Issued Date：2019/10/15Creep Corrosion
The chip of 7nm node technology is the most advanced-process product now in mass production
Adding cobalt (Co) in metal materials is the key, yet will copper (Cu) really be replaced by Co?
Looming AI (Artificial Intelligence) and big data is demanding chips to shrink to improve performance. Facing the 7nm advanced process, semiconductor production is pressed to make chips of better performance, with less power consumption, smaller dimensions, yet still compliant with reliability requirements.
Following the path set by Moore’s Law, the semiconductor 7nm process is now in mass production. In terms of material engineering, the key steps in breaking the bottleneck of the advanced process of 7nm or even narrower processes has to respond to the major evolution of metal material, especially with respect to transistor contacts and wires.
Here comes Co, the rising star metal now. But is it true that Co is replacing Cu as alleged by the industry?
The iST material analysis lab will walk you into the micro world of 7nm by testing and measuring mass-production chips of 7nm node.
Reduce RC delay to speed up chip operation
In terms of integrated circuits, the parameter playing a critical role in defining speed and performance of IC components is the resistance-capacitance (RC) delay time.
Getting into the 7nm process for semiconductor production is not only adding more layers of metal interconnect but also shrinking the distance between conductors. Transmission of electronic message in layered metal interconnects would significantly slow down the working speed of semiconductor components due to the RC delay. It’s critical to deal with this drawback.
Shrinking the IC process could add risks at the barrier of increasing resistance
Cu(Copper)and Al(Aluminum) are the two most frequently employed metals for metal interconnecting at the back end of line (BEOL) of semiconductors. Cu is better for metal interconnecting in advanced processes as it outruns Al in conductivity, however, the Cu atoms have a much greater diffusion coefficient in the dielectric layer than Al. The semiconductor process then employs the more compact TaN to replace the TiN of columnar crystal structure to prevent Cu drift and diffusion through the ILD and open circuits resulted.
On the other hand, the resistance coefficient of TaN outnumber TiN more than tenfold (see table 1) and result in rising metal interconnecting resistance caused by TaN barrier.
TaN barrier TiN barrier Resistance coefficient 200 ~ 350 mΩ-cm 22 mΩ-cm
Table 1: TaN and TiN resistance coefficient
Resistance of metal interconnecting is the sum of resistance of the Cu wire and TaN barrier. The effect of the former on ratio of resistance increase can be ignored when the wire is thick but not so when it is getting thinner and thinner after chips are shrunk to the extreme which, in turn, raises the rate of contribution in resistance by the TaN layer. This is illustrated in table 2: the rate by simplified measurement with parallel resistors suggests that it may soar over 40-fold when the diameter of Cu wire shrinks from 200nm to 20nm.
Resistance on metal wire = copper wire resistance + TaN resistance Existing process Copper wire dimension TaN layer dimension TaN layer resistance contribution 200nm 5nm 1 (the default value) 7nm advanced process Copper wire dimension TaN layer dimension 20nm 5nm > 40
Table 2: TaN layer resistance contribution: simplified calculation with parallel resistors
Due to Cu’s easy diffusion, it’s no good to reduce resistance by cutting the “thickness” of the TaN layer in the Cu process as the TaN layer may lose its function of barrier. It’s critical to get new material to replace the Cu wire or barrier layer in the 7nm IC process.
Selection of metal material played a key role in cutting resistance of 7nm ICs
How to reduce resistance of the TaN barrier? The key lies in the composition of metal material in the layer. Studies found that Co is ideal in mixing with the TaN barrier as it not only reduces resistance of the barrier but also thins it.
Double layer window maximizes performance of Co
The connection between metal wire and semiconductor components on silicon substrates is called “contact”, with Tungsten (W) playing the role of the connection and TiN the barrier. In the Cu metallization process, Co once again becomes the best alternative to reduce resistance of contact of W/TiN. The process of the double layer window is introduced to prevent the solid solution of Co in Cu and poorer metal conductors’ electron migration when replacing W/TiN with Co.
Test the 7nm process ICs and check whether Co is fully replacing Cu
After identifying reasons to employ Co, the iST material analysis lab is to pinpoint where in the 7nm process the Co material is used and whether Co is really replacing Cu completely.
Sample preparation operation
The analysis over products by the 7nm advanced process by iST material analysis lab.
Get market available mobile phone and tear down the Kirin 980 CPU from the PCB and analyze the relevant structure including X-ray analysis, removal of solder ball, packaging, and adhesive, IR (infrared ray) positioning grinding, etching, and CPU/DRAM dual-chip separation.
View Kirin 980 CPU with TEM
Analyze the FEOL (Front End of Line) and BEOL (Back End of Line) of 7nm ICs with Transmission electron microscope (TEM) and Energy-dispersive X-ray spectroscopy (EDS).
The iST material analysis lab observed the M1 and M2 metal layers with TEM and EDS, resolved FinFET, gate, and contact (see figure 2), and analyzed the elements distribution of Co and W (see figure 3).
Figure 2: STEM HAADF image of fin transistor, gate, contact, M1 and M2 structure
Figure 3: Co element shown in pink, and tungsten (W) in green; refer to figure 2 for distribution of Co and W in the structure
Based on figure 2 and 3, the lab found Co in the “contact” and “barrier layer”. The latter is fully covered by Co and becomes the material of barrier yet without fully replacing the W/TiN of the contact. This could be a result of different reactions between Co and the surrounding materials due to varying processes employed by the contact and barrier layer.
Results by TEM suggest that Co is to be the barrier of Cu and replace half of the contacts rather than replacing it. The conclusion: Co does not fully replace Cu in products made by the 7nm advanced process.
The electronic verification and analysis lab of iST is providing you with a full range of material analysis services including structure identification by TEM, EDS composition analysis, FIB precision editing, microscopic sample preparation, advanced process analysis, structural observation and engineering analysis, including nano/metal/ceramic/biomedical material analysis. To learn more about our services, just ring Jay Wang at +886-3-579-9909 EXT 6161 or email us at email@example.com.