1. How are MEMS Made? The Pressure Sensor Process Kit and Understanding Microfrabrication Processes in the Classroom
Matthias Pleil, Ph.D.
Principal Investigator – SCME
Research Associate Professor of Mechanical Engineering, University of New Mexico
Faculty – CNM Schools of Applied Technologies and Math Science and Engineering
Made possible through a grant from the National Science Foundation DUE

2. Session II - Outline MEMS Micromachining Overview
Pressure Sensor Process – Interactive Activity

3. MEMS Micromachining overview
Bulk
LIGA
This presentation is a brief overview of three widely used MEMS Micromachining Processes: Bulk Micromachining, Surface Micromachining and LIGA all of which require cleanrooms to reduce contamination during processing. These three micromachining processes enable the fabrication of micro-size components such as gears, cantilevers, probes, needles, and accelerometers.
Note: SEM of gears courtesy of Sandia National Labs, Cantilevers property of IBM, LIGA structures courtesy of HT MicroAnalytical, Inc.
Surface
MEMS Micromachining Learning Module

4. Surface Micromachining
Surface micromachining is a process that uses thin film layers deposited on the surface of a substrate to construct structural components for MEMS. The scanning electron microscope or SEM image shows microgears that were fabricated using surface micromachining.
These gears are very thin, approximately 2-3 microns in thickness, but can be hundreds of microns across. To give you a sense of scale, the gear teeth in this image are each smaller than the diameter of a human red blood cell, about 8-10 microns. These gears were made at Sandia National Laboratories using their SUMMiT V process.
[Image Courtesy of Sandia National Laboratories]

5. Surface Micromachining
Surface micromachining can be described as structural and sacrificial layers being deposited, patterned and etched on top of a substrate.
Structures have low aspect ratios (short and wide) and are sometimes referred to as 2.5 D (dimensional) - very thin structures and layers.
This process is based on CMOS manufacturing. (CMOS – complementary metal oxide semiconductor)
Surface Micromachining is a process that forms components and structures by depositing, patterning and etching a series of thin film layers on a silicon substrate.
Surface micromachining utilizes many of the same processes and techniques used in making computer chips, also known as the CMOS process.
However, there is a major difference between CMOS fabrication and MEMS Surface micromachining. In computer chips, CMOS devices move electrons, so there are no moving mechanical parts. In surface micromachining, the fabricated structures (e.g., cantilevers, gears, mirrors) move matter.
In order to move matter and to create moveable structures, spaces must be incorporated between moving components during the fabrication process. This is done with what is called, sacrificial layers. A sacrificial layer is placed between two structural layers to provide the spacer. Once the device is completed, the sacrificial layer is removed, releasing the part so it can move.
The top image illustrates structural and sacrificial layers for micro-cantilever fabrication. The bottom image is a side view of a gear similar to that shown in the previous slide – note how thin it is. These very thin structures have low aspect ratios and are sometime referred to as 2.5D rather than 3D.
[Image courtesy of Sandia National Laboratories]

6. Surface Micromachining
A micromechanical part is formed out of deposited thin films
Surface micromachined MEMS have at least one structural and at least one sacrificial layer.
This graphic illustrates the layers for fabricating a microcantilever. A sacrificial layer is deposited, patterned and etched, followed by a structural (material) layer. After the structural layer is deposited, patterned, and etched, the sacrificial layer is removed.
[Image courtesy of Southwest Research Institute. Copyright SwRI]
Surface micromachining is based on the deposition and etching of alternating structural and sacrificial layers on top of a substrate. The most commonly used substrate is silicon; however, less expensive substrates such as glass and plastic are also used. Glass substrates are used for MEMS applications such as DNA microarrays, implantable sensors, components for flat screen displays, and solar cells. Plastic substrates are used for various microfluidics applications and bioMEMS applications as well as for the fabrication of surface micromachined beams, diaphragms and cantilevers.
[Image courtesy of Sandia National Laboratories]

7. Surface Micromachining
What is a sacrificial layer?
Sacrificial layers are needed when building complicated components, such as moveable parts.
They can be used to separate layers as the structure is being constructed, then dissolved away at the end to free the structural layers so that they are free to move.
[Image Courtesy of Sandia National Laboratories]
Complicated components, such as movable gear transmission with chain drives, are possible because of the use of sacrificial layers. A sacrificial layer in surface micromachining is analogous to scaffolding used for bridge construction.
The lower image shows a cross-sectional view of a keystone bridge. This type of bridge is built by first constructing a scaffold made of wood. Cut stones are then placed on top of the scaffold. The final stone at the apex, called the keystone, is placed. Once this the keystone is in place, the scaffold can be removed and the bridge remains in place. The scaffold is temporary; its only purpose is to provide support during the construction process and it is later sacrificed. This is the same principle used in building micro-size structures using sacrificial layers.
When constructing MEMS there are many possible combinations of sacrificial and structural layers. The combination used is dependent upon the device(s) being constructed.

8. Surface Micromachining Materials
Materials are generally restricted to CMOS type materials
Sacrificial Layers
Silicon Dioxide
Structural Layers
Poly crystalline silicon (“Poly”)
Insulators
Silicon dioxide, Silicon Nitride
Coatings
SAM – Self Assembled Monolayer
Materials are generally the same as those used in CMOS processing. One of the most common sacrificial layers is silicon dioxide which is removed at the end of the process using Hydroflouric Acid solutions.
Poly crystalline silicon, referred to as “poly” is used as the structural layer. Poly can be doped so that it becomes a conductive structural layer.
Silicon nitride is often used as a thin film membrane or as insulator materials as well as a hard mask.
Self assembled monolayers are applied at different steps of the process to make surfaces hydrophobic or to reduce friction and wear of rubbing parts.
Microgears with alignment pin
[Image Courtesy of Sandia National Laboratories]

9. Surface Micromachining Process Outline
Start with Crystalline Silicon Wafers
Deposit (or grow) thin film material on the wafer surface
First layer acts as an insulator. It is a thermally grown silicon dioxide layer using oxygen gas
Or water vapor
A variety of Chemical Vapor Depositions (CVD) are used for depositing subsequent structural and sacrificial Layers
Metals are deposited using PVD (Physical Vapor Deposition) – evaporation or sputtering.
Si + O2 SiO2
Si + 2 H2O SiO2 + 2H2
The first step is to grow a layer of silicon dioxide on top of the crystalline silicon substrate. This is analogous to rust growing on an iron substrate (or iron oxide). Adding steam and heat make it grow faster. Subsequent layers of silicon dioxide as well as structural layers are deposited using chemical vapor deposition. Physical vapor deposition is used to deposit metals and metal alloys.

10. Surface Micromachining Process Outline
Pattern (Photolithography)
Coat wafer with photoresist
Expose resist to a pattern
Develop resist
Bake to harden resist
Etch (Wet and/or Dry Etch)
Deposit next film
Repeat Pattern, Etch, then Deposit again
Finally release structural layers by “dissolving” the sacrificial layers.
Package and test parts
After a thin film is deposited, photolithography is used to transfer a pattern from a reticle or mask to the thin film layer. Photolithography uses a thin coat of light sensitive material called “photoresist” which is developed after being exposed to the pattern. The photoresist is developed. If positive photoresist is used, the develop removes the resist which has been exposed to light. The remaining resist protects the underlying layer during the subsequent etch process.
[Image Courtesy of MATEC]

11. Photolithography and Etch
In the etch process the exposed material of the underlying layer is etched away in either a wet or dry etch process. Once the resist pattern has been transferred to the underlying layer, the resist is removed or stripped leaving the patterned material layer.
At this point another layer is deposited on top and the photolithography process is repeated with a different pattern or mask.
Pattern from mask is transferred into photoresist.
Photoresist pattern is transferred into underlying layer using an etch process.
After etch, the photoresist is removed.

12. Surface Micromachining - CMP
CMP or Chemical Mechanical Polishing is used after approximately one or two structural layers to flatten the bumpiness in the topography of the wafer’s surface.
MMpoly0
MMpoly2
Topology generated by discontinuities in MMpoly0
As you can see in these SEM images, after you have deposited, patterned and etched several layers, the subsequent layers get bumpier and bumpier. In other words, the topography gets worse at each layer. You can see what happens after even one additional layer – notice the bumps in the SEM side-views. SEM stands for Scanning Electron Microscope.

13. Surface Micromachining - CMP
Without CMP
We can mitigate this topography problem. Sandia National Laboratories developed a CMP process for MEMS which is similar to that used in CMOS manufacturing. A thick layer of sacrificial oxide is deposited then followed with a polish (CMP). The polish removes the topography making the top of the sacrificial layer very smooth. The next structural layer is then deposited. This structural layer will be flat on the bottom allowing the structure to move freely once the sacrificial layer is removed.
The image in the left shows the severe topography resulting if no CMP is done. Compare this to the image on the right. In this case a polish is done between the sacrificial and structural layer depositions.
With CMP
[Scanning Electron Microscope (SEM) images courtesy of Sandia National Laboratories]

14. Surface Micromachining - Pros and Cons
Leverages existing CMOS infrastructure
Batch Fabrication
Silicon (Poly) is a good mechanical material
Mechanical devices can be integrated with logic components [Graphic – 3 axes accelerometer. Courtesy of Sandia National Laboratories]
Cons
Limited in materials
Planer (2.5D)
Stiction issues
The upside of using Surface micromachining is that it is very compatible with CMOS processing, so mechanical devices can be built at the same time as the electronic logic circuits. Also, the cost is generally better since the MEMS industry can use the same equipment as the computer chip industry. Many MEMS start up companies purchase used equipment. The downside is that the mechanical devices are flat, have problems with stiction and do not lend themselves well to microfluidic applications.
The image shows a 3-axes accelerometer (middle components) incorporated into the same chip as their respective electronics.

15. Surface Micromachining – Components
SAW Sensors
Actuators
RF Switches
Inertial Sensors
Cantilevers
TRA’s
RF Switch
SCME
In spite of its disadvantages, surface micromachining is used to fabricate many MEMS components.
Components built using surface micromachining include the combdrives shown in the left image, and RF switch illustrated in the center and a gear and chain designed by Paul Tafoya, a MEMS student at Central New Mexico Community College. Other components built using surface micromachining include surface acoustical wave sensors, inertial sensors, and cantilevers.
Comb Drive
Sandia National Labs
Chain
Paul Tafoya

16. Bulk Micromachining
Bulk micromachining is a process that defines structures by selectively removing or etching into a substrate. Bulk Micromachining makes structures such as this Micro Cantilever Array possible. Here we see a series of silicon crystal based cantilevers fabricated by IBM. Cantilevers are beams supported at one end. A commonly known cantilever is a diving board. These cantilevers were fabricated by removing the bulk of the silicon substrate from under the structures.
[Image property of IBM]

17. Bulk Micromachining
Bulk micromachining defines structures by selectively etching inside a substrate, usually by removing the “bulk” of a material.
This is a subtractive process.
Take for example the cliff dwellings at Mesa Verde which were formed below the surface of the flat topped mesa. Man and nature have “bulk etched” these dwellings into the side of the cliff.
Micro-machined structures are formed into the wafer substrate in the same manner.
Bulk Micromachining is a selective subtractive process which usually removes the bulk of a material. The sculpturing of Mt. Rushmore and ancient Indian cliff dwellings are examples of bulk processing. Here the idea is “Take a cliff and remove everything that doesn’t look like a dwelling”
[Image printed with permission from Barb Lopez]

18. Bulk Micromachining
Backside of MTTC Pressure Sensor
Monocrystalline silicon wafers are mostly etched to form three-dimensional MEMS devices.
The silicon in the wafer substrate is specifically removed using anisotropic chemistries.
Sensors such as piezoresistive pressure sensors have been manufactured in high volume.
Bulk micromachined devices typically have high aspect ratios.
(100)
(111)
Silicon nitride
Bulk Micromachining is a process in which monocrystalline silicon wafers are selectively etched to form three-dimensional MEMS devices. This is a subtractive process in which the silicon substrate of the wafer is selectively removed.
Bulk micromachining takes advantage of the crystalline structure of silicon by using anisotropic etch processes. Here the etchant preferentially etches the (100) plane of the silicon – the beveled edges seen represent the (111) crystal planes which etch 400 times slower than the (100) plane. The upper photo was taken from the backside of a MEMS pressure sensor fabrication wafer. Bulk silicon is anisotropically etched up to the silicon nitride membrane which acts as an etch stop layer.
Using this bulk micromachining method, devices such as pressure sensors and microfluidic systems are manufactured in high volume. The bottom image shows a series of fluidic chambers and channels that have been bulk etched into a substrate.
Bulk micromachined devices typically have high aspect ratios (the ratio of height to width).
Omit the following from the script.
Credit for microfluidic chambers/channels: Microfluidic channels with high aspect ratio fluidic chambers [Image courtesy of Berkeley. Ref: C. Ionescu-Zanetti, R. M. Shaw, J. Seo, Y. Jan, L. Y. Jan, and L. P.  Lee (PNAS, 2005)]
Microfluidic channels with high aspect ratio fluidic chambers [Image courtesy of Berkeley, BioPOETS Lab. ]

19. Bulk Micromachining
Bulk Micromachining involves deposition, patterning and etching of structural and sacrificial layers.
It also includes bulk dry or wet etching of relatively large amounts of silicon substrate.
Structures include high aspect ratio fluidic channels, alignment grooves, pits.
Can create aspect ratios of 10 to (Aspect ratio: an object height to width)
MEMS pressure sensor (frontside/backside) [Images courtesy of MTTC/UNM]
Bulk Micromachining involves elements of surface micromachining. Thin films are deposited which are subsequently patterned using photolithography and then etched using dry or wet etchants. Layers of alternating structural and sacrificial layers are deposited, patterned and etched to produce complex 3D MEMS components.
To produce a bulk etched component, the process starts with a single crystal substrate on which a thin film of material that is inert to chemical etchants is deposited.
Bulk micromachined structures can be coupled with surface micromachined components such as thin membranes, valves, thin piezoresistors and cantilevers, to produce complex devices.
Here we see two types of structures: The top images show the electronic sensing circuit of a MEMS pressure sensor on the left and the reference chamber on the right. The sensing circuit has been patterned into a metal layer using surface micromachining. The image on the right is the backside of the pressure sensor that consists of a bulk etched chamber. You can see the sensing elements of the sensing circuit at the top of the chamber.
The bottom image is a microfluidic membrane valve with a bulk etched inlet and surface micromachined valve plate on top and a bulk – etched silicon proof mass used in inertial sensors.
[Image courtesy of Khalil Najafi, University of Michigan]

20. Bulk Micromachining
For silicon wafers, silicon dioxide or nitride films are most commonly used as an etch mask. The film is patterned to allow the removal of undesired portions the substrate or underlying layer. 
Anisotropic etching uses etchants like potassium hydroxide or KOH that etch different crystal planes at different rates.
The image is a view of the backside of a pressures sensor wafer. The light colored squares are the silicon substrate. The darker color is the silicon nitride mask that protects select areas of the substrate during the bulk etch. The exposed areas are etched.
The bottom graphic shows a cross sectional view of an Anisotropic and Isotropic etch processes.
A Silicon Nitride etch mask on the backside of a wafer used for pressure sensors. A subsequent etch process anisotropically removes the bulk silicon not protected by the nitride mask (the lighter areas).

21. Bulk Micromachining – Wet and Dry Etch
Two types of etching can be used in Bulk Micromachining
Wet Etch
Dry Etch
Both wet and dry etch can produce either
Isotropic Etch
Anisotropic Etch
Etch profiles will be different depending upon type of etch and the etchant
Depending on the process and material used, various etch profiles can be obtained. A myriad of wet and dry etching techniques have been developed over the years and are used to achieve almost any desired structural shape. Some of the typical structures are shown including grooves and slots that aide in assembly, nozzles that are used in inkjet print heads, cavities that create open volumes beneath membranes for pump and sensor applications, holes and grooves that allow fluids to pass through.
Anisotropic
Isotropic
Slow etching
crystal plane
Etch Mask

22. Bulk Micromachining – Wet Etchants
Chemicals used to anisotropically etch crystalline substrates:
Potassium Hydroxide (KOH)
Ethylene Diamine Pyrocatechol (EDP)
Tetramethyl Ammonium Hydroxide (TMAH)
Sodium Hydroxide (NaOH)
N2H4-H2O (Hydrazine)
In bulk micromachining wet etches can result in either isotropic or anisotropic structures depending upon the etchant and the material being etched. Generally, one needs to remove relatively large amounts of material, digging deep into a substrate such as silicon. This is a list of some of the common etchants used anisotropically etch crystalline substrates.
Some of the variables considered with selecting an etchant include cost, etch rates (how fast the etchant removes materials), resulting surface roughness, selectivity between the mask material and material to be etched, relative etch ratios between the different crystal planes, safety issues, and process compatibility are some of the variables used when selecting one etchant over another.

23. Bulk Micromachining – Dry Etch
Capable of higher resolution
Can generate isotropic or anisotropic etch profiles
Can attain critical dimensions (CD) < 3um
Tool sets used for dry etching are more expensive than wet etch
Types of dry etch
Reactive Ion Etching (RIE)
Isotropic Plasma Etching
Sputter Etching (ion milling)
Vapor Phase Etching
Dry etch typically makes use of reactive, vapor etchants or the bombardment of the exposed substrate by sputtering, with high energy particles. A dry etch process generally takes place in a plasma containment environment.
Dry etch processes are generally very well controlled and can be tuned to produce consistent, repeatable results. The equipment used can be cost prohibitive, running in the millions of dollars.

24. Bulk Micromachining – Components
Cantilever Arrays
Nozzles
Microfluidic channels
Needle arrays
AFM Probes
Membranes
Chambers
Through Wafer connections
Cantilever array
[Image courtesy of Seyet, Inc.]
Here are two examples of MEMS components which could only be possible through the development of bulk micromachining fabrication techniques as well as those listed on the left. The top graphic is a cantilever array used to identify a specific virus in a biological sample. The lower graphic is typical of the 3 axes accelerometer used in airbag deployment sensor.
[Image courtesy of Khalil Najafi, University of Michigan]

25. LIGA
Lithographie (Lithography), Galvanoformung (electroforming), and Abformung (molding)
LIGA was developed in the early 1980’s in Germany and stands for lithography, electroforming, and molding. It is also referred to as “Long Involved German Acronym”
LIGA is an additive, lithographic process which allows for the fabrication of complex, three dimensional structures with very high aspect ratios exceeding 100:1. These structures can have sub-micron size features with heights of several millimeters and widths of only a few microns. LIGA is also a type of HARMST process – High Aspect Ratio Micro-Structure Technology. LIGA molds allow for mass-production of micro-sized HARMST components. These components as well as other LIGA components can be fabricated using polymers, metals and moldable materials.
The SEM photo shows a mesh of material created by using LIGA, courtesy of HT MicroAnalytical.
[Image Property of MEMS Handbook, Volume 2]

26. LIGA
LIGA is a German acronym for X-ray lithography (X-ray Lithographie), Electroplating (Galvanoformung), and Molding (Abformung).
LIthographie Galvanoformung Abformung
In the early 1980s Karlsruhe Nuclear Research Center in Germany developed LIGA.
It allows for manufacturing of high aspect ratio microstructures.
High aspect ratio structures are very skinny and tall.
LIGA structures have precise dimensions and good surface roughness.
LIGA-micromachined gear for a mini electromagnetic motor
[Courtesy of Sandia National Laboratories]
LIGA uses the collimated x-rays produced by synchrotron radiation to illuminate thick x-ray sensitive materials such as PMMA (polymethylmethacrylate), also known as acrylic glass or Plexiglas. Once the PMMA is exposed under a lithographic mask using an x-ray exposure, it is developed (just like photoresist is developed in surface micromachining).
The results of LIGA lithography are tall, thin or deep structures. These structures are later filled using electroplating of nickel or other metals. The PMMA is then removed leaving the metal structures. These metal structures can be the desired structure (like a gear), or stamps and molds that are used to create thousands of structures in plastic. Hot plastic embossing and injection molding are used with the LIGA made molds.

27. The LIGA Process Parts Electroformed/ Electroplated and used directly
Electroplated Molds and Stamps
Basic Lithography? Almost
Light Source: X-ray Synchrotron
Photosensitive Material: PMMA (polymethyl methacrylate) or Plexiglass
The LIGA process creates electroplated molds, stamps, and components.
Let’s look at an overview of the LIGA process:
Once the PMMA is applied to the substrate or base, synchrotron radiation patterns the PMMA using a gold on beryllium mask and collimated synchrotron x-radiation. Like photoresist, the radiation modifies the PMMA so that the exposed material can be removed with a suitable or selective developer solution.
[Image courtesy of HT MicroAnalytical, Inc.]

28. The LIGA Process
With the use of a developer solution, the exposed PMMA is removed leaving a mold with high aspect ratio cavities, holes, or trenches. [Top graphic] This image shows the high aspect ratio cavity left in the PMMA after the develop process.
The cavities created in the develop step are filled with metal using a electroforming or electroplating process.
[Images courtesy of HT MicroAnalytical, Inc.]

29. The LIGA Process
Electroforming is similar to electroplating (graphic right).
Positive and negative electrodes are submerged in an electrolyte.
Metallic positive ions from the anode are attracted to the negatively charged cathode (substrate or PMMA structure).
Metal ions are neutralized on the substrate by the electrons of the cathode, reducing the ions to metallic form.
Process continues until the substrate is filled or coated to the desired thickness.
This graphic illustrates the electroplating process. Electroplating has been used for years to deposit metals on components such as faucets, fixtures, and sunglasses. The LIGA electroforming process has adapted the electroplating process so that thicker layers of metals can be deposited.

30. The LIGA Post Process Planarize, Strip PMMA, Release
Stamping or molding
Component level OR wafer scale assembly
Wafer scale bonding
Multi-layer structures
Packaging
Requires extensive, unique metrology
The electroformed metal is polished or planarized. In this example the polish step flattens the face of the gear. The PMMA is removed and then the part is released when applicable. Depending on the component, the remaining structure could be used to make molds or the end product. The graphic shows these three steps (CMP, strip, release) for a microgear.
[Images courtesy of HT MicroAnalytical, Inc.]

31. LIGA Structures PMMA 30m posts, 3m spacing, 300m tall
Precision miniature spring
Molded fiber Optic Cable holders
Cu – Angled structures
This slide shows some other examples of LIGA parts. Note the very high aspect ratios. LIGA can be used to build structures millimeters tall and a few micrometers wide. What are some applications which would require such tall structures?
[Images Property of MEMS Handbook, Volume 2]

32. LIGA – Bonded Structures
Bio-sensors
3 meter Gas Chromatography (GC)
Here are some more LIGA structures. Tall and thin structures make great heat exchangers. Such a device made at ANKA (not shown) can exchange 1KW of heat through a 1cm cube device.
100 micron channel heat exchanger
[Images Property of MEMS Handbook, Volume 2]

33. LIGA – Components HARMST Turbines Gears Springs Clips Needle Arrays
Shutters
Gratings
Packaging
This is probably the most famous LIGA SEM in the world – it has been attributed to the folks at ANKA in Germany.
LIGA Micromachining provides an extensive set of components for microstructure fabrications.
Unique structures are enabled.
Extensive unique metrology is required.
Multi-layer wafer-scale processing extends the additive approach to accommodate interfaces and packaging.
Ant with a LIGA micro-gear.
Image courtesy of Rorschungzentrum Karlsruhe, Germany

34. Summary
MEMS fabrication (also called micromachining) has allowed for the manufacturing of micro-sized devices which has also enabled nano technology, at lower cost and increased reliability when compared to macro-sized equivalent components. Such devices can be fabrication on top of substrates, within substrates, or molded and bonded depending on the micromachining processes used. Three widely used micromachining processes are
surface micromachining
bulk micromachining, and
LIGA (Lithography, Galvanoformung, and Abformung).
Surface, bulk and LIGA fabrication or micromachining processes have allowed for the construction of unique microsystems, devices and components. The number of devices that could be fabricated using these processes is limitless.

35. Questions?

36. Disclaimer
The information contained herein is considered to be true and accurate; however the Southwest Center for Microsystems Education (SCME) makes no guarantees concerning the authenticity of any statement. SCME accepts no liability for the content of this unit, or for the consequences of any actions taken on the basis of the information provided.

37. Acknowledgements
Copyright 2009 – 2010 by the Southwest Center for Microsystems Education and The Regents of the University of New Mexico.
Southwest Center for Microsystems Education (SCME)
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Albuquerque, NM
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Website: contact:
The work presented was funded in part by the National Science Foundation Advanced Technology Education program, Department of Undergraduate Education grant #
Revision – 01/25/10