Atomic Layer Deposition for Integrated Circuits and MEMS

The Atomic Layer Deposition (ALD) thin-film coating method has been used in the semiconductor industry for several years in the fabrication of a variety of electronic devices and components.

The increased level of system miniaturization and integration requires the deposition of thin films that are highly uniform, homogenous, conformal, dense, and free from defects, cracks, and pinholes on sophisticated electronic components.

When the required film thickness starts to approach the nanometre scale, conventional thin film deposition techniques such as CVD and PVD fail to meet this requirement, whereas ALD forms superior quality films even on the most intricate nanoscale geometries, thanks to its surface controlled, self-saturating film growth mechanism.

In addition to coating active material layers inside the IC components, the ALD method can also be used to encapsulate the devices against impurities, oxygen, and moisture, thereby ensuring long and failure-free operation. Today, numerous global semiconductor industries rely on Picosun’s production ALD technology in manufacturing advanced IC and electronic devices. Picosun is also involved in several international R&D initiatives in the field of ALD and IC.

Advantages of the ALD Technique

High-quality thin films with excellent chemical and structural purity, uniformity, and conformality can be manufactured using the ALD method, which involves gas-solid chemical adsorption reactions over the surface. These reactions are self-controlled and self-limiting, resulting in film growth by consecutive atomic layers.

A wide choice of materials can be deposited, including oxides, nitrides, fluorides, sulfides, pure metals (even noble ones), polymers, and graded, mixed, or doped layers. Various multi-layered nanolaminates can be deposited with the option of customizing the properties of the individual layers on the molecular level.

The ALD technique not only coats silicon wafers but also 3D objects, powder, porous, and high aspect ratio (HAR) samples. Since the film grows by sequential atomic layers, chemical composition and thickness of the film can be precisely controlled. The ALD process is repeatable and various materials can be deposited at low temperatures, allowing the use of also sensitive substrates such as plastics or papers.

ALD Applications

In IC applications ALD is typically used to manufacture:

• High-k oxide films
• Spacer oxide films
• Inter-poly dielectric oxide films
• Tunneling oxide films
• Blocking oxide films
• Gap-filling oxide films
• Passivation films
• Capping layer films
• Copper barrier and seed films
• Adhesion layer films
• Conductive metal films for interconnects (through-silicon-vias, TSV)
• Diffusion barrier films
• Ferroelectric materials
• Paramagnetic materials

For components and devices such as:

• Transistors (gate dielectrics & metals)
• Capacitors
• Memories
• Read heads

In MEMS applications, ALD is optimal for deposition of:

• Diffusion barriers
• Adhesion layers
• Charge dissipative layers
• Layers lowering frictional wear
• Optical layers
• Films to close nanoscale pores
• Biocompatible coatings
• Coatings for hermetical sealing
• Hydrophobic layers to decrease stiction
• Conformal, thermally conductive layers
• Conductive seed layers for plating purposes
• Etch masks and etch stop layers
• Conformal, electrically insulating layers

ALD is also entering into more “traditional” industrial production in applications such as:

• Protective coatings against wear and corrosion of metal objects
• Anti-tarnishing of coins and jewelry
• Decorative coatings
• Biocompatible coatings for medical applications
• Energy storage devices such as thin-film batteries
• Solar cell efficiency enhancement
• Powder-based catalyst materials
• Ecological, recyclable packaging materials

ALD in IC

In the IC industry, ALD is already a standard high volume manufacturing technique in HDD (hard disk drive), DRAM (dynamic random access memory), and CMOS (complementary metal-oxide-semiconductor) applications. In accordance with Moore’s law and constantly decreasing component dimensions ALD is becoming the enabling technology to realize e.g. more and more compact next-generation data storage devices. Advanced 3D integration and packaging of the systems and modules, on the other hand, utilize conductive TSV structures enabled by ALD.

Interconnecting the active layers in a 3D-integrated system requires highly conformal, ultra-thin metallization film that is able to cover uniformly deep trench structures with very high aspect ratios. Deep trench capacitors for memory devices also require high-quality metallization layers. ALD is the ideal method for producing these metal layers, thus enabling continuous shrinkage of device features. Currently, ALD metallization in a batch process is entering high throughput industrial production. Picosun offers various ALD processes for the deposition of conductive layers (metals/metallics) for IC applications.

High-k dielectrics such as HfO2, TiO2, Ta2O5, ZrO2, SrTiO3, and HfxSiyO, and other oxide films are another central group of ALD materials in IC. Ultra-thin ALD gate oxide and metal layers enable higher and higher transistor count on a chip and thus continuously improving the performance of ICT end products. ALD’s ability to produce nanolaminates with molecular-level designed layer structure also gives almost endless opportunities for fine-tuning and optimizing the electrical properties of the dielectric materials. For read heads for hard disk drives, ALD Al2O3 is the ideal passivation material due to its high electrical breakdown strength. Picosun provides oxide processes with leading film quality and process purity in batch for numerous IC production applications.

Figure 1. ALD has been used in the IC industry for several years. (Image Credit: Solid State Technology, November 2007; IEEE Spectrum 2007

ALD metal electrodes successfully entered into production over a decade ago. ALD aluminum oxide film is used as a passivation layer for reading heads in computer hard disks.

ALD in MEMS

Various conventional thin-film depositions methods such as CVD, spin coating, and sputtering suffer from the same drawbacks in MEMS technology as in IC component manufacturing: Constantly decreasing device features set so high standards for the film quality that they are very hard to reach with “traditional” coating techniques. High deposition temperatures are also often a restricting factor in MEMS manufacturing.

Both the abovementioned issues can be addressed with ALD, which enables high quality, uniform, and conformal film deposition on MEMS structures even at low temperatures.

ALD Al2O3 is utilized as an electric shocking preventive layer, etch mask and stop layer, hydrophobic, and adhesive layer. It can also be used as a protective layer for hermetic sealing of the devices.

ALD TiO2 works as an anti-stiction layer and protects the underlying structures against friction and wear.

ALD ZnO:Al can be used as charge dissipative layer and ALD SiO2 and Si3N4 as electrically insulating layers.

Of the abovementioned materials, Al2O3, SiO2, and TiO2 are also biocompatible, allowing their use in e.g. medical implant applications.

MEMS industry benefits especially from the ALD’s ability to coat outstanding quality thin film and nanolaminates inside trenches (Figure 2). Sharp enough interfaces within these structures are not possible to achieve with any other coating techniques.

A combination of ALD thin films of different materials can also act as reflective, anti-reflective, and black absorbing optical coating. Conformal film coverage provides superior film properties even in structures that are difficult to coat. Conversely, nanoscale pores can be closed using the ALD technique.

Picosun provides production-proven ALD solutions for various MEMS applications. Currently, Picosun ALD technology is in daily use at several global, leading MEMS manufacturers around the globe.

Figure 2. Above: SEM image of ALD SiO2 in trenches (picture credit University of Helsinki). Below: SEM image of ALD oxide nanolaminate in trenches, deposited in a PICOSUN™ ALD tool (picture credit VTT Technical Research Centre of Finland).

Conclusion

Today, ALD is a crucial, enabling technology in the IC and MEMS manufacturing, realizing both already existing products and enabling the device performance enhancement with constantly decreasing component features and increasing level of system integration.

PICOSUN provides leading quality, high throughput, and cost-efficient batch and cluster ALD tools for industrial manufacturing of IC and MEMS components (Figures 3 - 5) with years of unparalleled, production-proven experience in the field.

Figure 3. PICOSUN™ P-1000 batch ALD tool for ultra-high volume manufacturing for wafers up to 450 mm, large glass/metal sheets, and large batches of 3D objects.

Figure 4. PICOSUN™ P-300B batch ALD tool for high volume manufacturing for wafers up to 300 mm, large batches of 3D objects, and powder samples.

Figure 5. Picosun’s Picoplatform™ 300 vacuum cluster system for wafers up to 300 mm diameter, with full process automation and SEMI S2 compatibility.

This information has been sourced, reviewed and adapted from materials provided by Picosun Oy.

For more information on this source, please visit Picosun Oy.