Use of Dilute HF With Controlled Oxygen for Post Cu CMP Cleans
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267 Mat. Res. Soc. Symp. Proc. Vol. 566 0 2000 Materials Research Society
Polishing Damage
Scratches
Particles
Ions
Cu Figure 1: Cross section of a typical post Cu CMP device showing typical contamination and surface damage.
1.E+06 E E
C.2
C
I .E+05
I.E+04
1.E+03 320 HCI Flow (ml/min)
1.E+02 ý1423
5
20
90
H20 2 Flow (ml/min)
320
Figure 2: Removal of copper metal in SC-2 as a function of HCl and H 20 2 concentrations as measured by TXRF. function of concentration for HCl and HF. The pH of the HCI solutions in Figure 2 range from approximately -0.25 to 1.5. In this range, little effect of pH is seen on the total copper removed. Therefore, reaction (3), whose rate is expected to be proportional to [H -]2, is apparently not the rate-limiting reaction. While not investigated in this work, it is presumed that Reaction (3) is also not the rate-limiting reaction in the case of copper removal by DHF solutions near pH 2.
268
(1)
0 2(g) -- 0 2(aq)
2Cu(s) + 02 -+ 2CuO(s
(2)
CuO(s) + 2H+(aq) --. Cu++(aq) + H20
(3)
32.5
2
-.-e-DHF pH -
0.5
-
Assume complete disassociation of HCI Assume Ks for HF of 6E-4
4.5 -1
100
10 x1714
1000
Blend Ratio (x:1)
Figure 3: pH as a function for concentration for HCI and HF
The large increase in copper removal in Figure 2 between 0 and 5 ml/min H20 2 in the SC2 mixture indicates the possibility of controlling the copper etch rate by controlling the oxidizing species. Figure 4 shows the Pourbaix diagram for copper [4]. Corrosion of copper in acidic aqueous solutions is thermodynamically favorable when the oxidation-reduction potential (ORP) of the solution E is greater than approximately 0.1 V. The ORP of the acidic solution can be calculated using the Nernst equation applied to the chemical equilibria in equation 4 [5]. 02 concentrations ranging from saturated to I ppb (a typical lower limit to 02 concentration) give solution ORPs from 1.24 to 1.17 V. This range of ORPs and pH levels near 2 are marked by the small, dark rectangle centered above the word "corrosion" in the acidic portion of the Pourbaix diagram (Figure 4). This parameter space, used in this experiment, is entirely within the "corrosion" or etching regime. Therefore, etching of copper is thermodynamically favorable. E0 = 1.229 V
02 + 4H+ + 4e- + 2H 20
(4)
The reaction rates for the reactions in equations 1 and 2 are expected to be linear with 02 concentration. Given the diffusion coefficient D for 02 in water of 2x10' cm2/sec, it is possible to estimate the flux of 02 to the surface [6]. Assuming 5 ppm of dissolved 02 and a
269
10' cm boundary layer, the 02 flux at the surface would be 1015 atoms/cm 2esec. This flux is sufficient to support a 6-nm/min Cu etch rate. -2-1 0 1 2 3 4 5 6 7 8 9 10111213 1415162 2.2 2 1.8 1.8 1.6 1.6 1.4 1.4 2.2 2
1.2
1.2 1
S 0. 1 00.I C 0.6
0.8 0.6
0.4
0.4
• i•0.20
0.2 0
•0.2i
-0.2
0-0.4
-0.6 S -.0.B
0
-0.4
immunit-0.8
-1
-0.6
-1.2
-1 -1.2
-1.4 -1.6
-1.4 -1.6
-1.8
-1.8 -2-1 0 1 2 3 4 5 6 7 8 9 1011 1213 141516
pH
Figure 4: Pourbaix dia
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