Low Stress LPCVD Nitride with a Film Stress of less than 250 MPa Tensile is often used for building Membranes, Cantilever Beams and other mechanical structures. Low Stress Nitride is also very effective as a KOH. The characteristics of silicon-nitride films deposited with plasma-enhanced chemical vapor deposition (PECVD) can vary in composition and physical properties. An important parameter in the fabrication of micro-electro-mechanical systems (MEMS) devices is the control and tailoring of the film stress. In fact, silicon-nitride film stress can vary from tensile (up to 800 MPa) to compressive (-500 MPa), depending on the PECVD deposition conditions.
One of the basic building blocks in silicon wafer processing and MEMS processing is the ability to deposit thin films of material onto a wafer. One such deposition process is LPCVD Silicon Nitride, a thin film deposition process that coats all of the exposed areas of the wafer. The difference in mechanical properties between the nitride and the silicon wafer will introduce a "stress" in the wafer + nitride system.
Tensile and Compressive Stress: If there is an equal amount of nitride on both sides of the wafer it will be in a balanced state and remain flat.
If the nitride is removed from one side of the wafer the system will no longer be in equilibrium and the wafer will bow. If the wafer curves up toward the side with the nitride, the nitride is considered to be tensile. If the wafer curves down away from the side with the nitride, the nitride is considered to be compressive.
Selecting the Right Film Stress: Choosing the correct film stress can be crucial to yield and performance of the device. For instance, if Stoichiometric Nitride (800 Mpa Tensile) is used as a KOH mask, too thick of a film can lead to micro-cracking and film failure. Switching to a Low Stress NItride (200 Mpa Tensile) will eliminate micro cracking and improve yield.
Film Stress for MEMS: Having too low of a film stress can also cause problems for certain applications. Many MEMS foundry applications use silicon nitride as a membrane, diaphragm, or other suspended structure. Insufficient tensile stress can result in structures that can "sag" or "droop". Typically, the thinner the nitride, the more tensile stress is needed to keep a suspended structure flat.
Remember, the entire suspended structure must be considered a "system". If additional compressive films such as PECVD Oxide are added on top of the nitride, additional stress must be built into the nitride to compensate for it. The entire "system" must be tensile.
Applications Guide: Here are some of our nitride recommendations for select applications:
KOH Etch Mask: Nitride Thickness (500 to 3000 A)
Film type: stoichiometric Nitride On Oxide Stress
Target (-300/800 Mpa)
Thickness (500 to 10,000 A)
Film type: Low Stress Nitride Stress
Target (200 Mpa)
Thickness (500 to 30,000 A)
Film type: Super Low Stress Nitride Stress
Target (50 Mpa)
Suspended Structures:
Nitride Thickness (Less than 500 A)
Film type: Low Stress Nitride Stress
Target (350 Mpa)
Thickness (500 to 1000 A)
Film type: Low Stress Nitride Stress
Target (300 Mpa)
Thickness (1000 to 5,000 A)
Film type: Low Stress Nitride Stress
Target (250 Mpa)
Thickness (5,000 to 10,000 A)
Film type: Low Stress Nitride Stress
Target (200 Mpa)
Thickness (10,000 to 15,000 A)
Film type: Low Stress Nitride Stress
Target (150 Mpa)
Thickness (15,000 to 20,000 A)
Film type: Super Low Stress Nitride Stress
Target (100 Mpa)
Thickness (20,000 to 30,000 A)
Film type: Super Low Stress Nitride Stress
Target (50 Mpa)
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