METHOD FOR FILLING GAPS ON A SEMICONDUCTOR WAFER

Abstract: A method for filling gaps (1) on a semiconductor wafer (2) with a dielectric material (3, 8) employs a plasma enhanced chemical vapor deposition (PECVD) process with a temperature in the range of 500 to 700 DEG C. As a result of deposition gaps for e.g. shallow trench isolation or premetal dielectric techniques are filled homogeneously without any voids. The deposition may be improved with the application of a second radio frequency signal.

Description: Description Method for filling gaps on a semiconductor wafer The invention relates to a method for filling gaps on a semiconductor wafer with a dielectric material using a CVD processing apparatus for performing a CVD-deposition under plasma and vacuum conditions at elevated temperatures. Process technologies for the manufacture of integrated semiconductor circuits require the filling of gaps on the surface of the semiconductor wafer. The wafer incorporates a multiplicity of integrated circuit elements after certain processing steps executed. The requirement to fill gaps applies es- pecially to the manufacturing of semiconductor wafers with sub 0.25 micron design rules. The gaps may be located between transistors for isolation purposes according to the shallow trench isolation technique (STI) or may be located between polysilicon signal wires that preceed the metallisation proc- ess steps (pre etal dielectric, PMD) . The dielectric material to fill the gaps may be any known dielectric material as for example SI0 2 , BPSG, PSG, BSG, SiN, etc. In known processes for the manufacturing of ICs rather com- plex or ineffective CVD-deposition processes for filling gaps were used. For example a High Density Plasma- (HDP) -CVD- reactor based on a silane/argon gas mixture at around 350 to 400°C is used employing simultaneous oxide deposition and sputtering. Also, a HDP-CVD-process based on a silane-helium mixture at 600°C is used for gap filling. However, helium is a very light ion and its sputter efficiency is almost zero. This leads to a bottom up fill of the gap. Another process uses post deposition reflow, e.g. deposition of a glass on basis of boron phosphorous silane (BPSG) or tetra ethyl ortho silicate (TEOS) and subsequent anneal at 850°C. Further another process uses in situ reflow above the glass transition temperature which is greater than 750°C, e.g. in a Low Pressure- (LP) -CVD-reactor; the glass itself flows. Below the glass transition temperature at about 400 to 600°C a Sub Atmospheric Pressure- (SA) -CVD-reactor is used. A SACVD reactor commonly operates at pressures in the range from 20 to 600 Torr. HDP-CVD-reactors operate at very low pressure, typically in the range of 1 to 25 MilliTorr (mTorr) . The reaction chamber is rather expensive. SACVD-reactors use aggressive ozone molecules. The turnaround time for wafers is rather low so that a number of SACVD-reactors have to be operated in parallel. Thus, HDP-CVD- or SACVD-based processes are quite expensive . U.S. 5,643,640 discloses a fluorine doped phosphosilicate glass process (FPSG) using a Plasma Enhanced- (PE) -CVD- deposition for filling gaps or voids. The gaps may have a high aspect ratio. The process is carried out at a temperatur between 400 to 500°C. Using fluorine, however, has the drawback that the gate oxide may be contamined, damaged or the fluorine may further be oxidized. Fluorine doped deposition materials are not desired in the lower front end of line levels of subquarter micron process technology. It is an object of the invention to provide an improved method for filling gaps on a semiconductor wafer that provides a good void free filling of a gap with a dielectric material at moderate costs. This objective is solved by a method according to the features of claim 1. The method of the invention uses a plasma enhanced- (PE) -CVD- deposition apparatus at an elevated temperature above 500°C. Particularly the temperature is in the range of 500 to 700°C. In such PECVD-process there is no need for using halogen ions or radicals. The PECVD-deposition process according to the invention is applicable to any common dielectric material, e.g. SI0 2 , BPSG, PSG, BSG, SiN, and provides a conformal filling of the gap. A PECVD-reaction chamber operates at rea- sonable temperatures and vacuum pressures so that the costs of the reactor are below that of a HDP-CVD-apparatus . The throughput of a PECVD-reactor is higher than for a SACVD- reactor. Practically, a PECVD-chamber according to the invention will cost approximately half of a HDP-CVD-chamber and will have approximately double the throughput of a SACVD- chamber . The reaction temperature which lies preferably between 500 and 700°C is caused by a ceramic heating element located un- der the receptor carrying the semiconductor wafer. The vacuum is generated by a vacuum pump. The pressure is between 100 mTorr and 10 Torr. The plasma gas in the chamber is generated by a radio fre- quency signal. The signal has a frequency of more than 10 MHz, preferably 13.5 MHz. The radio frequency signal is coupled into the chamber opposite the front side of the wafer. According to a preferred embodiment of the invention a second radio frequency signal is coupled into the chamber, too. The second radio frequency signal has a frequency of less than 100 kHz, preferably about 10 kHz. This means that the two frequencies of the radio frequency signals differ by at least two orders of magnitude. The first radio frequency signal has a power of several hundreds of watt. The second radio fre- quency signal is applied with lower power. Both signals are applied through different coupling means into the chamber or through the same coupling means. The reaction gas composition in the chamber may be of any known type. Due to the high temperature an addition of halogen components or radical components as is ozone will not be necessary. The reaction gas or precursor gas composition comprises one of the following compositions: Dielectric material Precursor to be desposited Si0 2 TEOS, 0 2 , N 2 or SiH 4 , N 2 0, N 2 S iN S iH 4 , N 2 0 , NH 3 , N 2 BSG TEOS , TEB , 0 2 , N 2 or SiH 4 , B 2 H 6 , N 2 0, N 2 PSG TEOS , TEPO, 0 2 , N 2 or S iH 4 , PH 3 , N 2 0, N 2 BPSG TEOS , TEB, TEPO, 0 2 , N 2 or SiH 4 , PH 3 , B 2 H 6 , N 2 0, N 2 BSG/PSG/BPSG: boro-/phosphorsilicate-/borophosphorsilicate- glass TEB: Tri ethyl borate TEPO: Tri ethyl phosphate The semiconductor wafers to be processed by the method according to the invention have gaps on the surface of the semiconductor wafer to be filled. The gaps may be shallow trenches that had been etched into the silicon substrate, e.g. by any dry etching technique. The trenches serve to separate and isolate transistors of the same type from each other. This layout structure is commonly called shallow trench isolation (STI). Another area of application of the invention is to fill the gaps established by the polysilicon wiring structure. A multilayer structure composed of polysilicon, tungsten suicide and nitride is deposited on top of the wafer during the respective process steps. After patterning in order to generate local connections such as gate electrode connections and trench capacitor connections for DPAMs, gaps are left between the multilevel polysilicon wiring structure. The gaps are to be filled with dielectric material according to the PECVD- process of the invention. The gaps in the polysilicon structure have high aspect ratios up to 17. On the filled planar polysilicon-dielectric surface there is further deposited a metal layer for wiring. The above mentioned dielectric, therefore, is known as premetal dielectric (PMD) . Under the conditions of the PECVD-deposition process according to the invention the reactive molecules have a high enough mobility due to the elevated temperature so that they reach the bottom of the gap to deposit there. The corner effect, i.e. the phenomenon that molecules primarily deposit at the upper edge of the gap, is reduced by the higher mobility of the molecules due to the elevated temperature. The second radio frequency coupled into the reaction chamber causes a sputter effect at the upper edge of the gap. I.e. molecules tending to deposit at that location are loosened again by the application of the second radio frequency signal. The invention is further described with reference to the following drawings in which Figure 1 shows a gap filled according to prior art and Figure 2 shows a gap filled according to the invention. Respective elements in different figures are marked by the same reference symbols. The gap 1 to be filled is a gap between two CMOS-transistors of the same type that are located on the top surface of a semiconductor wafer 2. In order to prevent a coupling between the transistors the substrate be- tween the transistors is cut by a trench 1. This technique is called shallow trench isolation (STI) . Figure 1 shows the application of a prior PECVD-deposition process running at approximately 400°C. The gap fill material in both examples is composed of, for example, silicon dioxide (SI0 2 ) . During the course of the deposition process the dioxide grows on the wafer surface. This is illustrated by consecutive layers 3a, 3b, 3c, 3d showing the resulting thickness of deposited mate- rial at subsequent time instants. It is to be noted that under the normal PECVD-reaction parameters the deposition rate at the upper edge 4 of gap 1 is higher than at the side walls 5 or at the bottom 6 of gap 1. As a result, the SI0 2 grows faster at the upper edge 4 of the gap 1 than at the bottom 6. As a consequence, the gap is closed at the upper surface prior to a full and homogeneous filling of the gap. A void 7 is left in the center of the gap. The PECVD-deposition process according to the invention provides a homogeneous filling of gaps without any voids even for gaps with high aspect ratio. According to figure 2 the semiconductor wafer is inserted into the reaction chamber of a PECVD-deposition apparatus onto a receptor accommodating the wafer. The chamber is evacuated by a vacuum pump down to a pressure between 100 mTorr and 10 Torr during the deposition process. The chamber contains a reaction gas composition composed of TEOS, 0 2 , N 2 or SiH_j, N 2 0, N 2 for Si0 2 deposition. The chamber is heated up to a temperature of more than 500°C, e.g. 600°C. Heating is performed by a ceramic heating element under the receptor. A RF-signal of e.g. 13.56 MHz is coupled into the chamber to generate a plasma. Preferably, a second RF-signal with 100 kHz is also coupled into the chamber. The power for the first RF signal and for the second RF signal is 100 to 1000 Watt, respectively, for 200 Millimeter single wafer reactors. During the course of the deposition process the molecules deposit evenly on the surface of the wafer. The growth rate of the SI0 2 layer is approximately the same for all locations of the deposition. The growth rate at the upper edge of the gap 1, the side walls 5, and the bottom 6 is roughly in the same magnitude. That is why the corner rounding effect at the upper edge 4 disappeared. Due to their high mobility, the reaction gas molecules reach into the gap and equally deposit on its bottom and its side walls at approxi- mately the same rate. Thus, the gap is filled homogeneously with SI0 2 without any voids. In figure 2 the layers 8a, ..., 8e showing the deposited SI0 2 at consecutive times have no corner effect. However, at the end of the deposition a small notch centered over the gap 1 remains. The notch will be pla- narized during a subsequent step of polishing. The notch does not significantly influence the planarity of the surface of the wafer and can be neglected for further processing. The application of the second RF-signal improves the gap fill behavior. The second RF-signal provides a sputter effect so that material depositing at the corner region 4 is immedi- ately sputtered off again. In conclusion, the invention provides a specific PECVD- deposition process to fill gaps on the surface of a semiconductor wafer with a dielectric material in a homogeneous void free way. Due to the nature of PECVD the process equipment is relatively cheap compared to prior gap filling techniques and has a high throughput rate.

Claims: Claims 1. Method for filling gaps on a semiconductor wafer with a dielectric material comprising the steps of: - inserting said semiconductor wafer (2) into a reaction chamber of a plasma enhanced chemical vapour deposition processing apparatus, - performing a CVD-deposition under plasma conditions and under vacuum conditions to deposit said dielectric material (3a, ... 3d; 8a, ..., 8e) on said semiconductor wafer (2), - performing the step of CVD-deposition at a temperature above 500 °C. 2. Method according to claim 1, characterized in that said temperature in said reaction chamber of said apparatus is in the range from 500 °C to 700 °C. 3. Method according to claim 1 or 2, characterized in that the gas pressure in said reaction cham- ber of said apparatus is in the range of 100 mTorr to 10 Torr. 4. Method according to any of claims 1 to 3, comprising the step of coupling a first and a second radio frequency signal into said chamber and generating a plasma in response to said radio frequency signals, said second radio frequency signal having a frequency with at least two orders of magnitude lower than the frequency of said first radio frequency signal. 5. Method according to claim 4, characterized in that said first frequency is greater than 10 Megahertz and said second frequency is lower than 100 Kilo- hertz . 6. Method according to any of claims 1 to 5, characterized in that said chamber is filled with a reation gas composition comprising any gas of the group comprising TEOS, SiH 4 , TEB, B 2 H 6 , TEPO, PH 3 , 0 2 , N 2 0, NH 3 , N 2 during the step of CVD-deposition. 7. Method according to any of claims 1 to 6, characterized in that said gaps on said semiconductor wafer (2) to be filled with said dielectric material (3a, ..., 3d; 8a, ..., 8e) are formed between transistors on said semiconductor wafer (2) . 8. Method according to any of claims 1 to 6, characterized in that said gaps on said semiconductor wafer to be filled with said dielectric material are formed between portions of a metallization layer on top of said semiconductor wafer.

Inventors: KIRCHHOFF MARKUS, KIRCHHOFF, MARKUS
Applicants: KIRCHHOFF MARKUS, SEMICONDUCTOR 300 GMBH & CO KG, KIRCHHOFF, MARKUS, SEMICONDUCTOR 300 GMBH & CO. KG
Application year: 2000
Main classification: H01L21
IPC codes: H01L21, H01L23