1. Field of the Invention
The present invention relates to a method for filiing a hole with a metal. More particularly, the present invention relates to a method for filling a hole with a metal to form a metal plug in a contact hole or a via hole without a void or a recess therein.
2. Description of the Related Art
Generally, the memory capacity of a random access memory (RAM) chip is represented according to Moore's principle, which states that the memory capacity of a RAM chip increases about four times every three years. Whenever a new chip appears, the increase in capacity is achieved by reducing the chip size and lengthening of chip length. As the chip size is reduced, the distance between interconnect lines is shortened. However, the interconnect lines have an effect on each other. As a result, when the distance between the interconnect lines is below a predetermined value, a signal delay will occur in the semiconductor chip. To improve the signal processing speed of the semiconductor chip, a reduction to the specific resistance of a metal used as the interconnect line is required.
Traditionally, aluminum or aluminum alloy having the specific resistance of about 2.66 μΩ/cm has been used as the interconnect line. The IBM Corporation discloses a method for forming a metal wiring using copper having the specific resistance of about 1.65 μΩ/cm in 1988. The method for forming a metal wiring using copper has been studied. However, since copper is rapidly diffused in a silicon layer or most metal layers, a photolithography process may not be employed when forming a copper wiring. Accordingly, a damascene process is typically employed when forming a copper wiring.
Recently, copper has been widely used as the wiring in semiconductor devices. However, when the final wiring includes copper, an aluminum pad must be additionally used in the packaging process. To the contrary, when the final wiring includes aluminum, the aluminum pad is not used in the packaging process. Accordingly, the final wiring typically includes aluminum due to convenience and reduced cost. When the final wiring includes aluminum, copper is formed in a contact hole or a via hole and is connected to a lower conductive layer. Copper electrically connected between the lower conductive layer and an upper aluminum layer is formed through a single damascene process.
FIG. 1 is a scanning electron microscope (SEM) photograph illustrating a via hole filled with copper through a single damascene process, and FIG. 2 is a graph illustrating the recess depth of copper versus the distance apart from a dense pattern for forming a via hole.
Referring to FIGS. 1 and 2, the recess depth of a copper layer is dependent on the distance apart from a dense pattern for forming a via hole. The recess depth of the copper layer is augmented proportional to the distance apart from the pattern for forming the via hole. The recess of the copper layer in the via hole is caused by electrical failure between the copper layer and a wiring. As the via hole moves further apart from the dense pattern, the electrical failure becomes more severe.
In FIG. 1, a via hole I is positioned in the dense pattern, a via hole II is apart from the dense pattern at a distance of about 4 μm, a via hole III is apart from the dense pattern at a distance of about 7 μm, a via hole IV is apart from the dense pattern at a distance of about 10 μm, a via hole V is apart from the dense pattern at a distance of about 14 μm to about 15 μm, and a via hole VI is apart from the dense pattern at a distance of about 220 μm. The recess of copper layer is about 0 μm in the via hole I. The recess depth of the copper layer is increased according to the distance apart from the dense pattern in the via holes II and III, respectively. Furthermore, the recess depth of the copper layer is remarkably increased when the distance apart from the dense pattern is greater or equal to about 10 μm as in the via holes IV, V and VI.
FIG. 3 is a cross-sectional view illustrating a conventional method for filling a via hole with a copper layer through a single damascene process, and FIG. 4 is a graph illustrating occurrence ratio of a void or a recess in a copper layer according to sizes of via holes.
Referring to FIG. 3, a via hole or a contact hole is formed through an etching stop layer 12 and an insulating layer 30 formed on a semiconductor substrate 10 to expose a lower conductive layer 20 formed on the semiconductor substrate 10. A metal plug 40 is formed in the via hole or the contact hole. A void or a recess is formed on an upper surface of the metal plug 40. The void or the recess has a shape partially recessed on the upper surface of the metal plug 40.
In a process for forming the metal plug 40, the insulating layer 30, and the etching stop layer 12 are etched using a photoresist pattern (not shown) to form a via hole. A copper layer is formed in the via hole through a chemical vapor deposition (CVD) process or an electroplating process. The copper layer grows on a bottom face and a side face of the via hole, and an upper face of the insulating layer 30 in the electroplating process. The copper layer rapidly grows at an inlet of the via hole having a short diameter so that a cavity is formed in the copper layer. The cavity is exposed through a chemical mechanical polishing (CMP) process to form the void or the recess 42 on the metal plug 40.
The recess 42 formed in the metal plug 40 may be a potential cause for the electrical failure between the metal plug 40 and a wiring (not shown). An electrolyte remaining in the recess 42 may be evaporated and expanded during a successive annealing process. A gas expanded by high temperature may weaken bonding strength between the metal plug 40 and the upper wiring, and may peel off the upper wiring.
Referring to FIG. 4, in the conventional single damascene process, the void or the recess frequently occurs when the metal plug 40 is formed in the via hole having a diameter of below about 0.25 μm. The void or the recess may not occur when the metal plug 40 is formed in the via hole having a diameter of above about 0.25 μm. As described above, the occurrence ratio of the void or recess in the metal plug increases proportional to reducing the size of the via hole. Accordingly, when the copper metal, for example, is formed in the minute hole through the conventional single damascene process, a void or recess is frequently formed in the metal.
Embodiments of the invention address these and other disadvantages of the conventional art.