Patent Application
20250236509
Small oscillating mirrors implemented using micro-electro-mechanical systems (MEMS) technologies are commonly used to reflect imaging light for virtual- and/or mixed-reality scanning imaging systems in head-mounted display (HMD) devices. The mirrors are typically formed from blocks of semiconductor material by removing material from the block around and underneath the mirror while providing for a pair of torsional flexure beams that allow the mirror to oscillate around a lengthwise axis to provide the scanning function.
A torsional MEMS mirror module in a scanning display system that is housed in a display module assembly includes a silicon die that is die-attached to a substrate around only a portion of the perimeter of the die to provide for a cantilevered (i.e., fixed-free) die package in which one end of the die is partially fixedly-attached to the substrate while the non-attached end of the die is free from the substrate and unsupported. In an illustrative uniaxial scanner embodiment, the die includes a scanning mirror, which oscillates around a longitudinal axis formed by torsional flexure beams, and piezoelectric actuators to provide controllable beam steering of light from a source such as a laser.
The torsional flexure beams are typically subject to thermally-induced stress experienced during scanner operations from mismatches in the coefficient of thermal expansion (CTE) of the die, substrate, and display module assembly housing. While the scanning mirror and beam flexures are principally designed to operate in a torsional mode, other oscillations of the mirror, termed “ancillary modes,” are possible which include vertical, horizontal, and rocking modes. In conventional designs, the variation in ancillary mode frequency with temperature can be difficult to compensate for through feedback loop control. The present cantilevered die package provides for the die to be partially decoupled from the substrate and display module assembly housing to reduce the impact of CTE mismatch and improve the linearity of MEMS motion, thus improving image quality with feedback loop control across a wide environment temperature range. The approach can also prevent the control loop from the over-driving of the MEMS mirror at ancillary modes which may result in the MEMS flexure beam breakage.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure.
Like reference numerals indicate like elements in the drawings. Elements are not drawn to scale unless otherwise indicated.
Micro-electro-mechanical systems (MEMS) devices are commonly used in scanning micromirrors for beam scanning of a laser or other light source. MEMS devices typically comprise integrated circuit dies that are die-attached to a die carrier substrate. Modern MEMS scanners provide high operating frequencies with low-weight and small volume packages which make them well-suited to a variety of applications including, for example, medical imaging such as OCT (Optical Coherence Tomography), LIDAR (Light Detection and Ranging), and optical imaging display systems used in virtual- and mixed-reality head-mounted display (HMD) devices. Other use cases and applications for MEMS scanners using the present principles include, for example and without limitation, manufacturing such as additive fabrication (i.e., 3D printing) using stereo lithography, material processing such as laser marking, engraving, cutting, drilling, and welding, laser projection in entertainment and optical layout template (OLT) scenarios, galvanometer-based image capture, and microscopy, to name just a few of a wide range of MEMS scanning applications.
MEMS torsional scanning mirror designs in imaging applications comprise mirrors that oscillate about torsional flexure beams. For example, in raster-scanning applications requiring bidirectional dual-axis systems, MEMS torsional scanner systems typically utilize a pair of uniaxial scanning mirrors or a biaxial gimbal-mounted scanning mirror. Motion of a MEMS mirror is actuated by various driving methods using, for example, piezoelectric, electromagnetic, electrothermal, and electrostatic technologies.
A principal challenge for bidirectional scanning is the precise control of phase between scan lines and a high-quality control of mirror positioning and motions. When actuators impart the desired torsional oscillation of the mirror about its flexures, the mirror is subject to parasitic, or ancillary modes of oscillation including vertical, horizontal, and rocking (i.e., pitching) modes. Ancillary mode oscillations can result in off-axis mirror motions and cause unwanted beam deflections that can reduce image quality.
MEMS scanning systems can be designed to mitigate the negative effects of the ancillary oscillation modes to some degree. For example, the mirror and flexure beams can be designed to minimize physical cross-coupling between torsion and rocking modes, and/or the actuator drive signals can be modified to compensate for mirror position and motion resulting in the ancillary oscillation modes. Image post-processing may also be utilized in some applications to correct for reduction in image quality.
While the aforementioned mitigation techniques can provide satisfactory results in some cases, the inventors have recognized that ancillary oscillation modes are not thermally stable and their effects on mirror position and motion vary with temperature of the MEMS scanning system during operations. In particular, the frequencies of ancillary mode oscillation have been determined to be dependent on stress in the beam flexures. The inventors have further recognized that the flexure stress is susceptible to changes with MEMS scanner operating temperature because of mismatches in the coefficient of thermal expansion (CTE) of the silicon MEMS die, die-bond adhesive, and the underlying PCB substrate, as well as a display module assembly housing and associated mounting adhesive.
The expansion and contraction of the die, adhesive, and PCB substrate at different magnitudes and rates during operations of the MEMS scanner give rise to a temperature-dependent stress profile in the flexures that, in turn, causes temperature-dependent instability of the ancillary mode oscillation frequencies. Such temperature-dependent variability can be challenging to characterize and may result in undesired ancillary mode effects falling outside the range of constructive mitigation strategies.
The present MEMS mirror module described herein provides for thermally-stable ancillary modes of mirror oscillation by utilizing MEMS die packaging techniques that implement a cantilevered (i.e., fixed-free) die package in which one end of the die is partially fixedly-attached to the substrate while the non-attached end of the die is free from the substrate and unsupported. The cantilevered die package with fixed-free architecture effectively decouples the effects of CTE mismatch of the MEMS die, die-bond adhesive, and PCB substrate on temperature-dependent MEMS flexure stress. Ancillary mode oscillation frequencies changes with temperatures are thus limited to a smaller range relative to those experienced with conventional package designs.
The cantilevered die package used for the present MEMS mirror module advantageously reduces the contribution of CTE mismatch to ancillary mode thermal instability without limiting selection of materials to only those having similar CTEs. In addition, current MEMS manufacturing and assembly processes are readily adaptable for the cantilevered die package with minimal modifications.
Turning now to the drawings,
The optical display system 100 includes a display engine 115 that interoperates with a see-through optical combiner 120, through which the user 110 looks, to combine virtual images generated by the display engine with views of the real world. Typically, a separate optical combiner is provided by the HMD device for each of the user's eyes. For example, the display engine and left- and right-eye optical combiners can work together to support stereoscopic virtual images.
The display engine 115 includes a pair of uniaxial MEMS mirror modules that implement a raster scanning system by which light for virtual images from a light source 125 is guided to create an exit pupil for the display system 100 which is replicated (i.e., expanded) by the optical combiner 120. The MEMS mirror modules include a fast axis module 130 and a slow axis module 135. In typical applications, relay optics 140 such as lenses and reflectors are utilized between the fast axis and slow axis MEMS mirror modules to facilitate guiding of the virtual image light through the display engine. For example, the relay optics can shape the virtual image light beams and/or fold the light path within the display engine to implement a desired form factor.
The light source 125 is implemented using red, green, and blue (RGB) lasers in this illustrative example to support polychromatic virtual imaging using an RGB color space. In alternative embodiments, monochromatic virtual imaging may be utilized and/or different color spaces used. A controller 145 in the display engine provides control signals for the light source and MEMS mirror modules 130 and 135. Separate controllers may be alternatively utilized. The controller may also be alternatively incorporated into another controller or processor in the HMD device 105 such as a main or central processing unit (CPU) or other suitable processor.
The scanned virtual image light creates an exit pupil on an input element 205 to the optical combiner 120. The input element is disposed on a see-through waveguide 210. In this illustrative example, the exit pupil is expanded in the horizontal and vertical directions in the optical combiner to increase the size of the eyebox of the optical combiner for rendered virtual images. An expanding element 215 disposed on the waveguide expands the virtual image exit pupil horizontally while coupling the virtual image light downwards to an output element 220 that is disposed on the waveguide. The output element expands the exit pupil vertically and outcouples the virtual images to the user 110. In an illustrative example, the input, expanding, and output elements are implemented as diffractive optical elements (DOEs), reflective optical elements (ROEs), or a combination of DOEs and ROEs.
The MEMS mirror 340 is configured to provide torsional oscillations in a uniaxial arrangement in this illustrative example. The MEMS mirror is suspended over a cavity 440 in the module by a torsional flexure beam at each end of the mirror. A representative flexure beam 505 is shown in the enlarged view in
In this illustrative example, the PZT actuators are implemented using piezoelectric methodologies. A thin film layer of PZT 605 is disposed on an actuator substrate 610, as shown in
The PZT actuators are utilized in pairs to actuate the torsional oscillations of the MEMS mirror 340 (
The upward motions of the actuators in PZT set 1 provide a force through the respective actuator linkages 450 (
The PZT actuators in sets 1 and 2 operate according to drive signals that are received from the controller 145 (
While the torsional mode 1205 is the desired principal mode of MEMS mirror operation, unwanted parasitic vibrational modes, termed “ancillary modes” can be induced primarily by the saw-tooth ramp drive signals that actuate the motion of the MEMS mirror in the module. The drive signals typically include harmonics that are the same, or similar to, the ancillary mode frequencies. The ancillary modes include vertical, horizontal, and rocking (i.e., pitching) modes 1210, 1215, and 1220, as shown in
The ancillary modes also produce strain on the piezoresistive sensor 515, creating signals in the control loop to which the drive system responds. These unwanted signals are filtered out using notch filters that suppress the signal response in specific frequency bands. The ancillary mode frequency shifts with temperature due to CTE mismatch in the MEMS modules present challenges for control system design.
It is emphasized that the embodiments shown in the drawings and described in the accompanying text are illustrative and not limiting. Variations in the extent of the die-bond adhesive layer are utilizable to meet the requirements of a particular implementation of the present principles.
The die-bond adhesive layer 310 may comprise a variety of materials and be applied using different techniques. In the illustrative embodiments of the MEMS mirror module 300 shown in
Block 1805 includes providing a die carrier substrate including printed circuits having bond connecting pads to circuits in the printed circuit. Block 1810 includes providing a silicon die supporting a MEMS function, the die having a top surface including bond pads that are electrically coupled to internal circuits within the die, and further having a substantially planar bottom surface. Block 1815 includes die-attaching a portion of the bottom surface of the die to the die carrier substrate, in which the non-attached portions of the bottom surface are exposed and mechanically decoupled from the die carrier substrate.
The HMD device 105 further includes one or more processors 1908 which may include the functionality of the controller 145 (
Storage and memory system 1912 includes instructions stored thereon that are executable by the processors 1908. The storage system includes hardware storage devices, such as RAM, ROM, EEPROM, CD-ROM, solid state drives (SSDs) that are based on RAM, Flash memory, phase-change memory (PCM), or other types of memory, or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store desired program code means in the form of computer-executable instructions, data, or data structures and that can be accessed as required for the HMD device to implement the various features and functionality described herein.
It may be appreciated that the HMD device 105 is described for the purpose of example, and thus is not meant to be limiting. It may be further understood that the HMD device includes, in some embodiments, additional and/or alternative sensors, cameras, microphones, input devices, output devices, etc. than those shown without departing from the scope of the present arrangement. Additionally, the physical configuration of an HMD device and its various sensors and components may take a variety of different forms without departing from the scope of the present arrangement.
Various exemplary embodiments of the present MEMS mirror module with stress-decoupled vibrational modes are now presented by way of illustration and not as an exhaustive list of all embodiments. An example includes a MEMS (micro-electro-mechanical system) mirror module, comprising: a die carrier substrate including a die-bonding surface; a silicon die disposed on the die-bonding surface of the die carrier substrate, the die including a MEMS mirror that is suspended by torsional flexures; and a die-bond adhesive layer, disposed between a proximal end of the die and the die-bonding surface of the die carrier substrate, that fixedly attaches the proximal end of the die to the die-bonding surface of the die carrier substrate, and in which a distal end of the die is free-floating above the die-bonding surface of the die carrier substrate.
In another example, the die carrier substrate includes a printed circuit and a stiffener that is affixed to the die carrier substrate using an adhesive. In another example, the MEMS mirror module further includes wire bonds between the die and the printed circuit. In another example, the die-bond adhesive layer is disposed around a portion of a perimeter of the proximal end of the die. In another example, the die-bond adhesive layer is disposed along a portion of a perimeter edge of the proximal end of the die, the perimeter edge being parallel to a longitudinal axis of the torsional flexures. In another example, a length of the die-bond adhesive layer that is disposed along a portion of a perimeter edge of the proximal end of the die is between approximately 50 and 75 percent of a length of the die carrier substrate. In another example, the MEMS mirror module further comprises one or more piezoelectric actuators. In another example, the MEMS mirror is configured as a uniaxial scanning mirror. In another example, the MEMS mirror module is used in an optical scanning display system of a head-mounted display (HMD) device. In another example, the die has a coefficient of thermal expansion (CTE) that is different from a CTE of the die carrier substrate. In another example, the MEMS mirror includes a principal torsional mode of operation and a plurality of ancillary operation modes, and in which the free-floating distal end of the die provides for thermally-stable ancillary operation modes.
A further example includes a cantilevered semiconductor die package, comprising: a die carrier substrate having a die-attach surface; a semiconductor die having a fixed end that is fixedly die-attached to the die carrier substrate and a free end that is unattached to the die carrier substrate; and a die-bonding material forming a layer between the die carrier substrate and the die, the layer providing a mechanical connection between the die and the die carrier substrate, and the die-bonding material further functioning as a standoff that elevates the free end of the die above the die-attach surface of the die carrier substrate.
In another example, the die is a MEMS (micro-electro-mechanical system) die providing functions including one of sensor, oscillator, scanner, or actuator. In another example, the elevated free end of the die at least partially decouples the die and the die carrier substrate when the die and die carrier substrate undergo thermal expansion during electrical operation of the die. In another example, the die-bonding material functioning as the standoff has a “C” shape in plan view. In another example, the die-bonding material comprises one of adhesive bonding material, eutectic bonding material, or solder.
A further example includes a method for packaging a MEMS (micro-electro-mechanical system) module, comprising: providing a die carrier substrate including printed circuits having bond connecting pads to circuits in the printed circuit; providing a silicon die supporting a MEMS function, the die having a top surface including bond pads that are electrically coupled to internal circuits within the die, and further having a substantially planar bottom surface; and die-attaching a portion of the bottom surface of the die to the die carrier substrate, in which the non-attached portions of the bottom surface are exposed and mechanically decoupled from the die carrier substrate.
In another example, the die includes flexures and the mechanical decoupling of non-attached portions from the die carrier substrate decouples stress in the flexures from thermally-induced strain in the die carrier substrate and die. In another example, the method further includes performing wire bonding between respective bond pads on the die and printed circuit to create electrical interconnections between the die and the printed circuits. In another example, the MEMS function includes optical scanning using a mirror that is included in the die.
Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.
