represent the presence of metal. The corresponding region of a perfect chip is shown in Figure 4b. The purpose of the inspection is to test for short circuits, caused by an excess of metal, or open circuits, caused by a lack of metal. In this particular case, there is an open circuit in the bottom right square. For angled illumination of the chip, i.e. dark field illumination, a short circuit is identifiable as a dark pixel, and an open circuit as a light pixel, each pixel corresponding to a square of the region shown. The image data corresponding to the perfect chip of Figure 4b are inverted and either loaded electrically or read-in optically and stored in the FLC, as shown in Figure 4c. The chip under test is illuminated to produce an optical beam represented as "TEST" in Figure 4c, aligned with the stored image on the FLC. Thus the test image is preprocessed in the FLC by the stored inverted perfect image, and the optical beam emerging represents the difference image, for detection by the CCD layer. If the chip under test were perfect, then no signal would be received by the detector layer. However, in this example, the open circuit causes an unusual bright pixel, shown in Figure 4d, which is detected by the detector layer. The presence of this bright pixel is detectable, as is its address.

Other operations may be performed depending on the exact geometry of the novelty filter. For instance, for a device having amorphous silicon between the liquid crystal and the detector array, the "size" of the difference can be monitored before deciding whether to read-out a difference image.

In effect, the stored perfect image acts as a parallel exclusive OR (XOR) plane for electrical open circuits. Electrical short circuits are not detected, however, as the FLC plane acts as a blocking plane for areas where there is no metal in the perfect chip. Accordingly, the process is repeated in a second cycle, using bright field illumination of the test chip, with the stored image reversed, to provide the necessary information for electrical short circuits. Clearly, this type of testing under conditions of strict spatial, size and orientation constraints can be extended to other areas of interest. In a further embodiment now to be described with reference to Figure 5, the polariser/electrode layer is combined in the same plane as the detector layer. This combined layer is an interdigitated metal-semiconductor-metal detector layer (IMSM), in which the photosensitive semiconductor portions replace the regular diodes of the CCD structure described above. One pixel of this layer is shown in Figure 5, and consists of counter electrodes 16 and 18 connected respectively to elongate conductors 17 and 19 which are parallel and interdigitated. The comb structure of the interdigitated conductors constitutes an optical polariser and an electrical modulator pad for the liquid crystal layer to which it is adjacent. The voltages +V and -V on each pixel are varied according to its intended use.

When an image is being read, voltages are applied across the electrodes 16, 18 of each pixel which is addressed: the pixels may be addressed in series, to achieve shuttering, or in parallel. When a voltage is applied across the electrodes, photogenerated carriers are collected at each electrode to generate a current. Once the image has been detected, an appropriate voltage is applied to both electrodes 16, 18 of the detector, which then acts instead as a modulator pad, with a uniform electrical potential on the elements 17, 19 of the comb. The FLC and IMSM arrays then act as a novelty filter as described above, with the metal fingers acting as an optical polariser. Any difference image data can be obtained, for example by clocking the data from the pixels of the detector array, using conventional techniques.

All of the devices described above can be used for motion detection, using the device as a camera which records only changes in a scene.

This is useful for compressing information to meet constraints of a limited bandwidth for communications purposes, or storage constraints particularly if the stored time sequence has to be reviewed at a later time. The devices can also be used to detect the location of edges in an object, since edges represent the positions at which an image changes from dark to light or light to dark.

Edge detection apparatus is shown in Figure 6, which represents another embodiment of the invention. In this example, there is no longer a 1:1 correspondence between the pixels of the liquid crystal array and the pixels of the detector array. In this particular example, there are two liquid crystal pixels adjacent every detector pixel, thus doubling the number of liquid crystal pixels in one dimension. In an alternative example, there are four liquid crystal pixels for each detector pixel, so that the number is doubled in both rows and columns. Since the liquid crystal pixels are separately controllable to modulate their transmissivity, it is possible effectively to shutter i.e. to alternative dynamically, the optical beam which reaches each individual pixel of the detector plane, as shown in Figure 6 in the case of two pixels A' and B' adjacent a single detector pixel AB.

Adjacent points A and B of an object 20 in the far field are brought by a lens 21 to focus on the same detector element AB, but through different modulating pixels A' and B'. First the input scene is recorded on the detector plane with only the upper pixel of each pixel pair in a transmitting state and the lower pixels attenuating. This would record part A of the object 20 on the detector element AB. This first image is then applied electronically to the modulator pads for the lower pixel of each pixel pair, so that this lower group of pixels will act as a preprocessing plane, modulated with the first image. With the upper pixel group acting as a shutter, in the attenuating state, the same object 20 is imaged again; the portion B would be imaged through pixel B' of the liquid crystal array. The difference image then reaches the pixels of the detector array. Thus it is only where adjacent points A and B are different that there would be a corresponding signal from the relevant pixel of the detector array. The information reaching the detector array is therefore due solely to points of the input scene 20 which are adjacent and are different from the original input, i.e. the detector array receives only edge information. In order to detect all the edges, a second cycle of this process would be necessary, in order to detect changes in the opposite sense (dark to light edges instead of light to dark edges, for example).

Devices similar to that of Figure 6 can be used in a wide range of optical shuttering applications. High speed shuttering of an underlying sensor array such as a CCD is possible, and is described more fully in our copending patent application referred to above. Super resolution shuttering is possible, in which there may be larger numbers of liquid crystal pixels for each detector pixel, the number being determined, in relation to the available switching speed, in accordance with a trade-off between spatial and temporal multiplexing. In this way, the resolution of the image can be adjusted quickly according to time-varying requirements of this optical system. As also described in our co-pending patent application, the use of microlens arrays integrated with the electro-optical device increases the sensitivity of the device by concentrating incoming light onto the detector areas.

In the devices described above, the liquid crystal array is of FLC, mainly because of its advantages of non-volatility. However, some applications do not require non-volatility, and liquid crystals of more conventional structure can be used, for shuttering or for other applications.

Devices embodying the invention can be used for image processing in constrained geometries, for example in visual testing as part of quality control in manufacturing processes; security recognition devices; novelty filtering for image compression for example for video telephony; and professional video applications where high speed shuttering and variable resolution are useful.

Claims (25)

CLAIMS:

1. An integrated electro-optical device comprising a liquid crystal optical modulation layer and means for absrobing polarised light superimposed over an electronic detector layer.

2. A device according to Claim 1, in which the liquid crystal optical modulation layer and means for absorbing polarised light are responsive to electric control signals to vary absorption of polarized light.

3. A device according to Claim 2, in which the liquid crystal layer and means for absorbing polarised light are of the guest-host type using anisotropic dye.

4. A device according to Claim 1, in which the absorbing means is a polariser and is combined with the electronic detector layer.

5. A device according to Claim 4, in which the combined polarisation and detection layer comprises an array of interdigitated metal-semiconductor-metal detectors whose electrically conductive elements are structured to effect the optical polarisation.

6. A device according to Claim 4, in which the combined polarisation and detection layer comprises an optical polarisation layer adjacent the liquid crystal layer, and an adjacent detection layer.

7. A device according to Claim 6, in which the optical polarisation layer comprises an array of electrodes each of which has electrically conductive elements structured to effect the optical polarisation.

8. A device according to any one of the preceding claims, in which the detector layer comprises a single planar detector.

9. A device according to Claim 8, in which the detector comprises an amorphous silicon layer.

10. A device according to any one of Claims 1 or 7, in which the detector layer comprises a plurality of detectors.

11. A device according to Claim 10, in which the detectors are charge coupled devices.

12. A device according to any one of the preceding claims, in which the liquid crystal layer is pixellated by patterned electrodes.

13. A device according to Claim 12, in which for each of the detectors there is at least one corresponding adjacent pixel of the liquid crystal layer.

14. A device according to Claim 5 or 7, in which the electrically conductive elements are elongate and are arranged in parallel as a comb.

15. A method of reading an image previously stored in the liquid crystal optical modulation layer of a device according to any one of Claims 1 to 11, comprising transmitting a polarised optical beam through the liquid crystal optical modulation layer so that the emergent beam is detected by the electronic detection layer after being analyzed by the absorbing means.

16. A method according to Claim 15, in which the beam is modulated in accordance with a test image before it is transmitted through the liquid crystal optical modulation layer, thereby detecting the difference between the test image and the stored image.

17. A method according to Claim 16, in which the image is stored by optically imaging an object onto the liquid crystal optical modulation layer, detecting it and thereby storing it in the liquid crystal optical modulation layer.

18. A method according to Claim 16, in which the image is stored by electronically storing data representing the image and addressing the liquid crystal optical modulation layer with those data to store the image therein.

19. A device according to Claims 12 or 13, in which the electronic detector layer comprises a pixellated opto-electronic detector array, in which there is a plurality of pixels of the liquid crystal layer for each pixel of the detector array.

20. An imaging apparatus comprising a device according to Claim 19, and means for addressing the pixels of the liquid crystal layer as groups, each group consisting of one pixel of the said plurality for each pixel of the detector array, and electronic control means for selectively making the groups transmissive or absorptive so as to simulate linked optical shutters over all the pixels of the detector array.

21. An apparatus according to Claim 20 for detecting edges in an object or for detecting motion of an object by taking a sequence of images of the object, comprising optical apparatus for imaging the object in the plane of the liquid crystal layer.

22. A method of detecting the positions of edges in an object, using a device according to Claim 20 or 21, comprising controlling different groups of pixels of the liquid crystal layer to produce in sequence different images of the object spaced in the object plane in accordance with the spacing between the pixels of the said plurality of pixels, the method comprising detecting a first such image, storing that image in another group of pixels, transmitting a second image through the said other group of pixels already modulated in accordance with the first image, and reading the emergent light, which is representative of the difference between the first and second images, onto the detector array.

23. A method of detecting motion of an object using a device according to Claim 20 or 21, comprising controlling different groups of pixels of the liquid crystal layer to produce in sequence different images of the object, the method comprising detecting a first such image, storing that image in another group of pixels, transmitting a second image of the object through the said other group of pixels already modulated in accordance with the first image, and reading the emergent light, which is representative of the difference between the first and second images, onto the detector array.

24. A device substantially as described herein with reference to Figure 1, or Figure 2, or Figure 3, or Figure 5, or Figure 6, of the accompanying drawings.

25. A method of image processing, substantially as described herein with reference to Figure 1, or Figure 2, or Figure 3, or Figures 4aNd, or Figure 5, or Figure 6 of the accompanying drawings.