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3D-NAND Deposition and Etch Integration

Thursday, September 1st, 2016


By Ed Korczynski, Sr. Technical Editor

3D-NAND chips are in production or pilot-line manufacturing at all major memory manufacturers, and they are expected to rapidly replace most 2D-NAND chips in most applications due to lower costs and greater reliability. Unlike 2D-NAND which was enabled by lithography, 3D-NAND is deposition and etch enabled. “With 3D-NAND you’re talking about 40nm devices, while the most advanced 2D-NAND is running out of steam due to the limited countable number of stored electrons-per-cell, and in terms of the repeatability due to parasitics between adjacent cells,” reminded Harmeet Singh, corporate vice president of Lam Research in an exclusive interview with SemiMD to discuss the company’s presentation at the Flash Memory Summit 2016.

“We’re in an era where deposition and etch uniquely define the customer roadmap,” said Singh,“and we are the leading supplier in 3D-NAND deposition and etch.” Though each NAND manufacturer has different terminology for their unique 3D variant, from a manufacturing process integration perspective they all share similar challenges in the following simplified process sequences:

1)    Deposition of 32-64 pairs of blanket “mold stack” thin-films,

2)    Word-line hole etch through all layers and selective fill of NAND cell materials, and

3)    Formation of “staircase” contacts to each cell layer.

Each of these unique process modules is needed to form the 3D arrays of NVM cells.

For the “mold stack” deposition of blanket alternating layers, it is vital for the blanket PECVD to be defect-free since any defects are mirrored and magnified in upper-layers. All layers must also be stress-free since the stress in each deposited layer accumulates as strain in the underlying silicon wafer, and with over 32 layers the additive strain can easily warp wafers so much that lithographic overlay mismatch induces significant yield loss. Controlled-stress backside thin-film depositions can also be used to balance the stress of front-side films.

Hole Etch

“The difficult etch of the hole, the materials are different so the challenges is different,” commented Singh about the different types of 3D-NAND now being manufactured by leading fabs. “During this conference, one of our customer presented that they do not see the hole diameters shrinking, so at this point it appears to us that shrinking hole diameters will not happen until after the stacking in z-dimension reaches some limit.”

Tri-Layer Resist (TLR) stacks for the hole patterning allow for the amorphous carbon hardmask material to be tuned for maximum etch resistance without having to compromise the resolution of the photo-active layer needed for patterning. Carbon mask is over 3 microns thick and carbon-etching is usually responsive to temperature, so Lam’s latest wafer-chuck for etching features >100 temperature control zones. “This is an example of where Lam is using it’s processes expertise to optimize both the hardmask etch as well as the actual hole etch,” explained Singh.

Staircase Etch

The Figure shows a simplified cross-sectional schematic of how the unique “staircase” wordline contacts are cost-effectively manufactured. The established process of record (POR) for forming the “stairs” uses a single mask exposure of thick KrF photoresist—at 248nm wavelength—to etch 8 sets of stairs controlled by a precise resist trim. The trimming step controls the location of the steps such that they align with the contact mask, and so must be tightly controlled to minimize any misalignment yield loss.

A) Simplified cross-sectional schematic of the staircase etch for 3D-NAND contacts using thick photoresist, B) which allows for controlled resist trimming to expose the next “stair” such that C) successive trimming creates 8-16 steps from a single initial photomask exposure. (Source: Ed Korczynski)

Lam is working on ways to tighten the trimming etch uniformity such that 16 sets of stairs can be repeatably etched from a single KrF mask exposure. Halving the relative rate of vertical etch to lateral etch of the KrF resist allows for the same resist thickness to be used for double the number of etches, saving lithography cost. “We see an amazing future ahead because we are just at the beginning of this technology,” commented Singh.


Applied Materials Releases Selective Etch Tool

Wednesday, June 29th, 2016


By Ed Korczynski, Sr. Technical Editor

Applied Materials has disclosed commercial availability of new Selectra(TM) selective etch twin-chamber hardware for the company’s high-volume manufacturing (HVM) Producer® platform. Using standard fluorine and chlorine gases already used in traditional Reactive Ion Etch (RIE) chambers, this new tool provides atomic-level precision in the selective removal of materials in 3D devices structures increasingly used for the most advanced silicon ICs. The tool is already in use at three customer fabs for finFET logic HVM, and at two memory fab customers, with a total of >350 chambers planned to have been shipped to many customers by the end of 2016.

Figure 1 shows a simplified cross-sectional schematic of the Selectra chamber, where the dashed white line indicates some manner of screening functionality so that “Ions are blocked, chemistry passes through” according to the company. In an exclusive interview with Solid State Technology, company representative refused to disclose any hardware details. “We are using typical chemistries that are used in the industry,” explained Ajay Bhatnagar, managing director of Selective Removal Products for Applied Materials. “If there are specific new applications needed than we can use new chemistry. We have a lot of IP on how we filter ions and how we allow radicals to combine on the wafer to create selectivity.”

FIG 1: Simplified cross-sectional schematic of a silicon wafer being etched by the neutral radicals downstream of the plasma in the Selectra chamber. (Source: Applied Materials)

From first principles we can assume that the ion filtering is accomplished with some manner of electrically-grounded metal screen. This etch technology accomplishes similar process results to Atomic Layer Etch (ALE) systems sold by Lam, while avoiding the need for specialized self-limiting chemistries and the accompanying chamber throughput reductions associated with pulse-purge process recipes.

“What we are doing is being able to control the amount of radicals coming to the wafer surface and controlling the removal rates very uniformly across the wafer surface,” asserted Bhatnagar. “If you have this level of atomic control then you don’t need the self-limiting capability. Most of our customers are controlling process with time, so we don’t need to use self-limiting chemistry.” Applied Materials claims that this allows the Selectra tool to have higher relative productivity compared to an ALE tool.

Due to the intrinsic 2D resolutions limits of optical lithography, leading IC fabs now use multi-patterning (MP) litho flows where sacrificial thin-films must be removed to create the final desired layout. Due to litho limits and CMOS device scaling limits, 2D logic transistors are being replaced by 3D finFETs and eventually Gate-All-Around (GAA) horizontal nanowires (NW). Due to dielectric leakage at the atomic scale, 2D NAND memory is being replaced by 3D-NAND stacks. All of these advanced IC fab processes require the removal of atomic-scale materials with extreme selectivity to remaining materials, so the Selectra chamber is expected to be a future work-horse for the industry.

When the industry moves to GAA-NW transistors, alternating layers of Si and SiGe will be grown on the wafer surface, 2D patterned into fins, and then the sacrificial SiGe must be selectively etched to form 3D arrays of NW. Figure 2 shows the SiGe etched from alternating Si/SiGe stacks using a Selectra tool, with sharp Si corners after etch indicating excellent selectivity.

FIG 2: SEM cross-section showing excellent etch of SiGe within alternating Si/SiGe layers, as will be needed for Gate-All-Around (GAA) horizontal NanoWire (NW) transistor formation. (Source: Applied Materials)

“One of the fundamental differences between this system and old downstream plasma ashers, is that it was designed to provide extreme selectivity to different materials,” said Matt Cogorno, global product manager of Selective Removal Products for Applied Materials. “With this system we can provide silicon to titanium-nitride selectivity at 5000:1, or silicon to silicon-nitride selectivity at 2000:1. This is accomplished with the unique hardware architecture in the chamber combined with how we mix the chemistries. Also, there is no polymer formation in the etch process, so after etching there are no additional processing issues with the need for ashing and/or a wet-etch step to remove polymers.”

Systems can also be used to provide dry cleaning and surface-preparation due to the extreme selectivity and damage-free material removal.  “You can control the removal rates,” explained Cogorno. “You don’t have ions on the wafer, but you can modulate the number of radicals coming down.” For HVM of ICs with atomic-scale device structures, this new tool can widen process windows and reduce costs compared to both dry RIE and wet etching.


Tallness Makes Reliable Spindt Tip Cold Cathodes

Monday, November 30th, 2015


By Ed Korczynski, Sr. Technical Editor

MIT researchers have seemingly found a solution to a half-century-old engineering challenge:  how to make a reliable cold cathode array for vacuum electronic devices. While solid-state technology continues to replace vacuum tubes for most electronic applications—the most recent being light emitting diode (LED) luminaires replacing both incandescent and fluorescent light bulbs—there are still applications where vacuum-based devices provide unmatched performance. At the IEEE’s upcoming annual International Electron Devices Meeting (IEDM, Stephen Guerrera will present paper number 33.1 entitled “High Performance and Reliable Silicon Field Emission Arrays Enabled by Silicon Nanowire Current Limiters” on behalf of his team. Since these field emission arrays (FEA) are built in silicon, they can be integrated as cold cathodes with silicon ICs to function as compact RF amplifiers and as sources of terahertz, infrared, and X-ray energy.

Figure 1 shows a 3D illustration of the FEA structure, along with scanning electron microscope (SEM) close-up images of one tip cross-section and the tip array. The array of cold cathodes can be considered as a group of nanoscale electron guns. Each 10µm tall and 100-200nm diameter vertical silicon nanowire is topped by a 6-8nm diameter conical emitter tip.

FIGURE 1: (top) 3D schematic of the FEA device structure showing 50:1 aspect-ratio silicon nanowires, and (bottom) left-side SEM image of one tip cross-section, and right-side plan-view SEM of gate holes showing 1µm spacing and a gate aperture of 350nm.

As can be seen in the bottom left of Figure 1, the researchers used a variation on the now-standard “Bosch” deep reactive ion etch (DRIE, process to form the nanowires. The Bosch process uses vertical etching with side-wall dielectric deposition in alternating sequences such that cross-sections appear with characteristic scalloped profiles. It appears that the other unit-processes used by the MIT team to create this new device are likewise similar to industry standards.

However, while based on standard processes, the cost of using a Bosch etch and the other steps needed to fabricate the 50:1 aspect-ratio (AR) of these 200nm diameter wires is inherently high. In constrast, 5:1 AR structures can be formed using much less expensive processes, while 1:1 AR cones as used in original Spindt tips can be very inexpensive to make. Why do these nanowires need to be so tall?


Decades before organic light emitting diode (OLED) technology was to be the future of flat panel display (FPD) manufacturing, Field Effect Display ( FED) technology was explored as a more efficient replacement for liquid crystal displays (LCD). FED have typically been built using “Spindt Tip” Arrays, named after Charles A. Spindt who developed the technology for SRI International as originally published as “A thin-film field-emission cathode,” Journal of Applied Physics, vol. 39, no. 7, pages 3504-3505, 1968. Figure 2 shows how FED use multiple redundant Spindt Tips to light up the phosphor in each color sub-pixel. With ten or more connected in parallel, if one tip fails then the remaining tips in the sub-pixel cluster only have to handle a 10% increase in current…under another tip fails…and another tip will fail over time without a way to limit current.

FIGURE 2: Cross-sectional schematic of a pixel in a field effect display (FED), showing multiple redundant Spindt Tips driving a single phosphor dot.

Though inherently prone to reliability issues, the original Spindt Tip design is extraordinarily manufacturable. After layers of blanked film depositions, the top gate is patterned with uniform holes which serve as masks for both the etching of cavities and the deposition of tips. Each resulting cone-shaped tip concentrates the current to the point, allowing for efficient low voltage operation. The problem with concentrating current is that over time it tends to find a weak spot in grain boundaries resulting in decreased electrical resistance on one side of a tip, such that current flow over-concentrates and run-away heating causes the tip to fail.

In 2002 and still with SRI International, Spindt was co-author on “Spindt cathode tip processing to enhance emission stability and high-current performance” published by the American Vacuum Society [DOI: 10.1116/1.1527954]. The paper describes using the extracted field emission current to controllably heat and recrystallize the surface of Spindt tips to drive off impurities and smooth the tip surface, thereby producing more uniform physical arrays for more reliable functionality.

While alloys and anneals can improve the reliability, the run-away over-heating effect remains an inherent issue with conical tips. Dean et al. from Motorola worked on FEDs for many years, and found that individual tips emit from multiple nano-scale features with fluctuating current levels [DOI: 10.1109/IVMC.2001.939681]. It only takes one of these nano-scale features to preferentially line-up with a grain-boundary for it to draw relatively more current, and with electro-migration the feature can grow from the surface to be relatively closer to the gate compared to the rest of the tip. A protruding nano-spike create an extremely concentrated electrical field, which further concentrates the current flowing to the protrusion until it tends to physically explode.


Having exhausted all easier solutions, it now appears that using DRIE to create 50:1 AR vertical nanowires is the way to make reliable FEA. The nanowires act as current limiters to protect each emitter tip from run-away heating and arcing, and thereby design-in reliability unlike simple Spindt Tip cones. Since high-quality silicon epitaxial layers with controlled dopant levels to ensure uniform electrical resistivity can be commercially sourced, the resistance of nanowire arrays etched from such an epi-layer can be easily controlled. These device reportedly exhibit long lifetimes and low-voltage operation.

The team built emitter arrays as large as 1,000 x 1,000, and have shown ability to handle current density of >100 A/cm2, more than a hundredfold greater than any other field-emission cathode operated in continuous wave mode. These new devices combine the positive aspects of solid state semiconductors (high gain and low noise) with those of vacuum electronics (high power and efficiency). While not likely to appear in commercial FPDs, there seems to be demand for this technology in diverse communications, defense, and healthcare applications.


A Study Of Model-Based Etch Bias Retarget For OPC

Thursday, August 15th, 2013

Model-based optical proximity correction is usually used to compensate for the pattern distortion during the microlithography process. Currently, almost all the lithography effects, such as the proximity effects from the limited NA, the 3D mask effects due to the shrinking critical dimension, the photo resist effects, and some other well known physical process, can all be well considered into modeling with the OPC algorithm. However, the micro-lithography is not the final step of the pattern transformation procedure from the mask to the wafer. The etch process is also a very important stage. It is well known that till now, the etch process still can’t be well explained by physics theory. As we all know, the final critical dimension is decided by both the lithography and the etch process. If the etch bias, which is the difference between the post development CD and the post etch CD, is a constant value, it will be simple to control the final CD. But unfortunately this is always not the case. For advanced technology nodes with shrinking critical dimension, the etch loading effect is the dominant factor that impacts the final CD control. And some people tried to use the etch-based model to do optical proximity correction, but one drawback is the efficiency of the OPC running will be hurt. In this paper, we will demonstrate our study on the model based etch bias retarget for OPC.

To download this white paper, click here.