DISCO Technical Review Nov. 2018

Investigation of Cu and Ni Diffusion Amounts for Silicon Substrates
Sales Engineering dept.

Abstract
An investigation regarding the Cu and Ni diffusion amounts for silicon substrates was conducted. A Cu
and Ni reference solution was applied to a silicon substrate, heated to 350ºC for one or two hours, andthen measured using TXRF. As a result, it was discovered that Cu diffused into the silicon substrate tosuch a level that it could be detected on the side opposite the Cu side, while Ni diffused only several
hundred nm. It is assumed that preventative measures, such as gettering site forming for Cu, will berequired. However, the effects of Ni used in electroformed dicing blades are considered minimal.

Cu, which has a high diffusion coefficient,

1. Introduction
In semiconductor device manufacturing,

requires control even on the sidewalls andcontamination caused by metal impurities has abackside of Si substrate memory devices

large impact on decreases in device reliability.

especially, which are becoming thinner.

Use of heavy metals, such as Fe (iron) and Ni

Intentionally leaving processing damage as a(nickel), and alkali metals, such as Na (sodium),gettering site on the backside of the Si substrate

is avoided, and in particular, Cu (copper), when aafter thinning is used as a countermeasure for

Si (silicon) substrate is being used (Fig. 1).

capturing Cu.
On the other hand, electroformed dicing blades
are used in the dicing process, which singulates theSi substrates (Photo 1). These blades use Ni as thebond material. Essentially, the exposed section of
the diamond grit is what contacts the Si (Photo 2).
However, there are concerns of contaminationfrom metal impurities due to the possibility ofminute amounts of residual Ni in the Si substrate.
In this experiment, the amount of Cu and Ni
diffusion in the Si substrate was investigated undertemperature environment conditions harsher thanthose used in the back-end of the semiconductor
device manufacturing process.

Fig. 1. Diffusion coefficient and temperaturedependence on Si (1)1DISCO Technical Review Nov. 2018

2-2. Method of Measuring Cu Diffusion
Because Cu diffuses quickly on a Si substrate,
the amount of Cu was measured on the side where
the metal standard solution was applied and on
the opposite side.
Photo 1. dicing blades

2-3． Method of Measuring Ni Diffusion
Because Ni diffuses slowly at low
temperatures, the side with the metal standard
solution applied was measured. After heating thesubstrate in a clean oven and measuring theamount of Ni on the outermost surface, the Ni
was removed from the outermost surface using
HCl + H2O2 + H2O (hydrochloric acidperhydrate). Next, the naturally oxidized Si

Photo 2. Bond and diamond abrasive gritsurface, or SiO2 (silicon dioxide) layer, was
2. Method of Experimentremoved using DHF (dilute hydrofluoric acid), aThe method used to investigate the amount ofStandard Clean 1 was performed using NH4OH

diffusion in the Si substrate is shown below.

(ammonium hydroxide) and H2O2 (hydrogenperoxide solution) mixed in deionized water, and2-1. Method of Experimenta thin SiO2 (silicon dioxide) layer was formed on

Cu standard solution and Ni standard solution

the Si surface. Fine etching was performed

reagents for optical absorption analysis

repeatedly on the formed oxidization film and the(manufactured by Kanto Chemical Co., Ltd.) wereamount of Ni was measured every several tens ofeach diluted and applied to the backside of 725

nanometers.

µm thick 8-inch diameter Si substrates so that theconcentration was 1.00E13 atoms per square

3. Measurement Results

centimeter. After the substrates were natural-air

3-1. Amount of Cu Diffusion

dried, they were heated in an environment ofIn the Cu sample heated at 350°C for one hour,

350°C using a clean oven for both one and twoon the side opposite to where the Cu was applied,

hours each and subjected to forced diffusion. The

an average diffusion amount of 1.59E10 atoms

amount of Cu and Ni diffusion was then measured

per square centimeter and a maximum diffusion

using a TREX6000 manufactured by Technos

amount of 9.57E10 atoms per square centimeter

Corporation (TXRF: total reflection x-rayfluorescence).

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DISCO Technical Review Nov. 2018

3-2. Amount of Ni Diffusion

were measured (Fig. 2).

Because the diffusion speed was slow in the Ni
diffusion measurements, the amount of Ni wasmeasured while performing fine etching on asurface with metal standard solution applied.As isshown in the graphs in Fig. 4, Ni (Y-axis) waspresent in each process step (X-axis). However,for the sample subjected to thermal diffusion at350°C for one hour, almost all of the Ni hadFig. 2. Amount of Cu diffusion in 1 hour at 350°C (TXRF)

been removed in the cleaning of the outermost
surface where it had been applied. Fine etching
was performed repeatedly and the amount of Ni

In the Cu sample heated at 350°C for two

was measured every 10 nanometers.

hours, the average diffusion amount was 2.52E10

In the Y-log graph, even though it was a small

atoms per square centimeter and the maximum

amount, it is clear that the Ni diffused and wasdiffusion amount was 19.52E10 atoms per square

present inside the Si substrate. Taking the average

centimeter (Fig. 3).

Ni diffusion amount into consideration, theamount of Ni below a depth of 60 nm was lessthan 0.50E10 atoms per square centimeter.
It is also clear that the Ni diffused further into
the substrate in the sample that was subjected tothermal distribution for two hours at atemperature of 350°C than the sample that wassubjected to thermal diffusion for one hour at atemperature of 350°C (Fig.5). Taking the average
Ni diffusion amount into consideration, theFig. 3. Amount of Cu diffusion in 2 hours at 350°C (TXRF)

amount of Ni below a depth of 250 nm was lessthan 0.50E10 atoms per square centimeter.

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DISCO Technical Review Nov. 2018

Fig. 4. Amount of Ni diffusion in 1 hour at 350°C

such as in the NiSi (nickel solinoid) adjacent to

4. Discussion

gate electrodes(2)(3).

In the accelerated experiment of one hour in aharsher

Thus, based on the information above, we cantemperature environment than what is used in thededuce that the Ni in the electroformed dicing

back-end

deviceblades used in the dicing process has very little

manufacturing process), we observed that the Cu

impact on Si die, even when it adheres to thediffused through to the reverse side of the 725 µm

sidewall of the die.

350°C

environment

ofthe(whichisa

semiconductor

thick Si substrate. Even though this experiment
was performed under a harsher temperature

5. Conclusion

environment than that which is used in the actual

By forcibly contaminating Si substrates with Cu

manufacturing process, because of the highand Ni and investigating the amount of diffusion indiffusivity, it is thought that preventative measurestemperature environment conditions even harsher

for Cu contamination from the backside of the Si

than those used in the back-end of thesubstrate, such as gettering, are required.

semiconductor device manufacturing process, weverified that Cu has a high diffusivity and Ni has a

In contrast, it was evident that Ni only diffuses

low diffusivity.

several hundred nanometers even in an accelerated
experiment. Ni is already known as a stable

It was confirmed that Cu, which has a highcompound that is easy to use when reacting with Sidiffusivity, can be captured in a gettering site,

and is used on circuits on the front side of devices,which causes processing damage to remain on the4

DISCO Technical Review Nov. 2018

Fig. 5. Amount of Ni diffusion in 2 hours at 350°C

substrate. However, it is not clear yet whether
this is also possible for Ni. We will continue ourinvestigation and determine whether Ni can becaptured in a gettering site by processing damage.

References(1) Koji Kakuno 1993, “Science and Technology ofCrystal Defect Control in Semiconductors”, ScienceForum(2) Yoshifumi Nishi and Atsuhiro Kinoshita 2009,“Contact Resistance Reduction Technique Aimed atApplication in Advanced LSI”, Toshiba Review Vol. 64(3) Yasuaki Furui and Yukio Yasuda 1994, “SilicideFormation and Low Resistance Contact in Metal-SiliconInterfaces”, Applied Physics Vol. 63, (1994)

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