MEMS

Class 10

Design Realization Tools for MEMS

Mohammad Kilani

• Just as MEMS fabrication has its roots in the microelectronics fabrication infrastructure, the MEMS design realization infrastructure has its roots in the microelectronics infrastructure as well.

• However, the MEMS design realization requirements are significantly different. MEMS design involves complex geometric, three-dimensional moving mechanical devices similar to macroworld machine design.

• The result is a MEMS design realization environment that leverages a significant portion from microelectronics while plotting a new path to meet the new demands.

MEMS Design Tools

• The design of a device that is to be fabricated via LIGA, surface micromachining, or bulk micromachining requires a mask to be made for the patterning step of the fabrication process.

• The mask set is the interface between design engineer’s information (i.e., design), and the fabrication process. A mask is a two-dimensional design representation that will be patterned and etched into the working material.

LAYOUT

• Bulk micromachining and LIGA products require a typically small number of masks to produce a high aspect ratio MEMS part. Surface micromachining can require as many as 14 masks to produce a complex MEMS design.

• The three MEMS fabrication technologies share a common need to interface design information with the mask-making infrastructure; however, multi-level surface micromachining is more complex due to the number of masks required. Surface micromachining will be emphasized in this chapter because the complexities of design in this MEMS fabrication technology is a superset of the issues involved with the others.

LAYOUT

• The infrastructure for mask making is an established industry primarily servicing the microelectronics production complex. The two common data formats for the exchange of design layout information for use in the mask-making industry are GDSII stream format and the Cal Tech Intermediate Format (CIF)

• GDSII is a binary format and CIF is an ASCII format; both have become de facto standards, but GDSII is much more prevalent.

LAYOUT

• Due to the geometric simplicity needed for microelectronics, the GDSII file formation is merely a sequence of closed polygons that may be an approximation of the true geometry.

• However, current implementations of the GDSII file format and the supporting translators enable a large number of line segments, which enable a faithful representation of the desired geometry.

LAYOUT

• A closed polygon is a sequence of line segments that start and end at the same point. Obviously, the complete suite of two- and three-dimensional geometries (e.g., circles, ellipses, curves, cubes, spheres, cylinders) standard in the macroworld machine design infrastructure does not exist within the context of a GDSII file format

• The mask data preparation path between the CAD layout tool and the GDSII file used by the mask supplier includes a number of steps. Current MEMS layout tools have the ability to lay out complex geometric entities such as circles, ellipses, and curves; however, when the data are exported to a GDSII data file the geometric entities are approximated by closed polygons in a GDS file format.

LAYOUT

• The SUMMiT™ (Sandia’s ultraplanar multilevel MEMS technology) design tool suite utilizes the two-dimensional geometric layout capabilities of AutoCAD, which has the full set of geometric entities to facilitate complex mechanical design.

• However, to transfer these data to the mask-making process requires that data pass through a translator to put the data in the GDSII format. In this example, the data are exported from AutoCAD to a DXF file format that retains the geometric entity information and then to the GDSII file format, which does not retain the geometric entity information. The translation from the DXF format to the GDSII format can be accomplished with a translator such as ASM3500.

LAYOUT

• Geometric editing (e.g., changing the radius of a circle, radius of a fillet, etc.) of the design information can be easily done when the geometric entities are preserved. For example, the AutoCAD binary data file and DXF file utilized in the SUMMiT design tool suite retain the geometric entity information.

• Similar editing of design information utilizing a GDSII file format is severely inhibited because all curved geometry is merely approximated by a sequence of chords. Therefore, to edit a curved surface would involve the individual editing (i.e., repositioning) of a series of individual points that specify the curved surface.

LAYOUT

• Within the MEMS layout tool, layers corresponding to the masks that will be used in the fabrication processing are defined. Layout tools provide access to the drawing layers utilized for device layout as well as access to the standard components, design rule checking, and visualization tools.

• The Figure shows the detailed layer definitions within the SUMMiT design tool suite

LAYOUT

LAYOUT

• The table lists the definitions of the mask names, layout layer names, and GDS layer numbers utilized in the SUMMiT design tool suite.

LAYOUT

• The Figure illustrates the SUMMiT material layers deposited on the substrate.

• The mask name refers to the physical mask that will be utilized in fabrication, and the layer name refers to a drawing layer that will be used to define the design data used to make the physical mask.

• The additional layers shown in the Figure that are not listed in the table of the previous slide are utilized for layout construction and notation purposes.

LAYOUT

• There are several instances in which two drawing layers correspond to one physical mask.

• For example, the MMPOLY2 mask is composed of information from the MMPOLY2 and MMPOLY2_CUT drawing layers.

LAYOUT

• The two drawing layers are defined to provide ease in layout. For example, consider the layout for the seismic mass of an accelerometer. The design require a number of holes to reduce viscous damping and to enable the release etchant to remove the sacrificial material below.

• Because GDSII is limited to closed polygons, the geometry of the holes may formed by a continuous sequence of line segments that define the periphery of the mass. This form of layout for a device is circuitous and difficult to modify if changes are required.

LAYOUT

• If two layers are used in the CAD layout tool and then combined, a much easier and more editable layout of the MEMS device can be performed. The layout of the same seismic mass is achieved with two layers (i.e., layer and layer_cut).

• These layers can be combined via a logical XOR operation to form the mask description necessary for mask production. The XOR operation is typically performed by the mask production vendors.

LAYOUT

• Proper use of this technique requires that a master layer be defined (denoted by “a” in the Table.

• For example, MMPOLY2 is the master layer for the MMPOLY2 mask. The layer_cut is only valid when contained within the master layer.

LAYOUT

• Several different approaches to the layout of some typical geometric shapes using the two layer concept may be used.

LAYOUT

• The following subsections will discuss the layout details of five types of structures in SUMMiT, which span the spectrum of device layout frequently encountered.

• Although some specific details of these layouts may be unique to SUMMiT, the general techniques, issues, and concerns will be typical in comparable MEMS technologies.

SUMMiT Technology LAYOUT

• Anchoring or attaching mechanical layers to each other and to ground is the most basic function to be achieved in any surface micromachined device.

• Because surface micromachining is an alternating stack of two types of materials (i.e., structural and sacrificial), mechanical layers are attached by etching a hole or via in the sacrificial material; this enables the next mechanical layer material deposited to attach to the mechanical material below at the via.

• The figures show SEM images of various layers of SUMMiT anchored to each other. The attachment between the layers at the via produced by the SACOX_CUT can be seen between the adjacent layers. The term “SACOX_CUT” refers to a generic operation of opening a via in a sacrificial oxide layer to allow attachment of adjacent structural layers. SACOX#_CUT is the use of a SACOX_CUT on a specific SACOX# layer.

ANCHORING LAYERS

• A particular layer cannot be directly anchored to ground in the SUMMiT technology because deep SACOX_CUTs are not allowed. The SACOX_CUT enables the mechanical layers immediately above and below to be attached.

• For example, a SACOX3_CUT will enable MMPOLY3 and MMPOLY2 to be attached. The figure below shows a demonstration of the five layers of the SUMMiT process.

ANCHORING LAYERS

• The figure show a cross-section visualization and the masks for a post that extends from MMPOLY0 to MMPOLY4.

• Notice that the SACOX_CUTs are all on top of each other and the size of the SACOX_CUT becomes bigger at each higher level. The size increase enables the mechanical material at the higher level to attach to the shoulder of the via to the mechanical material on the level immediately below. This is denoted as a nested anchor.

ANCHORING LAYERS

• The nested anchor method can produce the smallest size post possible. However, a nested anchor will have encased silicon dioxide trapped inside the post as shown in the cross-section.

• The encased silicon dioxide is due to the inability of the etching processes to remove material completely at locations that have significant vertical topography. This artifact is known as a stringer. The residual stress of the entrapped silicon oxide in the post can cause slight deflections. this may be a design consideration, depending upon the application.

• A staggered anchor is a method of reducing the amount of encased silicon dioxide in an anchor; however, the required size of the post will increase.

ANCHORING LAYERS

• Rotational hubs are structures that enable 360° rotation similar to a wheel and axle. The ability to implement this structure at the microscale with no assembly necessary is an enabling feature for MEMS devices that require mechanisms.

• Two methods can be used to produce a rotational hub in the SUMMiT technology: a cap and post hub and a low-clearance hub.

• A cap and post hub can be implemented in any three-level surface micromachining technology, and it is the simplest hub design that can be utilized.

Rotational Hubs

• The masks and a cross-section of a cap and post hub implemented in the SUMMiT technology are shown. The central feature of this type of hub is a central post with a cap of sufficient diameter so that a rotating wheel will be constrained vertically.

• The figure’s cross-section shows the rotating wheel composed of MMPOLY1 and MMPOLY2, which are laminated together. Functionally, the rotating wheel could be only one layer instead of two. An MMPOLY3 cap is supported by a post of MMPOLY1 and MMPOLY2.

Rotational Hubs – Cap and Post Hubs

• The implementation of the cap and post structure is similar to the anchors discussed previously. The horizontal clearance for the rotating wheel is defined by the resolvable feature size of the lithography process in etching layers MMPOLY1 and MMPOLY2 at the rotating interface.

• The vertical clearance is defined by the thickness of the sacrificial oxide layer or the ability to produce structures such as dimples to constrain the vertical motion. Dimples are small “bumps” underneath surface micromachined layers that prevent broad area surface contact when the layers contact the substrate or each other. Dimples can also be used to minimize clearances.

Rotational Hubs – Cap and Post Hubs

• The low-clearance hub is a feature that SUMMiT was especially designed to implement.

• This hub utilizes the ability to perform anisotropic and isotropic wet etching in polysilicon and conformal deposition of thin films of sacrificial material accurately to control the clearance in the hub.

Rotational Hubs – Low Clearance Hubs

• The figures show the layout and cross-section of the low clearance hub, and a focused ion beam (FIB) cross-section of a low-clearance hub and pin joint fabrication in SUMMiT.

• A pin joint is very similar to a hub, but is not attached to ground. A pin joint enables linkages between rotating members to be used in rotational mechanism designs.

Rotational Hubs – Low Clearance Hubs

• The Figure shows a cross-section of the SUMMiT fabrication sequence for the low-clearance hub at several key points in the process. In part (a) SACOX1 has been deposited and patterned to produce dimples and to anchor MMPOLY1 to the MMPOLY0 film. MMPOLY1 has been deposited and patterned with the PIN_JOINT_CUT mask. A combination of anisotropic (dry etch) and isotropic (wet etch) has been performed to form the round features in SACOX1 beneath MMPOLY1.

Rotational Hubs – Low Clearance Hubs

(a)

(b)

(c)

(d)

• Part (b) shows the process after the SACOX2 layer has been deposited, patterned, and etched and the MMPOLY2 layer has been deposited. At this stage, SACOX2 can be seen to define the clearances in the internals of the low-clearance hub. The low-clearance hub lateral and vertical clearances in SUMMiT are 0.3 µm.

Rotational Hubs – Low Clearance Hubs

(a)

(b)

(c)

(d)

• Part (c) shows the cross-section after the MMPOLY2 etch has been performed. This etch can etch the laminated MMPOLY1 and MMPOLY2 layers, thus providing an even outside surface for the rotating wheel and etch release holes through the rotating wheel disc. Note that the MMPOLY2 etch stops on the SACOX2 layer in the internal hub features.

Rotational Hubs – Low Clearance Hubs

(a)

(b)

(c)

(d)

• Part (d) shows the cross-section of the released low-clearance hub and pin joint structure.

Rotational Hubs – Low Clearance Hubs

(a)

(b)

(c)

(d)

• The MMPOLY1 beam with a substrate connection is a simple structure illustrating the application of two features useful in design of a number of devices in the SUMMiT technology. The MMPOLY1 layer can be patterned in either of two ways in SUMMiT.

• The MMPOLY1 layer can be patterned directly using the MMPOLY1_cut mask and etch. The MMPOLY1 layer can also be patterned indirectly by using SACOX2 as a “hard” mask and etching with the MMPOLY2 etch. Part (c) below shows that the MMPOLY2 etch would etch the MMPOLY1 and MMPOLY2 layers except when the MMPOLY1 layer is protected by SACOX2. In this case, the SACOX2 layer was used as a “hard mask to stop the MMPOLY2 etch.

POLY1 BEAM WITH SUBSTRATE CONNECTION

• For the MMPOLY1 beam shown, the SACOX2 mask is used to define the MMPOLY1 beam via the MMPOLY2 etch. The MMPOLY1 beam is attached to the substrate using SACOX1_CUT.

• If a connection is to be established to the substrate for electrical grounding purposes, the NITRIDE_CUT mask is used to define the etch of the NITRIDE layer.

POLY1 BEAM WITH SUBSTRATE CONNECTION

• The concept of discrete hinges for MEMS was proposed for a number of different applications. Different variations and types of hinges have been developed and applied.

• The figures show SEM images of a “popup” mirror that utilizes two types of discrete hinges:

DISCRETE HINGES

Plate to plate hinge

Staple and pin hinge

• A staple and pin hinge allows a plate to be attached to the substrate and rotate about an axis parallel to the substrate.

• The Figure shows the layout and cross-section for a staple and pin hinge implemented in SUMMiT. The cross-section shows two posts that go up to the MMPOLY3 level, which bridges between the posts to form the staple.

• The pin is a narrow piece of laminate MMPOLY1 and MMPOLY2 that connects out of the plane of the cross-section to the movable plate. As in a cap and post hub, the staple and pin hinge clearances are defined by the width of the MMPOLY2 etch.

Staple and pin hinges

• A plate-to-plate hinge couples two plates and allows them to rotate about an axis parallel to the plane of the plates and the hinge to deflect off the substrate.

• The A-A cross-section reveals the major parts of this hinge design. An MMPOLY2 pin is connected out of the plane of the cross-section to plate 2. The pin is trapped by the staple and floor and rotates within these objects.

Plate-to-plate hinges

• A SACOX2 layer separates MMPOLY1 and MMPOLY2 and stops the MMPOLY2 etch so that an MMPOLY2 pin is formed.

• The staple is formed by utilizing SACOX3_CUTS to attach MMPOLY3 in two places to MMPOLY2.

• A subtle feature is the cutting of MMPOLY1 as defined by the MMPOLY1_CUT mask to separate the pin and the floor structures. Without the separation of the floor and pin structures via the MMPOLY1_CUT, this hinge would rigidly attach plate 1 and plate 2 (i.e., not functional).

Plate-to-plate hinges