GB1604206A - Ac driven liquid crystal light valve - Google Patents

Description

(54) AC DRIVEN LIQUID CRYSTAL LIGHT VALVE (71) We, HUGHES AIRCRAFT COMPANY, a corporation organized and existing under the laws of the State of Delaware, United States of America, of Centinela and Teale Street, Culver City, State of California, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The invention relates to a crystal light valve.

According to the present invention there is provided a liquid crystal light valve comprising first and second electrode layers between which are disposed a liquid crystal layer, a semiconductor body, an interface layer arrangement, and a dielectric layer, the interface layer arrangement being between the semiconductor body and the liquid crystal layer, and the dielectric layer being disposed between the interface layer arrangement and the semiconductor body, a voltage source arranged to apply a cyclic voltage waveform to the electrodes, and means for providing in the semiconductor body a spatial distribution of charge carriers, the dielectric layer cooperating with the electrodes so that during a portion of each cycle of said voltage waveform a depletion region extends at least substantially throughout said semiconductor body in such a manner that the spatial distribution of charge carriers is swept across the semiconductor body to be collected at the dielectric layer to activate the liquid crystal layer and provide a spatial representation of said charge distribution.

In one embodiment of this invention a silicon wafer is used as the semiconductor body, but other semiconductor materials can also be used.

The invention has the advantage that the spatial distribution of charge (representing information signals) is swept across the semiconductor body without substantial lateral spreading of charge carriers due to diffusion, as a consequence of the depletion region being established in the semiconductor body. Also, the light valve is AC driven, which improves the electrochemical stability of the liquid crystal layer. The signal representing charge carrier distribution can be introduced in the semiconductor body by optical images, X-rays, high energy electrons or brought in by a charge coupled device (CCD) arrangement.

The spatial resolution which is provided by means of field focusing during the depletion phase of the applied AC voltage can be further enhanced by an additional focusing arrangement which takes the form of a microgrid in the semiconductor body, the microgrid being defined by a dopant selected such that during said portion of the cyclic waveform the microgrid structure is depleted of mobile charge carriers so as to establish a concentration of immobile carriers which extend a focussing effect on said spatial distriburtion of charge carriers being swept to said dielectric layer.Thus, this microgrid forms on the substrate an array of resolution cells, which when depleted has a higher concentration of immobile charge carriers than the adjacent parts of the semiconductor body (because of its higher impurity concentration) and performs its focusing function by repelling the signal carriers towards the centers of the cells.

The invention will be described hereinafter by way of example and with reference to the accompanying drawings wherein: Figure I is a diagrammatic cross-sectional view of an MOS AC silicon liquid crystal light valve structure constructed in accordance with the present invention.

Figure 2 is a diagrammatic cross-sectional view of a CCD driven MOS AC silicon liquid crystal light valve with a microgrid constructed in accordance with the present invention.

Figure 3 and 4 show portions of two embodiments of a light valve according to the invention.

Figure 5 is a diagrammatic top view of the focusing microgrid on a liquid crystal light valve array.

Figure 6a-6d are waveforms representing the voltage bias supplies, the current through the liquid crystal layer and the voltage across the liquid crystal layer for a light valve according to the invention and with the microgrid structure for improved resolution.

Referring now to Figure 1, the shown alternating current (AC) liquid crystal light valve includes a high resistivity silicon substrate 10 on a surface of which there is an SiO2 gate insulator 12 followed by a light blocking layer 14, a dielectric mirror 16, a liquid crystal 18 and a transparent counterelectrpde 20. On the opposite side of substrate 10, there is a thin electrode 22, an SiO2 passivating layer 24, and an aluminum contact pad 25. A voltage source 53 is connected between contact pad 25 and counterelectrode 20. The silicon substrate 10 of the device illustrated in Figure 1 is p type with a typical resistivity higher than 1 KQ-cm. A high resistivity p semiconductor body is often designated wuith the symbol z while a high resistivity n material is often designated with the symbol v.Although a material is used for the illustration of Figure 1, the invention can be practiced equally well with a v type material except that the voltage and current polarities will have to be reversed. The structure 10 may have a thickness on the order of 3-10 mil. and should be chemomechanically polished on both sides to provide parallel, optically flat surfaces on each side of the substrate. On the substrate side, closest to the SiO2 layer, there are isolation channel stops 26 to isolate the active z region from the deleterious effects of minority carrier generation at the outside surfaces of the substrate and from the inversion charge typically present at SiO2 interfaces on p-type silicon.These channel stops are p+ regions, for a joe type substrate and they are formed by the heavy doping of predetermined regions of the substrate with a p-type impurity using a gaseous diffusion or ion implantation. The thin contact electrode 22 is formed on the silicon substrate through heavy doping and it is p+ for a joe type substrate. The purpose of this layer is to provide a uniform ohmic contact to the back of the substrate so that the electrical potential of the entire back surface of the substrate and at points inside the substrate can be raised or lowered in accordance with the applied bias voltage.

However, this layer must be transparent to light so that photons can pass without being appreciably absorbed. To meet this requirement, it is necessary to make the layer optically thin, i.e. of a thickness less than 1/absorption coefficient for photon wavelengths within the spectral range of 0.4 S X S 1.0 llm. In addition, in order that this layer provide a uniform electrical contact, it is preferable that it be degeneratively doped. The passivating layer 24 is typically a thin SiO2 layer to protect the surface of the silicon substrate. On the opposite side of the substrate 10, there is a gate dielectric layer 12 which is typically an SiO2 layer. This SiO2 layer 12 is the dielectric layer of an MOS capacitor formed on substrate 10.Adjacent this gate dielectric layer 12, there is a cermet light blocking layer 14 that is used in this light valve to absorb any projection light that may be transmitted due to the finite reflectivity of the dielectric mirror 16. In CdS light valves, there is a CdTe layer to provide the same function that cermet layer 14 provides for the light valve of Figure 1.

However, CdTe is inadequate for the light valve of Figure 1 because it absorbs light with a wavelength of less than 0.85 Fm, while the silicon substrate remains photosensitive to approximately 1.1 llm which is in the near infrared region of the optical spectrum. Cermet layer 14 consists of a stack of alternating metallic particles (such as, for example, Sn, In, Pb) and dielectric layers (such as, for example, Al203). The metallic particles when deposited in relatively thin films, will coagulate into a dense array of small metal islands which are electrically noncontinuous but they still retain their metallic optical properties at the wavelengths of visible light. The use of alternating layers of these metal drops separated by an insulator reduces even further the electrical conduction in the plane of a given film and permits the capacitive coupling of charge between the metal islands of adjacent films. If the spacing between these metallic islands is relatively large in comparison to the thickness of the insulating films, then the impedance (both DC and AC) for in plane charge transfer will be much greater than that between planes thereby producing an anisotropically conductive medium with the optical properties of a metal. When many alternating films are incorporated into a single layer, the opacity of the combined films is further enhanced by the multiple light scatterings from the many randomly situated metal islands.Since the islands are metallic and absorb light by the excitation of free electrons, this cermet layer is intended to provide low optical transmissivity over a wider spectral range than that achievable with semi-insulators (e.g., CdTe) since the latter are limited by the width of their forebidden band gap. Since the device of Figure 1 of the present application is designed for use with alternating current, it is not necessary that the cermet layer has a low DC conductivity perpendicular to the layer plane.

Layer 16 is a dielectric mirror such as a TiO2/SiO2 mirror. Region 18 is a liquid crystal electro-optic layer and regions 19 are dielectric spacers, typically SiO2, for the support of the liquid crystal. Counter electrode 20 is transparent to light and adjacent the liquid crystal layer 18.

This light valve utilizes the electro-optical properties of nematic liquid crystals in either the 45" twisted-nematic or the birefringent color-switching mode configurations to achieve optical modulation of the projected output light beam. Both of these phenomena manifest a voltage threshold VT below which the molecules of this layer retain their initial alignment parallel or perpendicular with respect to the electrode surfaces. If the rms voltage across the liquid crystal exceeds VT then the molecules will re-orient to an extent determined by the magnitude of the dielectric anisotropy for the particular liquid crystal used and the magnitude of the excess voltage (VLC - VT).

This effect induces an optical birefringence which can easily be used to retard the phase of a polarized light beam and thereby produce a color/intensity modulation in proportion to (VLC - VT).

In a particular silicon MOS light valve, a p-type silicon substrate with a resistivity of approximately 40KQ-cm, and a thickness of approximately 15mm was used. An SiO2 layer of 1000 A was formed on a surface of the substrate through thermal oxidation for the gate dielectric, a p+ back contact was formed using a boron diffusion step. Since a He-Ne layer beam was used to project the light valve output, a red-tuned, seven air (SiO2/TiO2) dielectric mirror was used to optically isolate the silicon substrate from the readout beam eliminating the need for a separate light blocking layer. A liquid crystal was aligned perpendicular to an electrode surface and then it was assembled with the remaining part of the light valve cell.

This device manifested both photosensitivity and good spatial resolution using green input light at intensities approximately 300 IlW/cm and various input voltage waveforms with frequencies between 100KHZ and 5 MHZ.

Referring now to Figure 2, there is shown a diagrammatic cross-sectional view of a CCD driven liquid crystal light valve. It receives a DC signal from the CCD and converts it into an AC signal (zero average charge) that activates the liquid crystal. It includes a transparent electrode 50, a liquid crystal layer 80, a multilayer mirror light blocking layer 120, a silicon dioxide layer (SiO2) 140, a high resistivity silicon (Si) wafer 160, a silicon epitaxial layer 180, a silicon dioxide layer 200, and a set of CCD electrodes 220. The combination of the multilayer mirror 100 and the light blocking layer 120 make up an interface means for the optical isolation of the liquid crystal from the light valve substrate and the CCD.

In the high resistivity silicon 160, or silicon substrate as it sometimes is referred to there is a microgrid structure 240 which is provided for the focusing of the signal charge carriers. This microgrid structure defines cell areas. Although this structure will be described with a p type silicon substrate, the invention is equally applicable with devices constructed with n type silicon substrates or substrates from other semiconductor materials using any type of conductivity determining impurity. In one particular example, the silicon substrate was p type with a < 100 > crystalographic orientation. The resistivity of the substrate can vary although for better resolution it would be preferable to be over 1 KQ-cm. Since this is a high resistivity p type material it is sometimes referred to as type Si.An AC power supply 260 is applied between transparent electrode 50 and the epitaxial layer 180.

During operation a CCD input register accepts serial input data, stores it and reformats it in subsequent parallel processing. This is done, for example, by having the CCD serial input register accept one line of information and after it is filled, the information is transferred in parallel into a CCD parallel array. Next, the serial register is filled with a new line of information while the first line of information is being shifted one step upward in the parallel array. Then the second line of information is transferred from the serial register into the first stage of the parrallel array. The same process is repeated until the parallel array contains an entire frame of information.Then the entire frame of information is transferred simultaneously through the light valve for a temporary storage into the liquid crystal layer where it is used to spatially modulate a laser readout beam. The light valve operates in an AC mode using an insulating SiO2 layer to prevent any DC current component from flowing through the structure and it electrically functions like an MOS capacitor.

The voltage waveform applied to this capacitor is preselected so that most of the time (as is shown in Figure 6, discussed at a later section) the SiO2 side 140 is biased positively with respect to the grounded epitaxial layer 180. For the remaining portion of the cycle the SiO2 layer 140 is grounded the power supply frequency is preselected to be the same as the frame frequency. During the positive part of the cycle the z region 160 is fully depleted and so is the microgrid structure 240. The depletion region also penetrates into a small portion of the epitaxial layer 180, with this penetration being deeper in the area between the microgrid regions. The regions of the microgrid act like buckets of immobile negative charge which repel the signal electrons.

Therefore, the mlcrogrid acts as a focusing grid to force the electrons toward the center of the region between the doped regions.

Charge carriers that are released from the control of the CCD electrodes diffuse into a portion of the epitaxial layer and they are then swept by the electric field across the depleted portion of the epitaxial layer, the entire thickness of the Jc-type substrate and reach the Si-SiO2 interface in the regions between the microgrid. Therefore, the presence of this charge changes the voltage drop across the liquid crystal, thus activating it. The charge carriers are stored at the SiO2/Si interface region between the microgrid until the bias on the SiO2 layer disappears which results in a collapse of the depletion region, at which time the excess minority carriers (which are electrons in this case) diffuse into the r: region, where they recombine during the course of the remaining portion of the bias cycle.

Referring now to Figure 3, there is shown a diagrammatic cross-sectional view of a portion of the light valve shown in Figure 2, that includes the light valve substrate with a CCD structure that provides the activating input signals. It includes a high resistivity z type Si wafer 160 with an SiO2 layer 140 on one side and a P type Si epitaxial layer 180 on the other. Next to the epitaxial layer 180 there is an SiO2 layer 200 on which there are CCD electrodes 220. A power supply 260 is connected across the epitaxial layer 180 and a transparent electrode 50.

Referring now to Figure 4, there is shown a diagrammatic cross-sectional view of a portion of a light valve that is photoactivated. It includes a high resistivlty z type silicon substrate 160 on one side of which there is an SiO2 layer 140 and in the substrate adjacent the SiO2 layer there is microgrid 240. On the other side of the substrate there is a transparent electrode 280 which is formed by the heavy doping (p+ region) of a relatively thin portion of the substrate with a p type impurity and adjacent to electrode 280 there is thin transparent SiO2 layer 300. A power supply 260 is connected between transparent electrode 50 and transparent electrode 280.Writing light 320 penetrates through the SiO2 and p+ layers 300 and 280 respectively and reaches to the sl type silicon substrate 160 which is sensitive to received radiation. In all other respects the operation of a photo-activated liquid crystal light valve is similar to the CCD driven liquid crystal light valve.

Referring now to Figure 5, there is shown a diagrammatic top view of a wafer for a liquid crystal light valve display with emphasis on the relative position of the microgrid regions or microchannel stops as they are sometimes referred to. A moderately doped grid structure 240 is formed in a silicon wafer 160 to define an array of resolution cells 420 which are Je type silicon. A p+ channel stop 360 provides a type of field isolation. The microchannel grid 240 is formed either by ion implantation or diffusion and has a smooth surface for good liquid crystal alignment. The doping level and the depth of the grid are such that it is depleted during normal operating conditions. Since the grid is like buckets of immobile negative charge that repel electrons the transferred electrons are focused to the center of the cell.

Referring now to Figures 6a-6d, there are shown voltage and current waveforms and an equivalent circuit that illustrates the operation of a light valve constructed in accordance with this invention. In Figure 6a there is shown a power supply waveform applied by power supply 260 of Figure 2.

The voltage is 0 during time T2 (accumulation phase) and V (typically 50-100 volts) during time T1 (depletion phase). T1 is selected to be much larger than T2. The liquid crystal current is proportional to the derivative of the voltage and has a waveform as shown in Figure 6b. The current pulses located between the power supply pulses are the signal pulses. Assuming that the liquid crystal layer 80 has an electrical equivalent circuit as shown in Figure 6c with an RC constant approximately equal to 5 msec and assuming that T2 is 5 msec, then the voltage across the liquid crystal has the form presented in Figure 6d.

WHAT WE CLAIM IS: 1. A liquid crystal light valve comprising first and second electrode layers between which are disposed a liquid crystal layer, a semiconductor body, an interface layer arrangement, and a dielectric layer, the interface layer arrangement being between the semiconductor body and the liquid crystal layer, and the dielectric layer being disposed between the interface layer arrangement and the semiconductor body, a voltage source arranged to apply a cyclic

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. The voltage waveform applied to this capacitor is preselected so that most of the time (as is shown in Figure 6, discussed at a later section) the SiO2 side 140 is biased positively with respect to the grounded epitaxial layer 180. For the remaining portion of the cycle the SiO2 layer 140 is grounded the power supply frequency is preselected to be the same as the frame frequency. During the positive part of the cycle the z region 160 is fully depleted and so is the microgrid structure 240. The depletion region also penetrates into a small portion of the epitaxial layer 180, with this penetration being deeper in the area between the microgrid regions. The regions of the microgrid act like buckets of immobile negative charge which repel the signal electrons. Therefore, the mlcrogrid acts as a focusing grid to force the electrons toward the center of the region between the doped regions. Charge carriers that are released from the control of the CCD electrodes diffuse into a portion of the epitaxial layer and they are then swept by the electric field across the depleted portion of the epitaxial layer, the entire thickness of the Jc-type substrate and reach the Si-SiO2 interface in the regions between the microgrid. Therefore, the presence of this charge changes the voltage drop across the liquid crystal, thus activating it. The charge carriers are stored at the SiO2/Si interface region between the microgrid until the bias on the SiO2 layer disappears which results in a collapse of the depletion region, at which time the excess minority carriers (which are electrons in this case) diffuse into the r: region, where they recombine during the course of the remaining portion of the bias cycle. Referring now to Figure 3, there is shown a diagrammatic cross-sectional view of a portion of the light valve shown in Figure 2, that includes the light valve substrate with a CCD structure that provides the activating input signals. It includes a high resistivity z type Si wafer 160 with an SiO2 layer 140 on one side and a P type Si epitaxial layer 180 on the other. Next to the epitaxial layer 180 there is an SiO2 layer 200 on which there are CCD electrodes 220. A power supply 260 is connected across the epitaxial layer 180 and a transparent electrode 50. Referring now to Figure 4, there is shown a diagrammatic cross-sectional view of a portion of a light valve that is photoactivated. It includes a high resistivlty z type silicon substrate 160 on one side of which there is an SiO2 layer 140 and in the substrate adjacent the SiO2 layer there is microgrid 240. On the other side of the substrate there is a transparent electrode 280 which is formed by the heavy doping (p+ region) of a relatively thin portion of the substrate with a p type impurity and adjacent to electrode 280 there is thin transparent SiO2 layer 300. A power supply 260 is connected between transparent electrode 50 and transparent electrode 280.Writing light 320 penetrates through the SiO2 and p+ layers 300 and 280 respectively and reaches to the sl type silicon substrate 160 which is sensitive to received radiation. In all other respects the operation of a photo-activated liquid crystal light valve is similar to the CCD driven liquid crystal light valve. Referring now to Figure 5, there is shown a diagrammatic top view of a wafer for a liquid crystal light valve display with emphasis on the relative position of the microgrid regions or microchannel stops as they are sometimes referred to. A moderately doped grid structure 240 is formed in a silicon wafer 160 to define an array of resolution cells 420 which are Je type silicon. A p+ channel stop 360 provides a type of field isolation. The microchannel grid 240 is formed either by ion implantation or diffusion and has a smooth surface for good liquid crystal alignment. The doping level and the depth of the grid are such that it is depleted during normal operating conditions. Since the grid is like buckets of immobile negative charge that repel electrons the transferred electrons are focused to the center of the cell. Referring now to Figures 6a-6d, there are shown voltage and current waveforms and an equivalent circuit that illustrates the operation of a light valve constructed in accordance with this invention. In Figure 6a there is shown a power supply waveform applied by power supply 260 of Figure 2. The voltage is 0 during time T2 (accumulation phase) and V (typically 50-100 volts) during time T1 (depletion phase). T1 is selected to be much larger than T2. The liquid crystal current is proportional to the derivative of the voltage and has a waveform as shown in Figure 6b. The current pulses located between the power supply pulses are the signal pulses. Assuming that the liquid crystal layer 80 has an electrical equivalent circuit as shown in Figure 6c with an RC constant approximately equal to 5 msec and assuming that T2 is 5 msec, then the voltage across the liquid crystal has the form presented in Figure 6d. WHAT WE CLAIM IS:

1. A liquid crystal light valve comprising first and second electrode layers between which are disposed a liquid crystal layer, a semiconductor body, an interface layer arrangement, and a dielectric layer, the interface layer arrangement being between the semiconductor body and the liquid crystal layer, and the dielectric layer being disposed between the interface layer arrangement and the semiconductor body, a voltage source arranged to apply a cyclic

voltage waveform to the electrodes, and means for providing in the semi-conductor body a spatial distribution of charge carriers, the dielectric layer cooperating with the electrodes so that during a portion of each cycle of said voltage waveform a depletion region extends at least substantially throughout said semi-conductor body in such a manner that the spatial distribution of charge carriers is swept across the semiconductor body to be collected at the dielectric layer to activate the liquid crystal layer and provide a spatial representation of said charge distribution.

2. A liquid crystal light valve according to claim 1 wherein said interface layer arrangement includes a dielectric mirror arranged to reflect light incident on the liquid crystal layer after passage of the incident light through the liquid crystal layer, and a light blocking layer for blocking any of said incident light which passes through the mirror from reaching the semiconductor body.

3. A liquid crystal light valve according to claim 1 or 2 wherein said charge distribution providing means comprises a charge coupled device formed in said semiconductor body.

4. A liquid crystal light valve according to claim 3 wherein said charge coupled device includes an array of charge transfer electrodes formed in an epitaxial layer in said semiconductor body, the epitaxial layer constituting one of said electrode layers.

5. A liquid crystal light valve according to claim 1 or 2 wherein said semiconductor body is photosensitive such as to provide said spatial charge distribution in response to an incident pattern of light.

6. A liquid crystal light valve according to any preceding claim including a microgrid structure formed in the semiconductor body adjacent said dielectric layer, said microgrid structure being defined by a dopant selected such that during said portion of the cyclic waveform the microgrid structure is depleted of mobile charge carriers so as to establish a concentration of immobile carriers which extend a focussing effect on said spatial distribution of charge carriers being swept to said dielectric layer.

7. A liquid crystal light valve substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

8. A liquid crystal light valve substantially as hereinbefore described with reference to Figure 2 or 3 or 4 of the accompanying drawings.