Patent Description:
Scanning systems are commonly used to form an image or pattern in a media or an imaging plane for displays printing, three-dimensional (3D) printing, metal engraving, and selective laser melting.

Referring to <FIG>, scanning systems <NUM> generally include a spatial light modulator (SLM <NUM>) to modulate light from a light source <NUM>, a collimate lens <NUM> to form and direct multiple parallel beams from the SLM onto a scan mirror <NUM>, and an imaging lens <NUM> to magnify and project modulated light from the scan mirror onto an imaging plane <NUM>. The scan mirror <NUM> generally rotates around two axes to scan the modulated light over the imaging plane <NUM> to form an image or pattern.

One problem with conventional scanning systems, particularly those used to form an image or pattern on a two-dimensional (2D) imaging plane using multiple beams from a SLM <NUM>, is that due to characteristics of imaging lenses used in these systems off-axis beams of multiple beams cannot scan parallel resulting in a distortion error that cannot be compensated for by increasing or controlling the speed of the scan mirror. That is a beam which has angle θ against an optical axis <NUM> of the imaging lens <NUM> settles at a position that is at a distance of fθ from a center of a swath or image <NUM> of the SLM <NUM> projected onto the imaging plane <NUM>. This distance from the center of the image <NUM> increases as a function of the angle θ moving outward along the x-axis resulting in stretched or elongated, distorted image along the x-axis. Furthermore when the scan mirror is also rotated along a y-axis there is also distortion along the y-axis. Referring to <FIG>, a dashed line <NUM> represents the desired or ideal location of a grid of images <NUM> of the SLM <NUM> projected and scanned along the x-axis on the imaging plane <NUM> while solid black lines <NUM> indicates a grid of the actual location of images of the SLM.

Accordingly, there is a need for a scanning system and a method for operating the same to compensate for distortion due to multi beam scanning optics.

A multi-beam scanning system providing scanning in a plurality of linear swaths is disclosed in <CIT> B <NUM>. Another multi-beam scanning system is disclosed in <CIT>.

A multi-beam scanning system and a method of operating the same to compensate for distortion are provided.

Embodiments of the present invention will be understood more fully from the detailed description that follows and from the accompanying drawings and the appended claims provided below, where:.

Embodiments of scanning systems including MEMS-based spatial light modulators (SLMs) and multi-beam scanning optics, and methods for operating the same to compensate for distortion are disclosed. In the following description, numerous specific details are set forth, such as specific materials, dimensions and processes parameters etc. to provide a thorough understanding of the present invention. However, particular embodiments may be practiced without one or more of these specific details, or in combination with other known methods, materials, and apparatuses. In other instances, well-known semiconductor design and fabrication techniques have not been described in particular detail to avoid unnecessarily obscuring the present invention. Reference throughout this specification to "an embodiment" means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase "in an embodiment" in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.

The terms "over," "under," "between," and "on" as used herein refer to a relative position of one layer with respect to other layers. As such, for example, one layer deposited or disposed over or under another layer may be directly in contact with the other layer or may have one or more intervening layers. Moreover, one layer deposited or disposed between layers may be directly in contact with the layers or may have one or more intervening layers. In contrast, a first layer "on" a second layer is in contact with that second layer. Additionally, the relative position of one layer with respect to other layers is provided assuming operations deposit, modify and remove films relative to a starting substrate without consideration of the absolute orientation of the substrate.

<FIG> is a schematic block diagram of multi-beam scanning system capable of being operated to compensate for distortion according to an embodiment of methods of the present disclosure. Referring to <FIG>, the system <NUM> generally includes a spatial light modulator (SLM <NUM>) to modulate light from a light source <NUM>, such as a laser, illumination optics <NUM>, which can include numerous elements such as lens integrators, mirrors and prisms, designed to transfer light from the light source <NUM> to the SLM, a collimate lens <NUM> to direct multiple beams of modulated light from the SLM onto a scan mirror <NUM> and a imaging lens <NUM> to project modulated light onto a one or two dimensional imaging plane <NUM>. Generally, as in the embodiment shown the scan mirror <NUM> is capable of movement along at least two non-parallel axes to form a substantially linear swath of illumination across a two-dimensional (2D) imaging plane <NUM>, and to scan the linear swath across the 2D imaging plane in a direction orthogonal to a long axis of the linear swath. The imaging lens <NUM> can include an f-theta (fθ) lens to provide a flat field at the imaging plane <NUM> of the scanning system <NUM>.

Additionally, the scanning system <NUM> further includes a controller <NUM> to control operation of the light source <NUM>, provide image data and drive signals to SLM <NUM> and to control the scan mirror <NUM>. As explained in greater detail below, the controller <NUM> is configured to provide image data to each of a number of SLM pixels in the SLM <NUM>, including providing compensated image data to at least some address pixels of the SLM pixels generating beams of modulated light beam distal from an optical axis of the imaging lens <NUM> to compensate for distortion along the long axis of the linear swath illuminated across the 2D imaging plane. Preferably, the scanning system <NUM> further includes a memory <NUM> coupled to or integrated with the controller <NUM> to store, for example in a lookup table, compensating data derived from an algorithm executed in the controller for each address pixel in the SLM <NUM>. Data derived from the algorithm and stored in the memory for each address pixel is combined or concatenated with image data with image data received for each SLM pixel to derive the compensated image data.

More preferably, the scanning system <NUM> further includes a dot clock or clock <NUM> coupled to or integrated with the controller <NUM> and the controller is further configured to delay drive signals to the address pixels generating beams of modulated light distal from an optical axis <NUM> of the imaging lens <NUM> as the linear swath is scanned across the 2D imaging plane in the direction orthogonal to the long axis of the linear swath to compensate for distortion along a long axis of the direction of the scan.

One type of MEMS based SLM suitable for use in a multi-beam scanning system according to an embodiment of the present disclosure is a ribbon-type SLM, such as a Grating Light Valve (GLV™), commercially available from Silicon Light Machines, in Sunnyvale CA.

An embodiment of a ribbon-type SLM will now be described with reference to <FIG>. For purposes of clarity, many of the details of MEMS in general and MEMS optical modulators in particular that are widely known and are not relevant to the present invention have been omitted from the following description. The dimensions and the relative dimensions may not correspond to actual reductions to practice of the invention.

Referring to <FIG> in the embodiment shown the SLM <NUM> includes a linear array <NUM> composed of thousands of free-standing, addressable electrostatically actuated ribbons <NUM>, each having a light reflective surface <NUM> supported over a surface of a substrate <NUM><NUM>. Each of the ribbons <NUM> includes an electrode <NUM> and is deflectable through a gap or cavity <NUM> toward the substrate <NUM> by electrostatic forces generated when a voltage is applied between the electrode in the ribbons and a base electrode <NUM> formed in or on the substrate. The ribbons <NUM> are driven by a drive channel <NUM> in a driver <NUM>, which may be integrally formed on the same substrate <NUM> with the array <NUM>.

A schematic sectional side view of a movable structure or ribbon <NUM> of the SLM <NUM> of <FIG> is shown in <FIG>. Referring to <FIG>, the ribbon <NUM> includes an elastic mechanical layer <NUM> to support the ribbon above a surface <NUM> of the substrate <NUM>, a conducting layer or electrode <NUM> and a reflective layer <NUM> including the reflective surface <NUM> overlying the mechanical layer and conducting layer.

Generally, the mechanical layer <NUM> comprises a taut silicon-nitride film (SiNx), and flexibly supported above the surface <NUM> of the substrate <NUM> by a number of posts or structures, typically also made of SiNx, at both ends of the ribbon <NUM>. The conducting layer or electrode <NUM> can be formed over and in direct physical contact with the mechanical layer <NUM>, as shown, or underneath the mechanical layer. The conducting layer or ribbon electrode <NUM> can include any suitable conducting or semiconducting material compatible with standard MEMS fabrication technologies. For example, the conducting layer <NUM> can include a doped polycrystalline silicon (poly) layer, or a metal layer. Alternatively, if the reflective layer <NUM> is metallic it may also serve as the conductive layer <NUM>.

The separate, discrete reflecting layer <NUM>, where included, can include any suitable metallic, dielectric or semiconducting material compatible with standard MEMS fabrication technologies, and capable of being patterned using standard lithographic techniques to form the reflective surface <NUM>. In the embodiment shown, a number of ribbons are grouped together under to form a large number of MEMS pixels <NUM> driven by a much smaller number of drive channels <NUM>.

Another type of MEMS-based optical modulator for which the distributed mirror of the present invention is particularly useful is a Planar Light Valve or PLV™ from Silicon Light Machines, Inc. Referring to <FIG> through 4D, a planar type light valve or PLV™ <NUM> generally includes two films or membranes having light reflecting surfaces of equal area and reflectivity disposed above an upper surface of a substrate (not shown in this figure). The topmost film is a static tent member or face-plate <NUM> of a uniform, planar sheet of a material having a first planar light reflective distributed mirror or reflector <NUM>, for example taut silicon-nitride covered on a top surface with one or more layers of material reflective to at least some of the wavelengths of light incident thereon. The face-plate <NUM> has an array of apertures <NUM> extending from the top distributed mirror <NUM> of the member to a lower surface (not shown). The face-plate <NUM> covers an actuator membrane underneath. The actuator membrane includes a number of flat, displaceable or movable actuators <NUM>. The actuators <NUM> have second planar distributed mirror or reflector <NUM> parallel to the first planar distributed mirror <NUM> of the face-plate <NUM> and positioned relative to the apertures <NUM><NUM> to receive light passing therethrough. Each of the actuators <NUM>, the associated apertures <NUM> and a portion of the face-plate <NUM> immediately adjacent to and enclosing the aperture form a single, individual modulator <NUM> or diffractor. The size and position of each of the apertures <NUM> are chosen to satisfy an "equal reflectivity" constraint. That is the area of the second distributed mirror <NUM> exposed by a single aperture <NUM> inside is substantially equal to the reflectivity of the area of the individual modulator <NUM> outside the aperture <NUM>.

<FIG> depicts a cross-section through two adjacent modulators <NUM> of the light valve <NUM> of <FIG>. In this exemplary embodiment, the upper face-plate <NUM> remains static, while the lower actuator membrane or actuators <NUM> move under electrostatic forces from integrated electronics or drive circuitry in the substrate <NUM>. The drive circuitry generally includes an integrated drive cell <NUM> coupled to substrate or drive electrodes <NUM> via interconnect <NUM>. An oxide <NUM><NUM> may be used to electrically isolate the electrodes <NUM>. The drive circuitry is configured to generate an electrostatic force between each electrode <NUM> and its corresponding actuator <NUM>.

Individual actuators <NUM> or groups of actuators are moved up or down over a very small distance (typically only a fraction of the wavelength of light incident on the light valve <NUM>) relative to first planar distributed mirror <NUM> of the face-plate <NUM> by electrostatic forces controlled by drive electrodes <NUM><NUM> in the substrate <NUM><NUM> underlying the actuators <NUM>. Preferably, the actuators <NUM> can be displaced by n* λ/<NUM> wavelength, where λ is a particular wavelength of light incident on the first and second planar distributed mirrors <NUM>, <NUM>, and n is an integer equal to or greater than <NUM>. Moving the actuators <NUM> brings reflected light from the second planar distributed mirror <NUM> into constructive or destructive interference with light reflected by the first planar distributed mirror <NUM> (i.e., the face-plate <NUM>), thereby modulating light incident on the light valve <NUM>.

For example, in one embodiment of the light valve <NUM> shown in <FIG>, the distance (D) between reflective layers of the face-plate <NUM> and actuator <NUM> may be chosen such that, in a non-deflected or quiescent state, the face-plate, or more accurately the first distributed mirror <NUM>, and the actuator (second distributed mirror <NUM>), are displaced from one another by an odd multiple of λ/<NUM>, for a particular wavelength λ of light incident on the light valve <NUM>. This causes the light valve <NUM> in the quiescent state to scatter incident light, as illustrated by the left actuator of <FIG>. In an active state for the light valve <NUM>, as illustrated by the right actuator of <FIG>, the actuator <NUM> may be displaced such that the distance between the distributed mirrors <NUM>, <NUM> of the face-plate <NUM> and the actuator <NUM> is an even multiple of λ/<NUM> causing the light valve <NUM> to reflect incident light.

As noted above, one problem with conventional scanning systems, particularly those used to form an image or pattern on a 2D imaging plane using multiple beams from a SLM, is that due to characteristics of fθ imaging lenses off-axis beams cannot scan parallel resulting in distortion. Referring to <FIG>, the area <NUM> represents a swath or projected image of a SLM <NUM> on the optical axis. Point or location 120a is the image of the n-th pixel of the SLM <NUM>. When the scanning mirror <NUM> rotates as to scan along the x-axis, trajectory of the n-th pixel which is off the x-axis does not move parallel because the location 120b is proportional to the angle against the optical axis <NUM>. The projected SLM image can also be scanned along the y axis and the point or location 120c represent the n-th pixel when the scanning mirror <NUM> rotates as to set n-th pixel at next to the (-n)-th pixel. The dashed line <NUM> represents the desired or ideal location of a grid (trajectory of the image of the SLM) projected on the imaging plane <NUM> while solid black lines <NUM> indicates the actual location. In this embodiment the cell size along the x axis of the grid correspond to the size of a projected single pixel and is same as resolution of the imaging system. The cell size along the y axis shows the pitch which SLM can be driven between on state and off state at constant time interval. The frequency of this constant time interval is referred to hereinafter as a "Dot clock. " During scanning the projected pixel on the x axis (<NUM>th pixel in this embodiment) moves at a constant speed and the pitch along the x axis is constant. However, for the pixels off the x axis the pitch is uneven. The distorted, actual location of a pixel (xn, yn) on the imaging plane <NUM> of a particular beam in the image is given by the equations below, where: f is the focal length of the f-theta lens <NUM>; θx is the scanned angle along the x axis; θy is the scanned angle along the y axis; αn is the incident angle of the n-th pixel on the scanning mirror; and βn is the scanned angle of the n-th pixel against optical axis.

A method of operating a scanning system of <FIG> including a MEMS-based SLM and multi-beam scanning optics to compensate for distortion will now be described with reference to <FIG> and <FIG>.

Briefly, the method compensates for distortion by providing smaller controlled pitch for both scanning direction and pixels arranged direction on an image plane than a required resolution for forming an image. For example, resolution of a 3D printer using selective laser sintering (SLS) requires a resolution with pixels of about <NUM> for building 3D parts, while magnification of a projection lens of the scanning system of can be configured to have a pixel pitch of about <NUM> on the image plane. Thus, the actual size of a projected single pixel of the SLM doesn't need to resolve <NUM>; <NUM> provides sufficient resolution for 3D printing using SLS. This difference between the resolution or pixel size the scanning system is capable of providing and that required for a particular application, i.e., 3D printing using SLS, enables increase control of pitch along the pixel arranged direction to compensate for distortion.

As for distortion in the scanning direction, a clock for controlling on/off states of pixels of the SLM can be increased by three times from the original dot clock frequency to increase controlling pitch. For example in a conventionally operated scanning system used for 3D printing the dot clock is operated at a frequency selected to draw dots or pixels on the image plane every <NUM>. To compensate for distortion in the scanning direction a scanning system operated according to the method of the present invention uses an increased clock, referred to hereinafter as a sub-clock, which can change image data every <NUM>. These increased controlling pitches for both directions are generally called "address pitch" or "address size. " The grid written by address pitch is called "address grid. " This address grid is also distorted due to characteristics of imaging lens. By using the address grid the distorted image can be compensated.

<FIG> illustrates a scanned swath <NUM> of light modulated by a MEMS-based SLM across an imaging plane <NUM>. A grid shown in dashed lines <NUM> illustrates the desired or ideal resolution of pixels projected on the imaging plane. An actual, uncompensated pattern of pixels is shown by a grid shown in solid lines <NUM>, and is a result of distortion characteristics of the fθ imaging lens <NUM> and off-axis beams, such as that shown in <FIG>. A linear array of square boxes <NUM> on the center of the imaging plane <NUM> is the projected image of entire pixels of SLM. A cell of the grid <NUM> is a minimum feature or resolution of the image plane <NUM> to form images and it consists of at least three (<NUM>) address pixels along the vertical direction. Each pixel is shown as a square box in this drawing but actual pixel image does not need to have the same size as the box shown because required resolution is the size of the grid <NUM>. By scanning the array <NUM> along horizontal direction smaller grid (it is not shown in the drawing) is to be formed within the swath <NUM> As stated above, the grid is called address grid along the vertical direction. The address grid is also distorted. There are arrays 508a and 508b on the upper and lower sides. These arrays are projected images of SLM when they are moved along the vertical direction by the scanning mirror. These arrays 508a and 508b form other swaths on the upper and lower sides by scanning horizontally. As the higher location pixels are projected from the horizontal center axis and the farther pixels are scanned from the vertical center axis, deviation from ideal location in a vertical direction gets larger.

The column <NUM> which is written on/off pattern (cross-hatched cells are on and white cells are off) is the example of compensated image. Distortion in a vertical direction or longitudinal axis of the grid <NUM> caused by the fθ imaging lens <NUM> is monotonically increases as a function of the scan mirror angle of the scanning system <NUM>. In the column <NUM>, pixels (or address grids) in the fourth cell of the grid <NUM> from the center cell cannot fill out the fourth cell of the ideal grid <NUM> which is in the on state region, so the outer pixel in the fourth cell of the grid <NUM> should be set on-state to fit the ideal grid <NUM>. The equation yn (θx, θy) as shown above tells where a pixel belong to in the ideal grid <NUM>. Thus pixels can be driven by compensated image data that vary as a function of the scan mirror <NUM> angle. The compensated image data can be stored as a function of the angle of the scan mirror <NUM> in the memory <NUM> coupled to or integrated with the controller <NUM>.

Distortion along the vertical or longitudinal axis in the swathes which are made by projected SLM image 508a and 508b outside of center swath <NUM> is increasing further because angles against optical axis are getting bigger. Optionally it is important to overlap at least some pixels of 508a of the neighbor swath of the swath <NUM> with pixels of <NUM> in the center swath <NUM>, as shown in <FIG> because it is hard to get the same shape between the horizontal scan line of the upper-side end pixel of <NUM> and the one of lower-side end pixel of 508a because horizontal rotating axis generally does not locate in the same plane where the vertical rotating axis exists. Ideally these axes should be in the same plane, pupil plane of the fθ imaging lens. This difference makes such an error. In addition, this of overlapping pixels also improves stitching between SLM pixels 508a through 508b.

To compensate for distortion in a horizontal direction, or along a direction of the scan (indicated by arrow <NUM>), drive signals to each pixel of <NUM> can be delayed by providing an on/off signal. Referring to <FIG> and <FIG>, the scanning system further includes a dot clock or clock <NUM> used to generate a dot clock signal shown as arrows in <NUM> used by the controller <NUM> to generate an on/off signal <NUM> to delay drive signals to the pixels <NUM> generating beams of modulated light distal from the optical axis <NUM> of the imaging lens <NUM> as the linear swath <NUM> is scanned across the 2D imaging plane <NUM> in a direction orthogonal to the long axis of the linear swath to compensate for distortion along an long axis of the scan direction <NUM>. Generally, dot clock is proportional to the cell size of the grid <NUM> and resolution of delay depends on the design how much the clock should be divided into. The resolution of delay in <FIG> is one third of the dot clock and we call it "address clock. " On/off signal <NUM> shows the one of the pixel which is pointed out by the arrow in <FIG>. Distortion in a direction of horizontal axis is that cell size of grid <NUM> is getting shorter as scan angle is increasing. To compensate for it the signal to change from on to off is delayed by one digit to fit the ideal grid <NUM>. As with the compensated image data used to compensate for distortion along the longitudinal axis of the linear swath <NUM>, these delays for each pixel of <NUM> are substantially constant for the scan system <NUM> at a given scan speed, and thus can be stored in a look table in the memory <NUM>, which is accessed by the controller during operation of the system, rather calculated for each address pixel during each scan.

<FIG> is a flowchart illustrating a method of operating the above described scanning system including a MEMS-based SLM and multi-beam scanning optics to compensate for distortion. Referring to <FIG>, the method begins with illuminating a spatial light modulator (SLM) including a number of SLM pixels arranged parallel or co-axially along long axes of the pixels and driven by a single channel, wherein each pixel includes a number of address pixels (<NUM>). Next, image data and drive signals are provided to the number of SLM pixels to modulate light incident on the SLM to generate beams of modulated light from the number of SLM pixels and address pixels (<NUM>). Preferably, as described above, providing image data to each of the number of SLM pixels includes providing compensated image data to at least some of the address pixels generating beams of modulated light beam distal from an optical axis of the imaging lens to compensate for distortion along the long axis of the linear swath. The beams of modulated light are scanned over a two-dimensional (2D) imaging plane to form a substantially linear swath of illumination using a collimate lens, a scan mirror moved about a first axis, and an imaging lens (<NUM>). Next, the linear swath is scanned across the 2D imaging plane in a direction orthogonal to a long axis of the linear swath using the scan mirror moved about a second axis (<NUM>). Optionally, the method further includes delaying drive signals to the address pixels generating beams of modulated light distal from an optical axis of the imaging lens as the linear swath is scanned across the 2D imaging plane in the direction orthogonal to the long axis of the linear swath to compensate for distortion along an long axis of the direction of the scan (<NUM>).

Thus, embodiments of a multi-beam scanning system, and methods for operating the same to compensate for distortion have been described.

Claim 1:
<NUM>. A method of operating a multi-beam scanning system (<NUM>) forming an image on a two dimensional (2D) image plane (<NUM>, <NUM>), comprising:
illuminating (<NUM>) a spatial light modulator (SLM) (<NUM>) including a number of SLM pixels (<NUM>, <NUM>) arranged in parallel in a pixels arranged direction along a long axis of the SLM, each SLM pixel having a first pixel pitch;
providing (<NUM>) drive signals including image data to the SLM pixels to modulate light incident thereon to generate beams of modulated light reflected from the SLM;
scanning (<NUM>), in a first scanning direction (x, <NUM>), the beams of modulated light across the 2D imaging plane to form a substantially linear swath (<NUM>) using a collimate lens (<NUM>), a scan mirror (<NUM>) moved about a first axis, and an imaging lens (<NUM>); and
scanning (<NUM>) the linear swath across the 2D imaging plane in a second scanning direction (y) orthogonal to a long axis of the linear swath to form a plurality of linear swaths, using the collimate lens, the scan mirror moved about a second axis, and the imaging lens,
wherein the method compensates for distortion along the long axis of the linear swaths caused by the imaging lens for beams off the optical axis of the imaging lens by providing a controlled pitch which is smaller than a required resolution for forming the image, in both the pixels arranged direction by providing said first pixel pitch to be smaller than the required resolution, and in the first scanning direction on the image plane, to provide an address grid of individually addressable address pixels having said controlled pitch, and by providing compensated image data to said address pixels as said drive signals including image data, and
wherein address pixels in neighboring linear swaths scanned onto the 2D imaging plane overlap.