HDP-FSG Integration in Multilevel Interconnect Devices
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include methyl
silsesquioxanes, and
hydrogen
silsesquioxanes (HSQ). The latter has been widely investigated due to its low dielectric constant, in the 2.9-3.3 range, and good planarization [1]. On the other hand, CVD fluorosilicate glass deposited in a high density plasma (HDP) reactor has shown good gap-fill capability and film stability for a dielectric constant as low as 3.5 [2]. Despite a higher bulk dielectric constant than HSQ, FSG for 0.18pm applications has exhibited a dielectric constant that is competitive with integrated HSQ. We will show in this paper how this is possible. Specifically, we will discuss integration with CMP and metallization. EXPERIMENTAL HDP-FSG was deposited in a 200mm HDP-CVD reactor using SiFSSiHSO 2/Ar chemistry. CMP of the FSG was performed using the same slurry as for undoped oxides. PVD Ti/TiN/Al was deposited on the Endura® platform. Adhesion of post-CMP FSG to metallization was determined after subjecting the structure to six annealing cycles. Annealing was performed in a tube furnace at a temperature of 410°C for 30 minutes in a nitrogen ambient. Fluorine concentration was measured by a Nicolet FT-
443 Mat. Res. Soc. Symp. Proc. Vol. 564 © 1999 Materials Research Society
IR spectrophotometer. Fluorine content was measured using the ratio of the Si-F peak height, occurring ~937cm- 1, to the Si-O stretching peak height measured at 1,095 cm-'. Stability of FSG was determined by pressure cooker testing (PCT). In this test, a lpm FSG film is deposited, and its FT-IR spectrum taken. Then the wafer is subjected to PCT at 120'C for 2 hours under 85% relative humidity and a pressure of 12-15psig. Post-PCT FT-IR spectrum is analyzed for moisture (-H, -OH) incorporation. A quadrupole-based secondary ion mass spectrometry (QuadSIMS) was used to measure both the H and F content and depth profiles in the FSG film. The sputter was carried out by Cs primary ion beam with a beam energy of 4 to 6 keV and a current of 90-100nA, depending on the particular application. Each sample is sputtered through the FSG thickness and F or H concentration is calculated by averaging F concentration over the depth profile RESULTS AND DISCUSSION The HDP-USG that is deposited at a temperature of 4000 C is characterized by a dielectric constant of 4.1. Because of the high density of the ionized species, the low chamber pressure, and the high ionic bombardment during deposition, the HDP oxide has the lowest impurity content of any CVD oxide and a compressive stress around 150-18OMPa. By comparison, TEOS oxide stress is in the range of 50-10OMPa, compressive, while SOG and SACVD oxides are generally tensile. The high rigidity of this HDP oxide coupled with high aspect ratio Al lines can cause stress-induced voiding [3,4,5]. Indeed, with the shift to 0.25pm production, instances of stressinduced voiding have been observed. Migration to lower deposition temperatures (
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