Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating selected area
Reexamination Certificate
2001-06-29
2004-03-23
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating selected area
C205S296000, C205S157000, C205S220000, C204S22400M, C106S001180, C106S001260, C427S443100
Reexamination Certificate
active
06709563
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates a copper-plating liquid, a plating method and a plating apparatus, and more particularly to a copper-plating liquid, a plating method and a plating apparatus useful for forming copper interconnects by plating a semiconductor substrate with copper to fill copper in fine recesses for interconnects formed in the surface of the substrate.
2. Description of the Related Art
In recent years, instead of using aluminum or aluminum alloys as a material for forming interconnection circuits on a semiconductor substrate, there is an eminent movement towards using copper (Cu) which has a low electric resistance and high electromigration resistance. Copper interconnects are generally formed by embedding copper into fine recesses formed in the surface of a substrate. There are known various techniques for producing such copper interconnects, including CVD, sputtering, and plating. According to any such technique, a copper is deposited on the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 39A through 39C
illustrate, in a sequence of process steps, an example of producing such a substrate W having copper interconnects. As shown in
FIG. 39A
, an oxide film
2
of SiO
2
is deposited on a conductive layer
1
a
formed on a semiconductor base
1
on which semiconductor devices are formed. A contact hole
3
and a trench
4
for interconnects are formed in the oxide film
2
by the lithography/etching technique. Thereafter, a barrier layer
5
of TaN or the like is formed on the entire surface, and a seed layer
7
as an electric supply layer for electroplating is formed on the barrier layer
5
.
Then, as shown in
FIG. 39B
, copper plating is performed onto the surface of the substrate W to fill the contact hole
3
and the trench
4
with copper and, at the same time, deposit a copper film
6
on the oxide film
2
. Thereafter, the copper film
6
on the oxide film
2
is removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film
6
filled in the contact hole
3
and the trench
4
for interconnects and the surface of the oxide film
2
lie substantially on the same plane. An interconnect composed of the copper film
6
, as shown in
FIG. 39C
is thus formed.
The seed layer
7
is generally formed by means of sputtering or CVD. In the case where the copper film
6
is formed by electroplating with copper, a copper sulfate plating liquid, which contains copper sulfate and sulfuric acid, has generally been used as a plating liquid.
With the recent trend towards finer interconnects, the trenches for interconnects or plugs are becoming to have a higher aspect ratio. This poses the problem that a seed layer cannot be sufficiently formed by, e.g. sputtering, in the bottom portion of the trench, thus failing to form a uniform seed layer Thus, as shown in
FIG. 40A
, there is a likelihood that the thickness t
1
of the seed layer
7
formed on the side wall of the trench near the bottom portion thereof becomes {fraction (1/10)} or less of the thickness t
2
of the seed layer
7
formed on the side wall of the trench near the surface of the substrate. When electroplating with copper is carried out to fill with copper into such a trench by using a copper sulfate plating liquid, an electric current hardly passes through the extremely thin portion in the seed layer
7
, causing to the formation of an undeposited portion (void)
8
shown in FIG.
40
B. An attempt to overcome this drawback by increasing the overall thickness of the seed layer
7
so as to thicken the extremely thin portion would not be successful, since in electroplating with copper for filling such trench, copper would deposit thick around the opening of the trench to close it, resulting in the formation of a void.
On the other hand, a copper-plating liquid, which comprises a base such as copper sulfate and, as additives, a complexing agent and a pH adjusting agent for maintaining the liquid pH within a neutral range, has been developed. Such a copper-plating liquid, however, is generally too unstable for practical use. Moreover, the pH adjusting agent generally contains an alkali metal such as sodium and potassium. A plating liquid containing an alkali metal, when applied to a semiconductor substrate, causes electromigration to deteriorate the semiconductor. There is also known a copper-plating liquid comprising a copper cyanide. However, since cyanides are harmful to human health, it is required to avoid using such a plating liquid from operational and environmental viewpoints.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above drawbacks in the prior art. It is therefore an object of the present invention to provide a copper-plating liquid which is free from alkali metals and cyanides, and which can reinforce the thin portion of a seed layer and ensures complete filling with copper in fine recesses having a high aspect ratio formed in the surface of a substrate, and also to provide a plating method and a plating apparatus which utilize the copper-plating liquid.
Tn order to achieve the above object, the present invention provides a copper-plating liquid free from an alkali metal and a cyanide, comprising divalent copper ions and a complexing agent. The inclusion of a completing agent in the copper-plating liquid can enhance the polarization as a plating bath and improve the uniform electrodeposition property. This enables reinforcement of the thin portion of a seed layer and uniform filling with copper into the depths of fine recesses, such as trenches and holes, having a high aspect ratio. Further, the deposited plating is dense, and is freed from microvoids formation therein. Furthermore, the copper-plating liquid of the present invention, which does not contain any alkali metal nor cyanide, does not cause deterioration of a semiconductor which would otherwise be caused by electromigration due to the presence of an alkali metal and, in addition, does meet the demand for avoiding the use of a cyanide.
Preferably, the plating liquid further contain a pH adjusting agent selected from agents not containing an alkali metal nor a cyanide, such as sulfuric acid, hydrochloric acid, phosphoric acid, choline, ammonia and tetramethyl ammonium hydroxide. By using such a pH adjusting agent according to necessity, the plating liquid may be maintained within a pH range of 7-14, preferably at a pH range of about 8-11, more preferably at a pH range of 8-9.
The concentration of divalent copper ions in the plating liquid should preferably be in the range of 0.1-100 g/l, more preferably in the rage of 1-10 g/l. A copper ion concentration below the above range lowers the current efficiency, thereby lowering the precipitation efficiency of copper. A copper ion concentration exceeding the above range worsens the electrodeposition property of the liquid. The concentration of the complexing agent should preferably be in the range of 0.1-500 g/l, more preferably in the range of 0.1-200 g/l, furthermore preferably in the range of 20-200 g/l. When the concentration is lower than the above range, an adequate complexing with copper can hardly be made whereby sediments are likely to produce. When the concentration is higher than the above range, on the other hand, the plating can take on the so-called “burnt deposit” state and thus the appearance is worsened and, in addition, the treatment of waste liquid becomes different. Further, when the pH of the plating liquid is too low, the complexing agent cannot effectively combine with copper, thus failing to provide a complete complex. On the other hand, too high a pH of the plating liquid can bring about the formation of a variant form of complex which makes a sediment. The above described preferred pH range can obviate these drawbacks.
The plating liquid may also contain at least one additive selected from organic acids, amides, glycerin, gelatin, heavy metal ions, thiazoles, triazoles, thiadiaz
Kimizuka Ryoichi
Kobayashi Takeshi
Nagai Mizuki
Okuyama Shuichi
Ebara Corporation
King Roy
Nicolas Wesley
Wenderoth , Lind & Ponack, L.L.P.
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