1. Field of the Invention
This invention relates to methods for characterization of the mechanical properties of thin films and test structures for performing the same, for example thin films to be used in small scale electromechanical devices such as microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS) devices.
2. Description of the Related Art
MEMS include micro mechanical elements, actuators, and electronics. Although the term MEMS is used through the specification for convenience, it will be understood that the term is intended to encompass smaller-scale devices, such as NEMS. Micromechanical elements may be created using deposition, etching, and/or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. One type of MEMS device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. As described further herein, an interferometric modulator may comprise a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. One plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed.
In one aspect, a method of characterizing a deposited film layer includes providing a plurality of test structures formed on a substrate, where each test structure includes a film layer spaced apart from an electrode by a cavity, the film layer being configured to be actuated via the electrode to move into the cavity, where the film layer is supported by at least a residual portion of a sacrificial layer, actuating one or more of the plurality of test structures, determining the response of the actuated test structures, and characterizing mechanical characteristics of the film layer based on the response of the actuated test structures.
In another aspect, a package including a precursor stack on a substrate for use in characterizing a film layer includes a substrate, a patterned electrode layer formed over the substrate, where the electrode layer is patterned to define a plurality of test structure regions, each the test structure region including an electrode, a dielectric layer formed over the patterned electrode layer, and a sacrificial layer formed over the dielectric layer, where the precursor stack on the substrate is packaged prior to patterning of the sacrificial layer.
In another aspect, a test structure for characterizing a deposited thin film layer includes an optical stack formed over a substrate, the optical stack including: a driving electrode, and an insulating layer, and a support formed over the optical stack, where the support includes a substantially contiguous substantially annular portion of a sacrificial layer on which the thin film layer is deposited, the substantially annular portion of the sacrificial layer extending about and defining a cavity due to a portion of the sacrificial layer being removed, where the deposited thin film layer is spaced apart from the optical stack by the cavity.
In another aspect, a method of characterizing a deposited thin film layer includes providing a plurality of test structures formed on a substrate, where the test structures include a film layer spaced apart from an electrode by a cavity, the film layer being configured to be actuated via the electrode to move through the cavity, actuating the test structures via the electrodes, determining the response of the test structures, and determining a mechanical stiffness of the film layer.
In another aspect, a test structure for characterizing a film layer includes means for reflecting at least a portion of incident light, means for deforming the reflecting means to move into a cavity between the reflecting means and the deforming means, and means for spacing the reflecting means apart from the deforming means, where the spacing means extends substantially continuously around a perimeter of the cavity.
In another aspect, a program storage device is provided, the program storage device including instructions that when executed by a processor perform a method including actuating a test structure, the test structure including a thin film spaced apart from an optical stack by a cavity, where the film layer is supported by at least a residual portion of a sacrificial layer, where actuating the test structure include causing the film layer to deflect into the cavity, determining the response of the test structure, and characterizing mechanical characteristics of the film layer based on the response of the actuated test structures.
The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. MEMS devices of similar structure to those described herein can also be used in non-display applications such as in electronic switching devices.
The mechanical characteristics of deposited thin films play a critical role in the design and operation of MEMS and other devices which include such thin films. It is desirable to accurately characterize the mechanical characteristics of these thin films, such as the residual stress within the thin films and the change in the mechanical properties over time, under load, and at varying temperatures. By providing a test structure which interferometrically modulates incident light, the position of a movable layer can be determined both on the basis of the electrical characteristics of the test structure and the optical characteristics of the test structure. Rapid and accurate determinations of the position of the modulator may be made. In addition, the design of the test structure may be configured to simplify analysis of the mechanics of the test structure, as well as to facilitate fabrication of the test structure.
One type of interferometric modulator display comprising an interferometric MEMS display element is illustrated in
The depicted portion of the pixel array in
The optical stacks 16a and 16b (collectively referred to as optical stack 16), as referenced herein, typically comprise several fused layers, which can include an electrode layer, such as indium tin oxide (ITO), a partially reflective layer, such as chromium, and a transparent dielectric. The optical stack 16 is thus electrically conductive, partially transparent, and partially reflective, and may be fabricated, for example, by depositing one or more of the above layers onto a transparent substrate 20. The partially reflective layer can be formed from a variety of materials that are partially reflective such as various metals, semiconductors, and dielectrics. The partially reflective layer can be formed of one or more layers of materials, and each of the layers can be formed of a single material or a combination of materials.
The layers of the optical stack 16 may be patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective layers 14a, 14b may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of 16a, 16b) deposited on top of posts 18 and an intervening sacrificial material deposited between the posts 18. When the sacrificial material is etched away, the movable reflective layers 14a, 14b are separated from the optical stacks 16a, 16b by a defined gap 19. A highly conductive and reflective material such as aluminum may be used for the reflective layers 14, and these strips may form column electrodes in a display device.
With no applied voltage, the gap 19 remains between the movable reflective layer 14a and optical stack 16a, with the movable reflective layer 14a in a mechanically relaxed state, as illustrated by the pixel 12a in
The processor 21 may also be configured to communicate with an array driver 22. The array driver 22 may include a row driver circuit 24 and a column driver circuit 26 that provide signals to a display array or panel 30. The cross section of the array illustrated in
In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row 1 electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row 2 electrode, actuating the appropriate pixels in row 2 in accordance with the asserted column electrodes. The row 1 pixels are unaffected by the row 2 pulse, and remain in the state they were set to during the row 1 pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used.
In the
The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,
The interferometric modulators may function as direct-view devices, in which images are viewed from the front side of the transparent substrate 20, the side opposite to that upon which the modulator is arranged. The reflective layer 14 optically shields the portions of the interferometric modulator on the side of the reflective layer opposite the substrate 20, including the deformable layer 34. This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. Such shielding allows the bus structure 44 in
The operation of a MEMS device which includes a movable layer is dependent in large part upon the mechanical characteristics of the movable layer. In order to accurately predict the behavior of such a MEMS device, it is very helpful to accurately assess the characteristics of the various individual or combined layers in a MEMS device, and in particular the layer or layers that make up the movable layer. Because the mechanical properties of the MEMS device affect the operation of the MEMS device, the design of the MEMS device is dependent upon accurate measurements of these mechanical properties. In particular, the properties of interest in designing MEMS devices include the relationship between stress and strain (e.g., Young's modulus) as a function of temperature, as well as measurements of creep and fatigue.
One conventional method of characterizing thin films is described with respect to
Another conventional method of characterizing thin films involves the use of a nano-indenter. This method utilizes a test structure including a movable layer disposed over a cavity through which the movable layer can be deflected. The test structure may be similar to the test structure of
Both of these conventional testing methods have significant limitations. Neither of the methods are well suited to characterizing the properties of the film at high temperatures, as the external equipment utilized in the testing processes, such as the nano-indenter or the optical characterization equipment, may need to be exposed to the high temperature environment. These methods also typically test the properties of the film at a single location, and generally cannot be used to determine the variation of these properties across a substrate, for example as the properties of a sputtered film may vary across the substrate. Furthermore, these methods, particularly the etching back of a silicon layer, can be challenging and costly.
In one embodiment, a test structure may be used which functions as an interferometric modulator.
In the illustrated embodiment, the optical stack comprises an electrode layer which may be patterned to form a driving electrode 122a and a non-driving portion 122b, separated by gaps 128 which serve to isolate the driving electrode 122a from the non-driving portion 122b. A dielectric layer 124 may be deposited over the patterned electrode layer. A black mask 126 may be disposed underneath a portion or all of the supports 130 as shown. In one embodiment, the electrode layer may comprise a partially reflective thickness of chromium or another conductive and reflective material, such as molybdenum, a molybdenum-chromium alloy, titanium, or any other suitable material. In other embodiments, the electrode layer may comprise a partially reflective layer, alternately referred to as an absorber, in addition to a conductive layer. By providing a partially reflective layer within the optical stack, the test structure can function as an interferometric modulator, facilitating measurement of the state of the test structure.
In
The deformation of the movable layer 110 to a fully actuated position can be seen in
One method of fabricating such a test structure 100 is described with respect to
In
In
In
In
In
It will be understood that significant modifications may be made to the above process. For example, rather than using portions of the sacrificial layer 132 as supports 130, support structures may be formed from a different material using additional process steps. These alternate support structures 130 may provide additional rigidity, as well as additional electrical isolation between the electrode layer and the movable layer. Other modifications may also be made to the support structures 130.
It will also be understood that the above fabrication steps need not be sequentially performed at the same location. For example, it may be desirable to interrupt the fabrication process at the point corresponding to
In operation, the test structure 100 may be actuated via the application of a voltage between the driving electrode 122a and the movable layer 110. If the applied voltage is less than the actuation voltage, the movable layer 100 may deform towards the driving electrode 122a, but will not collapse to contact the optical stack 120. On the other hand if the applied voltage is greater than the actuation voltage, the movable layer 110 will collapse to a position such as the one shown in
Determination of the position (deflection) of the movable layer 110 can be done in at least two ways. The electrical characteristics of the test structure 100, such as the capacitance of the movable layer 110 and the driving electrode 122a, may be measured. Alternatively or in addition, optical characteristics, such as the wavelengths of light reflected at various portions of the test structure 100 may be used to determine the size of the gap between the movable layer 110 and the partially reflective layer within the optical stack 120. In an embodiment in which the movable layer 110 is to be fully actuated, the electrical characteristics of the test structure 100 can be used to provide a rapid and automatable determination of whether the movable layer 110 has collapsed against the optical stack 120. In embodiments in which the movable layer 100 is not fully actuated, the position of the movable layer 110 can be used to provide an accurate measurement of the position of the movable layer 110, both over the driving electrode 122a and across the entire test structure 100. In certain embodiments, the test device can be attached to an image capturing system, such as a camera, which can preserve a record of the reflected light in a given state (e.g., the reflectance of light and/or the various colors generated by light interference of the movable layer 110 and the optical stack 120), and can enable the analysis and determination of the position of the test structure 100 to be performed after the testing is completed.
It will also be understood, as discussed above, that an array of these test structures 100 may be fabricated so as to enable the characterization of the mechanical properties of the movable layer 110 at different locations on the substrate 102 onto which the movable layer 110 is deposited. This can be used to provide both an indication of the uniformity of the properties of the deposited thin film across the substrate 102 as well as to enable alterations to the design to be made to compensate for such variations in the mechanical properties of the movable layer 110. These devices may also be used as process control monitors, fabricated alongside other MEMS devices and used to provide an indication of the mechanical characteristics of the movable layer 110, such as the stresses within the movable layer 110.
In one embodiment, the actuation voltage of a test structure such as test structure 100 of
The release voltage of the test structure 100 may be determined in a similar manner, by beginning with an actuated test structure and gradually decreasing the applied voltage until the movable layer 110 releases by moving away from the optical stack 120. Because actuation of the movable layer 110 represents a point at which the electrostatic force attracting the movable layer 110 towards the optical stack 120 overcomes the mechanical restoring force of the movable layer 110, the actuation of the movable layer 110 may result in a quick collapse (or release) of the movable layer 110 once the movable layer 110 passes the equilibrium point. It may be desirable that this actuation be quasi static, rather than dynamic, and that the escape of air or other fluid from the cavity within the test structure 100 not cause a significant damping effect on the movement of the movable layer 110. This can be accomplished through the provision of apertures in the movable layer 110, the support structure 130 of sufficient number or size. Such apertures in fluid communication with the cavity 140 can be provided in a variety of locations in the test structure 100. By making the actuation quasi-static, the actuation of the movable layer 110 is fast, enabling an accurate determination of actuation or release voltages (also referred to herein as transition voltages), and the actuation process can be accurately measured and analyzed.
The actuation voltage Va can be used to provide an accurate determination of the stress within the movable layer 110, utilizing the following equation:
where Va is the magnitude of the actuation voltage in volts, and K/A is the restoring force constant of the movable layer 110 per unit area, measured in (N/m)/nm2, or GPa/nm. The remaining variables are either known amounts or figures which can be readily measured. In particular, ∈0 is the permittivity of free space (a known constant), and ∈r is the effective dielectric constant of the oxide layers, a figure which can be readily determined based upon the materials comprising the various layers within the optical stack and the thicknesses of those layers. tdie is the thickness of the dielectric layers within the optical stack (in nm), and g0 is the size of the air gap (the space between the movable layer 110 and the oxide layer 120, whether the movable layer 110 is deflected or not) at the offset voltage, as measured in nm. For certain embodiments of test structures 100, g0 may be close to the undriven air gap when the offset voltage is sufficiently low, such as when the offset voltage is less than 1V.
The stiffness of the movable layer 110 is related to the stress within the movable layer 110, and may be used to determine the same. Generally, the stiffness K of such a test structure 100 may be generally related to the stress σ via the following relationship:
K=ησt+βE (Eq. 2)
where t is the thickness of the movable layer 110, and η is dependent upon the diameter of the cavity 140. In the second term, which represents the bending stiffness, the constant β is dependent upon the geometry of the test structure, and E represents Young's modulus. However, for a thin film structure such as the test devices described herein, the bending stiffness is much smaller than the residual stress stiffness represented by the first term, and can be ignored in favor of the first term. In order to accurately determine the stiffness of the movable layer 110, finite element analysis may be utilized, taking into account the size and shape of the cavity 140, the size and shape of the bottom electrode 122a, the thickness of the movable layer 110, and the height of the cavity 140.
The release voltage may be used in a similar manner to calculate the stiffness of the movable layer 110, which can then be used to determine the stress in the movable layer 110. The release voltage Vr is related to the stiffness via the following equation:
where gdown the down state air gap, measured in nm, and where α is a constant.
The stiffness may also be calculated via the application of a voltage V which is less than the actuation voltage. For a square wave voltage having a magnitude V and an offset Voffset, the relationship of the voltage V to the stiffness is given by the following equation:
where Voffset is the offset voltage, and where g is the measured air gap when the voltage V is applied. The proper Voffset may be determined in a number of ways, such as by applied an offset voltage such that the air gap remains constant for a particular voltage. In one particular embodiment, a photodiode may be positioned adjacent the test structure 100 such that light reflected by the test structure is incident upon the photodiode. The voltage across the photodiode may be monitored, such as via an oscilloscope. When the offset voltage is improperly set, the photodiode will exhibit a swinging response having the same frequency as the applied square wave. The proper offset voltage is selected to be the offset voltage at which the voltage detected from the photodiode is stable.
In another embodiment, a number of voltages V lower than the actuation voltage may be applied, and the air gap g measured for each of the voltages V. The electrostatic force Fe being applied to the movable layer for a given voltage V can be calculated using the following equation:
The electrostatic force is related to the mechanical stiffness of the membrane by the following relationship:
F
e
=K(g0−g) (Eq. 6)
where the movable layer 110 is treated as a linear spring with a spring constant k, and wherein the electrostatic force is proportional to the change in the air gap. A plot can be generated of the electrostatic force as a function of the air gap, and then the value of the mechanical stiffness k can be determined from the plot, via an appropriate analytical method.
Once a value for k has been obtained, the actuation voltage may be estimated via Equation 1, above. In certain embodiments, the actuation voltage may be independently determined via the process described above, and the measured actuation values have been shown to be very close to the actuation voltage values determined via the present process. The present process also advantageously permits a determination of the actuation voltage in an embodiment in which full activation of the test structure 100 may be undesirable.
Finally, finite element analysis may be used to calculate the stress within the movable layer 110. As noted above, the finite element analysis may take into account a variety of factors, including the size and shape of the cavity 140, the size and shape of the driving electrode 122a, the height of the cavity 140, the movable layer 110 thickness, and the determined actuation voltage. For a particular embodiment in which the bottom electrode 122a has a radius of 75 μm, the cavity has a height of 2400 Angstroms, and the movable layer has a thickness of 1300 Angstroms, the stress is related to the actuation voltage Va and the release radius Rrelease (the radius of the released area which forms the cavity) by the following exemplary equation:
The particular equations generated by the finite element analysis may vary depending on the modeling used. Further, the above methods are not limited to being performed on a single test structure 100, but may be performed on multiple test structures 100 simultaneously or in series, in order to determine the mechanical characteristics of the movable layer 110 at various locations on the substrate 102. In a particular embodiment, a pattern of test structures 100 formed on the substrate 102 may be tested using one of the methods disclosed herein so as to generate various values for stress within the deposited movable layer 110 at the various locations on the substrate 102. The process conditions for depositing the movable layer 110 may also be varied during deposition of the movable layer 110, and the results compared to test structures 100 formed under different process conditions.
In other embodiments, mechanical characteristics other than the actuation voltage or stresses within the movable layer 110 may be determined. In one embodiment, fatigue behavior can be characterized by cycling the test structure 100 via the application of a periodic voltage which is configured to actuate and release the test structure 100. In particular embodiments, a square wave with an appropriately set offset voltage may be used, or triangular wave may be used, although other waveforms can be used, as well. The actuation voltage may be measured periodically as the test structure 100 is cycled, and the data may be arranged in any useful format, such as a plot of actuation voltage as a function of time or cycles. Further analysis may include the determination of the stress for the given actuation voltages, in order to characterize the stress of the movable layer 110 as a function of time or cycles. In a particular embodiment, the actuation voltage may be measured at discrete points during the process, or may in the case of a triangular or similar wave, be continuously measured by correlating the actuation of the test structure 100 with the applied voltage.
In another embodiment, the stress relaxation of the test structure 100 under load may be characterized, also referred to as creep. In one embodiment, the test structure 100 may be maintained under a constant load for a period of time, either by application of a DC voltage or a square wave or other suitable waveform. The voltage may be sufficient to fully actuate the movable layer 110, or may simply deform the movable layer 110. The actuation voltage may be periodically determined, in order to determine the stress relaxation within the movable layer 110 over time after being exposed to a given load. As the stress level is different for unactuated structures than it is for partially or fully activated structures, the stress relaxation may be measured for multiple stress levels, including unactuated or partially actuated test structures. In one embodiment, stress relaxation in an unactuated movable layer 110 may be measured by determining the residual stress in the movable layer 110, permitting the movable layer to return to an unactuated state for a period of time, and subsequently making a second determination of the residual stress movable layer 110.
As noted above, such measurements may be performed at desired temperatures. In certain embodiments, it may be desirable to form a series of test structures 100 under the same process conditions, and then perform one of the above tests sequentially on the test structures 100 at different temperature conditions, in order to determine the effect that temperature has on a mechanical characteristic such as creep rate or fatigue.
Any number of other mechanical properties may be characterized using such a test structure 100. For example, the coefficient of thermal expansion may be determined based upon the change in deflection of the movable layer 110 at particular temperatures. Such a determination need not require the application of a voltage across the test structure 100. Another measurement which can be made is a determination of Young's modulus of elasticity for the deposited film.
Generally, the stress is a function of factors such as the modulus of elasticity, the temperature, the thickness of the film, the size of the test structure 100, and the time for which the movable layer 110 has been under load. By varying any one of these factors while holding the others constant, the relationship between that factor and the mechanical characteristics of the thin film of the movable layer 110 can be determined. By characterizing the thin film deposited as the movable layer 110 in this manner, the design and operation of MEMS devices utilizing the deposited thin film layer can be optimized.
In certain embodiments, a program storage device may be provided, comprising instructions that when executed by a processor perform one of the above testing methods, or perform certain steps of one of the above testing methods. The program storage device may comprise, for example, a computer-readable medium comprising such instructions.
Thus, it will be understood that test structures and testing methods described herein may be utilized in a variety of ways, and may be modified as appropriate for use in particular embodiments. It will also be recognized that the order of layers and the materials forming those layers in the above test structures are merely exemplary. Moreover, in some embodiments, other layers, not shown, may be deposited and processed to form portions of a MEMS device or to form other structures on the substrate. In other embodiments, these layers may be formed using alternative deposition, patterning, and etching materials and processes, may be deposited in a different order, or composed of different materials, as would be known to one of skill in the art.
It is also to be recognized that, depending on the embodiment, the acts or events of any methods described herein can be performed in other sequences, may be added, merged, or left out altogether (e.g., not all acts or events are necessary for the practice of the methods), unless the text specifically and clearly states otherwise.
While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device of process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US07/21004 | 9/28/2007 | WO | 00 | 3/26/2010 |