A method of making a dielectric layer

Abstract: A dielectric layer of first interconnection for electronic semiconductor devices, specifically CMOS circuits, comprises a first thickness (11) of tetraethylorthosilicate which is overlaid by a layer (12) of self-planarizing siloxane. That layer provides a surface structure which is permissive of the subsequent conventional masking and electric contact attaching steps.

Description: This invention relates to a method of forming a dielectric layer of first interconnection for electronic semiconductor devices. As is known, in the course of making CMOS devices, subsequently to forming the gate electrodes and the source and drain regions of the device, and related re-oxidation, a dielectric layer of an isolating oxide is deposited to a thickness in the range of 0.8 to 1.0 micrometers. That dielectric layer, commonly referred to as dielectric of first interconnection, separates the surface layer of polycrystalline silicon from the metallic paths of electrical interconnection. Once the gate of the device has been defined, the device shows a predetermined map. The successive deposition of the dielectric layer of first interconnection follows this map enhancing, however, its features to the point that the subsequent masking, contact forming, and metal deposition steps are affected by it and result in a low yield of current manufacturing processes due to a high rate of faulty devices. In addition, surface irregularities on the mapping represent a major hindrance to a reduction in size of the interconnection paths to the metallization mask. To obviate this drawback, the prior art has proposed, for CMOS devices, the use of a double dielectric layer formed of a first thickness of silicon dioxide over which a layer of borophosphosilicate is deposited. Borophosphosilicate (i.e. borophosphosilicate glass or BPSG) allows the underlying map to be softened if the semiconductor is subjected to heat treatment at 900-1000°C for about 30-60 minutes. In accordance with a method known as classic planarization, for example as described in the "5th International IEEE V-Mic Conference" No. 357, June 13-14, 1988, the borophosphosilicate is coated with a photoresist layer and plasma attacked with selectivity 1:1. A fairly planar surface is thus obtained; however, the reproducibility of this process is poor and the process itself non-uniform, which is inconsistent with the requirements of very large volume integrated circuit manufacturing processes. Also, the extent of the planarization to be achieved thereby is heavily dependent on the underlying mapping, and in addition, two extra steps have in all cases to be provided in the manufacturing process. A second prior technique, as described for example in the "5th International IEEE V-Mic Conference", No. 293, June 13-14, 1988, consists of superimposing, on the borophosphosilicate, a layer of siloxane, or so-called spin-on-glass (SOG), which shows to be less sensitive to the underlying mapping during the selective attack step. However, here too does the manufacturing process involve extra steps. A further prior approach is described in the "4th International IEEE V-Mic Conference", No. 61, June 15-16, 1987, and consists of depositing the siloxane directly onto the map surface, and hence on the layer of polycrystalline silicon, and of applying a heat treatment under a nitrogen medium followed by deposition of a borophosphosilicate layer. Further final heat treatment at 920°C is found to soften the profile contour of the end surface. However, not even this approach has proved quite effective to yield planarized structures, despite any advantages to be derived therefrom. The publication "Proceedings 1988 VLSI Multilevel Interconnect Conference, 13th - 14th June 1988, pages 419-425, IEEE, New York, USA" discloses the forming of a dielectric layer to isolate different levels of interconnection in semiconductor devices. This dielectric layer comprises two layers. The lower layer is of a tetraethylorthosilicate (TEOS) based oxide on which a layer of self-planarizing spin-on glass (SOG) is superimposed. EP-A-0204631 discloses the use of siloxane for a SOG deposition of a self-planarizing layer in dielectric isolating interconnection levels of semiconductor devices. The technical problem underlying this invention is to provide a novel method of making a self-planarizing dielectric layer of first interconnection. This problem is solved by a method as defined in the appended claim. The features and advantages of a method according to the invention will be apparent from the following detailed description of an embodiment thereof, to be taken by way of illustration and not of limitation in conjunction with the accompanying drawing. The drawing shows schematically a side elevation cross-sectional view of the structure of an electronic semiconductor device incorporating a dielectric layer of first interconnection formed by a method according to the invention. With reference to the drawing view, generally and schematically shown at 1 is an electronic semiconductor device, specifically a CMOS device. comprising a P-channel MOS transistor 2 and an N-channel MOS transistor serially connected to each other. The device 1 comprises a semiconductor substrate 3 which is very slightly doped with impurities of the P type and in which a pod 4, commonly referred to as an N-well, is formed which is doped oppositely frm the substrate. Inside the N-well pod 4 there are formed two zones 5 and 6, both doped P-type, which are adapted to form the source and drain of the transistor 2 as well as to bound a so-called channel region 7 overlaid by a layer 8 of an isolating oxide, the so-called gate oxide, with a gate 8a of polycrystalline silicon. At the peripheral margins of the N-well pod 4, laterally of the source 5 and drain 6 zones, there are provided opposing isolating zones 9 overlaid by a layer 10 of silicon oxide, the so-called field oxide. Beginning with this configuration, a layer or thickness 11 of a tetraethylorthosilicate based oxide is deposited on the device 1 using an LPCVD reactor at a temperature in the 700-750°C range. With this technique, it becomes possible to obtain a high covering degree such that the difference between the thickness 11 of this oxide over the gate 8a and the source 5a and drain 6 amounts to less than 10%. Subsequent heat treatment at 900°C under a nitrogen medium for about 10 minutes imparts the layer 11 of tetraethylorthosilicate with excellent electrical properties while retaining its covering capability unaltered. Preferably, the thickness of the layer 11 is only by 50 nm (500 Å) smaller than the thickness of the polycrystalline silicon of the gate 8a. The layer 11 is overlaid by a layer 12 of self-planarizing siloxane. That layer is formed by using a dispenser, known as spinner, whereby the siloxane is dispensed as diluted in alcohol which is then subjected to heat treatment for conversion into a dielectric. More specifically, the siloxane layer 12 is placed in contact with a heated plate to 200°C for 1 minute. Thereafter, it is subjected to a first heat treatment step at 425°C under a nitrogen atmosphere at a pressure of 13.33 Pa (100 mTorr) for 60 minutes, and for a like time period, to a second heat treatment step at a temperature of 600°C. Additional treatment at 900°C under a steam medium at atmospheric pressure for 30 minutes will complete the process required to convert the diluted siloxane into a dielectric material. The end product of the process just described is a planar structure which can then be passed to conventional masking, contact masking, contact attachment, resist removal, interconnection material and final passivation oxide layer deposition steps. It is advisable that the electrical interconnection be made using a first layer of titanium to prevent so-called electric poisoning from the siloxane. Laboratory tests have revealed, moreover, excellent adhesion of the final passivation dielectric layers to the siloxane. The dielectric layer of first interconnection obtained by the method according to the invention affords a high degree of planarity on any types of semiconductor devices. Another advantage is that this dielectric layer can improve the output of the contact and metallization layer lithography process and increase the lithographic resolution of metallization while favoring more compact interconnection structures. A further advantage is that this dielectric layer enables the interconnection structures to be designed independently of the substrate mapping, and facilitate automated designing.

Claims: A method of making a dielectric layer on a surface of a silicon-gate MOS semiconductor device, as it results after the source and drain regions and the polysilicon gate electrodes have been formed thereon, for providing electrical isolation for a metallic interconnection layer to be formed on said dielectric layer, said method comprising the steps of: depositing on said surface a first layer (11) of a tetraethylorthosilicate (TEOS) based oxide using an LPCVD reactor at a temperature of 700-750°C, heating said first layer at 900°C under a nitrogen atmosphere for about 10 minutes, depositing on said first layer by a spin-on technique a self-planarizing siloxane layer (12), heating said siloxane layer to 200°C for 1 minute, subsequently, subjecting said siloxane layer to a first heat treatment step at 425°C under a nitrogen atmosphere of 13.33 Pa (100 mTorr) for 60 minutes followed by a second heat treatment step at a temperature of 600°C for 60 minutes and performing an additional heat treatment at 900°C under a steam medium at atmospheric pressure for 30 minutes.

Inventors: GUALANDRIS FABIO, MASINI LUISA, GUALANDRIS, FABIO, MASINI, LUISA
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1989
Main classification: H01L21
IPC codes: H01L23, H01L21, H01L29