Semiconductor devices are very small electronic components that may be used to form integrated circuits, the basic operational portions of the now-familiar electronics products called chips. A chip is, generally speaking, a semiconductor die that has been encapsulated in a hard plastic or similar enclosure. Sometimes a number of interconnected dice are housed in the same enclosure. Before encapsulation, leads, traces, or other conductive devices external to the die itself are added to provide electrical connections between integrated circuits on the die and other electronic devices external to the chip. The chips are then often mounted onto printed wire boards and installed in such appliances such as personal computers, mobile telephones, and media players. Each chip performs a specified set of functions useful to the appliance, which appliance may use only one or a large number of chips.
The dice in chips are very small, flat pieces of silicon or a similar material, frequently less than a square centimeter in area. In this small area are fabricated thousands, even millions of the small electronic components referred to above. The fabrication process, many parts of which are now automated, involves selectively layering and removing insulating and conducting materials in predetermined patterns to form the parts needed for each of the individual semiconductor components. Completed components may then be interconnected with each other to form integrated circuits. Rather than make a single die at a time, it is more efficient to fabricate a number of them simultaneously. For this purpose a thin wafer is sliced from an ingot formed of the selected substrate material. A wafer may be used to make over a hundred dice, which are separated for individual use late in the fabrication process.
The electrical appliances mentioned above have become very popular with consumers, in part because of their small size and consequent portability. With their popularity, however, have come demands from the market place for even smaller devices that are even more capable. To accomplish this, the tiny semiconductor devices formed in the fabrication process must become even smaller and more tightly packed together. This effort results not only in greater challenges during the fabrication process itself, but gives rise to certain electrical problems, such as current leakage, that detrimentally effect of the performance of the device.
One common semiconductor device is called the transistor. A transistor is a small switch that can control the flow of electricity without the need for any moving parts. One such transistor shown in FIG. 1. FIG. 1 is an elevation (side) view illustrating in cross-section an exemplary transistor 10. Transistor 10 includes a gate structure 11 formed on substrate 12. Gate structure 11 is made up of a number of different component parts. A thin layer of dielectric material, such as an oxide, separates gate electrode 14 from substrate 12. This separating layer may be referred to as gate dialectic 13. Gate electrode 14 is made of a conducting material, for example a metal. More recently, crystalline polysilicon, or simply poly, has been used instead of metal. Disposed above gate electrode 14 in this example is a contact 15, typically made of metal, which may be used to make electrical connections between gate electrode 14 and an interconnect to another device (not shown). Spacers 16 and 17, formed of a dielectric material, are disposed on either side of the gate structure 11.
Gate structure 11 is the portion of transistor 10 that controls the flow of electricity. The current itself flows through the substrate between source 18 and drain 19 through channel 20 when a small voltage is applied to the gate structure 11. Source 18 and drain 19 are each formed in the substrate 12 by a local implantation of ions, such as those of boron or phosphorus. This process of ion implantation is sometimes known as doping. Source 18 and drain 19 are in turn connected, for example, to a voltage source and to a ground (not shown), respectively. Metal contacts 21 and 22, disposed on, respectively, source 18 and drain 19, provide a site for terminating such external electrical connections. Other external connections are made to dedicated bond pads formed on the die, which are in turn coupled to the integrated circuits and individual components that have also been formed there.
FIGS. 2a through 2d are a sequence of side views illustrating in cross-section the configuration of a semiconductor device 30 at various selected stages of fabrication. In FIG. 2a, it may be seen that a buffer oxide layer 34 has been formed over, and in this case directly over the substrate. The substrate may, for example, be silicon, and the oxide layer a silicon dioxide material. A hard mask 36 has been formed directly over the buffer oxide layer 34. Typically, the hard mask may for example be made of silicon nitride or some other suitable material. In the sequence illustrated here, no operational components have yet been formed, but an isolation structure will now be formed to separate those planned operational components that will be formed later.
In this example, the isolation structure is formed by first etching a recess 38 into the substrate 32, and in the oxide layer 34 and hard mask 36 disposed above it. This configuration is illustrated in FIG. 2b. Once the recess 38 has been formed, an oxide material 40 is deposited, in this case filling trench 38 and covering the surrounding portions of semiconductor device 30. This configuration is shown in FIG. 2c. The portion of oxide layer 40 that is disposed within the substrate 32 portion of recess 38 is now referred to as isolation structure 39. To finish the fabrication process, the remainder of oxide layer 40 is removed, for example by a CMP (chemical metal polishing) process. The resulting configuration of semiconductor device 30, now including isolation device 39, is depicted in FIG. 2d. 
The process described above in reference to FIG. 2 is not uncommon and has proven fairly reliable. As mentioned above, however, there is strong market pressure to produce smaller appliances, which in turn require even smaller chips. The devices on these chips are often being reduced in size, a phenomenon sometimes referred to as scaling, to such an extent that existing processes are now producing less then optimum results. For example the recess 38 depicted above (in FIG. 2c) now needs to be so small that reliable oxide deposition is difficult to achieve. This is especially true where the lateral dimension of the recess, or trench, has been narrowed, but without reducing, or perhaps even with increasing the depth (as viewed in FIGS. 2a through 2d). Deep, narrow trenches are said to have a high aspect ratio. High-aspect ratio trenches are useful, but improperly-formed isolation structures may not perform their required function, and the device's performance may as a result be significantly degraded.
Needed, then, is a method of forming an isolation structure that may be used, as continued downward scaling reduces the size of such features, to create a reliable isolation structure without significantly increasing the cost of fabrication. The present invention provides just such a solution.