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Surface micromachining and Process flow part 1

Surface micromachining The Si wafer functions like the big green flat plate.

Some Jenga pieces are removed. The ones that remain form the MEMS structure.

+

= Surface micromachining

Review of surface micromachining process

Surface micromachining example – Creating a cantilever Deposit poly-Si (structural layer—the Jenga pieces that remain)

Deposit SiO2 (sacrificial layer— the Jenga pieces that are removed) Remove sacrificial layer (release)

Etch part of the layer.

Often the most criti Silicon wafer (Green Lego® plate)

Reminder of the surface micromachining process

side view

top view silicon oxide

metal

Process flow for surface μ-machined cantilever

1. . 2. .

Mask 1 (negative resist)

Top view (4)

3. .

Top view (5)

4. . 5. . 6. .

Mask 1 (positive resist)

Top view (6)

Process flow for surface μ-machined cantilever Top view (7)

7. . 8. .

Mask 2 (negative resist)

Mask 2 (positive resist)

9. . 10.. 11.. 12..

Top view (9)

Top view (10)

Top view (11)

History and processes

• Surface micro-machining (SMM) • Developed in the early 1980s at the University of California at Berkeley • Originally for polysilicon mechanical structures • Other processes include o Sandia National Lab’s SUMMIT (Sandia’s Ultraplanar Multi-level MEMS Technology)  five levels possible with four poly layers o MEMS CAP’s polyMUMPs (Multi MEMS Processes)  three layers of poly with a

Photo of a PolyMUMPs surface-micromachined micro-mirror. The hinge design allows for out-ofplane motion of the mirror.

Requirements and advantages • Three to four different materials required in addition to the substrate o Sacrificial material (etch rate Rs) o Structural mechanical material (etch rate Rm) o Sometimes electrical isolators and/or insulation materials (etch rate Ri) • Many SMM processes are compatible with CMOS (complementary metal oxide silicon) technology used in microelectronics fabrication. • Can more easily integrate with their control electronics on the same chip efficient and • Many SMM inexpensive processes have developed their own sets of standards

Rs >> Rm > Ri

Best results are obtained when structural materials are deposited with good step coverage. Chemical vapor deposition (CVD) or Physical vapor deposition (PVD) If PVD Sputtering

Common material/etchant combinations for surface μ-machining

Structural material

Sacrificial Material

Si/Polysilicon

SiO2

Al

Photoresist

Polyimide Si3N4

Phosphosilicate glass (PSG) Polysilicon

Etchant Buffered oxide etch (BOE) (HF-NH4F ~ 1:5) Oxygen plasma HF XeF2

Problems and issues

Stiction

moistu re

Stiction Stiction = static + friction Stiction = stick + friction

An example of an unfavorable scaling

surface tension  L  ~ 2 restoritiv e force F L

1 ~ L

An example of a portmanteau

Ways to reduce stiction •Coat (cubrir) surface with a thin hydrophobic layer in order to repel liquid •Dry surfaces using supercritical CO2. Removes fluids without allowing surface tension to form. C •Use “stand-off bumps” on the underside of moving parts. Pillars prop up (soportar) movable parts

Problems and issues

“Dimple” resulting from a stand-off bump on the underside of the cold arm

Polysilicon hotarm actuator created using surface μmachining

Te toca a ti Explain (with words, drawings, or both) how standoff bumps might be created.

Lift-off Usually included as an “additive technique” by most authors 1. Photoresist is spun on a wafer and exposed to create pattern Resist has either straight side walls, or better, a reentrant shape. 2. Material deposited through the photoresist mask using a line-of-sight method, such as evaporation 1.Shadowing takes place, 2.Part of the photoresist sidewalls must be free of deposited material (+) or (-) • Photoresist stripped leaving behind only C resist? material deposited through the opening. material to is the lifted of.thickness. Thickness of the deposited material mustUnwanted be thin compared resist Most often used to deposit metals, especially those that are hard to etch using plasmas

Typical process steps for surface micromachining

• • •

modeling and simulation design a layout design a mask set thin film formation (by growth or deposition)

1 2 3 4 mask set

lithography

C etching

die separation

C packaging

release

This is where process flow becomes complicated.

Die separation and packaging • Must separate the individual devices • Often saw or scribe the wafer

• Provide MEMS device with electrical connections • Protect MEMS from the environment • Sometimes must also provide limited access to environment (e.g., pressure sensor, inkjet print heads) die separation

• Packaging a difficult engineering C problem packaging

• Largest cost of producing many (most) devices

packagi ng

More on packaging

Wafer-level packaging

Die-level packaging

packagi ng

More on packaging Schematic of a packaged MEMS pressure detector showing some of the requirements unique to MEMS

Process integration (Process flow) We have learned much about the many materials and techniques for used processing materials to create devices, including • Nature of crystalline silicon • Adding material o Doping o Oxidation o Deposition  PVD  CVD • Photolithography • Bulk Micromachining • Surface Micromachining

How do we put these things together to create a device?   Specifically: •How do we choose which steps we need? •How do we choose the order of the steps? •How do we communicate this order of steps in the field?

Process integration (Process flow)

List of process steps in the correct order with the accompanying lithography

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Process integration (Process flow)

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Bulk μ-machined pressure sensor Thin Si diaphragm changes shape when pressure changes on one side relative to the other.   Piezoresistors (implemented using p+ diffusion) sense the deformation.   Aluminum wires send resistive electrical signal off the chip.   n+ diffusion is used as an etch stop for the backside etch.   Oxide + Nitride provides

Process flow, 1 The first for determining the process flow is to decide which steps we need. What are the basic steps necessary to build the diaphragm?    •Etch backside (Need to protect front of wafer during backside etch) •Add SiO2 and nitride layers •Etch area above diaphragm to give diaphragm ability to move easily •Create an “etch stop” layer o Reverse bias p-n junction will stop etch o Start with p-type wafer o Dope n-type layer or grow n-type epilayer (layer produces with

Process flow, 1 The first for determining the process flow is to decide which steps we need. What are the basic steps necessary to build the sensor?    •Add diffusion to get piezoresistor •Add wires so that piezoresistor can be connected to external world •Note that wires must be metal (Could use diffusion if the distance is short)

Process flow, 1 The first for determining the process flow is to decide which steps we need. What processing steps are required to produce entire device?    •Deposit/pattern oxide and nitride •Deposit/pattern Al for pads •Backside etch •n-type diffusion for etch stop •p-type diffusion for resistors/wires

Each of these steps results in more steps in the detailed process flow. But to begin, let’s determine the order in which the steps must be placed.

Process flow, 1 Order of steps What impacts our decisions on choosing an order?    1.Geometry The oxide must be deposited before the nitride. 2.Temperature High T processes must go first. High T processes can cause dopants to further diffuse and metals to melt and flow. Which processes are high T? •Oxidation •CVD (unless PECVD) •Drive-in for diffusion

3. Mechanical stress If a following step can cause a device to break, you may want to rethink the order if you can. This is why release steps are often (though not always) done last. 4. Interaction of chemicals If an etch will attack another material, you

Process flow, 1 Order of steps Let’s choose an order    1.n-type doping 2.Oxide: Can be done before doping of resistors if oxide is thin. (Boron will implant through thin oxide but not if oxide is thick!) 3.Dope resistors 4.Deposit nitride Do we do backside etch or metallization next? A long backside etch will attack metal, and so we must do backside etch first. Can we pattern nitride and oxide on both front and back at the same time? Yes, but etching both sides at the same time will etch all the way through the silicon and you will not have a diaphragm! And so we do them at

Mask 1

Process flow, 1 Order of steps Let’s choose an order    5.Backside etch: Before etching backside, we must cut the nitride and SiO2 using Mask 2. Nitride and SiO2 on topside protects topside of wafer. 6.Front side etch: Etch 6 nitride and oxide on topside of wafer

5.Metallization: How does the metal connect to the doping? Must cut through the nitride and oxide first. Holes are called “vias” or “ cuts”. Must pattern oxide and nitride on topside of 7 wafer to create cuts.. •Metallization: Add aluminum for vias and pads

Podemos combinarlos, ¿no?

Mask 3

Mask 2

2, Detailed process flow A detailed process flow is the list of all steps necessary for the process people to implement the device. It should include each of the following: 1. All steps in the proper order, including when to clean the wafer 2. Any chemicals necessary 3. Thicknesses of materials • •

These choices come for modeling. The “process people” can turn chemicals and thicknesses into times necessary for etches, depositions, etc.

4. Equipment necessary It is the responsibility of the process flow person to think about which equipment is necessary for each step. Why? Because if you need a high temperature deposition to follow a metallization, you need a PECVD to do it or your metal will flow. The process flow person knows the entire process and makes design decisions. 1. MASKS for photoligthography

Detailed process flow Let’s revisit each of the basic steps that we came up with and see what is really involved. You will notice that many of the steps actually turn into several steps when coming up with the detailed process flow. For this exercise, we will ignore dimensions and chemicals. However, note that these are also important components of the design flow. 1. n-type doping a. No mask is required since it covers the entire wafer b. This could be done by purchasing a wafer with an epilayer or it requires 2 steps i. implantation ii.drive-in 2. Oxide i. No mask is required since it covers the entire wafer. ii. Note that oxide will grow on both sides of the wafer. If you do not want it on the backside of the wafer, you must protect the backside of the wafer. iii. In this case, we do want oxide on both sides of

Detailed process flow Mask 1

3. Dope resistors and wires a. Mask 1 – what does it look like? (Assume positive resist.) b. This step requires 4 total steps a. Photolithography so that ion implantation only goes where you want it to go b. Ion implantation c. Remove photoresist (Must be done before drive-in. Why?) d. Drive-in a. Deposit nitride i. No mask is required since it covers the entire wafer ii. Depending on the process, you may need to process both sides of the wafer. a. PVD often only deposits on one side of the wafer.

Mask 1

Detailed process flow Mask 2

5. Backside etch a. Mask 2 – what does it look like? (Assume positive resist.) b. Must align Mask 2 with Mask 1 so that the resistors are on the edge of the diaphragm.  Alignment marks c. This step requires 5 steps a. Photolithography to determine where you want the backside etch to start b. Etch nitride c. Etch SiO2 d. Etch Si (Nitride and the SiO2 used as a “hard mask” for the long Si etch.) e. Remove photoresist

Mask 2

Detailed process flow Mask 3

6. Cuts/Diaphragm cut 7. Mask 3 – what does it look like? (Assume positive resist.) 8. Must align Mask 3 with Mask 1 so that wires connect to resistors. Alignment marks 9. This step requires 3 total steps a. Photolithography to determine where you want material removed for the metal b. Etch the nitride and oxide c. Remove the photoresist

Mask 3

Detailed process flow Mask 4

7. Metallization 8. Mask 4 – what does it look like? (Assume positive resist.) 9. Must align Mask 4 with Mask 1 so that metal does not etch away. Alignment marks 10. This step requires 4 total steps a. Deposit the Aluminum b. Photolithography to determine which Al you want to remove c. Etch unwanted Al d. Remove the photoresist

Mask 4

3, Final process flow These steps can be combined to create a final process flow. One additional requirement in process flows is to include information about when to clean the wafer. Some general guidelines are: •Always start with an RCA clean and an HF dip to get rid of every possible •All future cleans are usually RCA cleans without an HF dip. HF may etch away your MEMS structures. •Always strip photoresist and clean before high temperature processes. •Always clean before depositing a new layer.

Final process flow Final Process Flow for Bulk Micromachined Pressure Sensor Starting material: 100mm (100) p-type silicon, 1×1015 cm-3 boron

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