Thin dielectric layer on a substrate and method for forming such a layer

Abstract: The thin dielectric inorganic layer overlays a substrate, has a thickness of ≦ 20 nm and a defect density of ≦ 0.6 defects/cm² determined by BV measurements. To form e.g. such a layer a layer having the desired composition and thickness is formed on a substrate, followed by an ion implantation with a dose of >10¹⁵ ions/cm² into the substrate through the layer and a subsequent anneal in an inert atmosphere at a temperature of ≧ 500°C for a predetermined time.

Description: The invention relates to a thin dielectric inorganic layer with a low defect density and to a method to form such a layer. Especially in the semiconductor technology field it has always been a problem to produce thin dielectric layers having a sufficiently high breakdown voltage not only locally but over the whole layer surface. A remarkable expenditure was therefore spent to develop methods to form high quality dielectric layers or to improve the dielectric properties of already existing layers. In DE-A-2 314 746 a method is described where during the thermal oxidation of silicon a chlorine compound, i.e. CCl₄, is injected with the oxidizing agent. The thickness of the layers grown by this method was in the order of 50 nm. The method is able to increase the breakdown voltage and to stabilize the flatband voltage. A similar method is described in the article "Reduction of Charge in Silicon Dioxide Layers by Use of Chlorine Ion Implantation" by M. Briska, et al. published in the IBM Technical Disclosure Bulletin, volume 23, number 7A, December 1980, page 2768. The chlorine is implanted with a dose of 2 x 10¹³ ions/cm² into a 220 nm thick anodic silicon dioxide layer which is then annealed for 10 minutes in nitrogen at a temperature of 1000°C. With this method positive metal ions, like sodium, aluminium and iron in the silicon dioxide layer can be reduced considerably, with the consequence that breakdown voltages are increased and leakage problems reduced. In the article "MOSFET" Devices with High-Gate Dielectric Integrity" by A.B. Fowler, et al. published in the IBM Technical Disclosure Bulletin, volume 17, number 1, June 1974, p.270 it is described that a high temperature (1100 - 1300°C) hydrogen anneal greatly reduces the breakdown defect density in a subsequently grown thin silicon dioxide layer the thickness of which is not defined. In this article it is also mentioned, that to achieve a high doping in the area underneath the thin silicon dioxide layer ion implantation through the thin oxide layer is a useable alternative. When the thickness of the dielectric layers was reduced further in order to comply with the increasing microminiaturization especially in the information technology it was found, that it became disproportionately more difficult to keep the defect density at a low level. The reasons are not known exactly. On the one hand the suspicion was mentioned that an ion implantation through a thin dielectric layer was detrimental to it. On the other hand in US-Patent 4 001 049 a method is described for improving the dielectric breakdown of an e.g. 20 nm thick SiO₂-layer by implanting for example argon or hydrogen ions with a dose of > 1.8 x 10¹³ (for argon) or > 3.4 x 10¹⁶ ions/cm² followed by an anneal at temperatures between 200 and 800°C. However, it is emphasized in the patent that the ions must be of insufficient energy to penetrate completely through the silicon dioxide layer and - though certainly the average breakdown voltage is increased by the method - the defect density achieved is still not satisfactory. In another method described in the article "Insulator Film Stack for Very Small Area Capacitors" published in the IBM Technical Disclosure Bulletin, volume 28, number 3, August 1985, page 1361 & ff., a transition metal oxide, like hafnium oxide, is inserted, in a three layer stack of dielectric films which allows the creation of charge storage capacities equivalent to 6 to 10 nm of thermal SiO₂ in a total actual stack thickness of 12 to 18 nm providing capacitors with improved voltage breakdown and higher charge storage capacity per unit area characteristics. However, this method besides being rather complicated and being used for special purposes only does not achieve a thorough solution of the high defect density problem in very thin dielectric layers either. It is therefore one object of the invention to provide a dielectric layer having a thickness in the order of less than 20 nm and a low defect density based on a predetermined minimal breakdown voltage. It is another object of the invention to provide a method for forming very thin dielectric layers having a low defect density with regard to a predetermined minimal breakdown voltage that has to be exceeded. It is a further object of the invention to provide a method for forming such layers reproducibly in a manufacturing environment. It is still another object of the invention to provide a method for forming such layers with comparatively low costs. These objects are achieved by a method as disclosed in claim 1 and by a structure as disclosed in claim 4. Defect density in this context means the number of defects per cm² with the defects being short circuits or E BD values of < 9 MV/cm. In relation to their thickness the inventive layers have a remarkably low defect density. In fact the inventive layers can be distinctly less thinner than 20 nm, e.g. less than 10 nm and still have a defect density of 0.2 defects per cm². Especially two aspects of the inventive method are surprising: That ion implantation - contrary to the apprehension - can achieve such an improvement in defect density and that only a rather high dose, i.e. > 10¹⁵ atoms/cm² can accomplish that result. The inventive method can be practised with, Si, ions. According to the inventive method a ≦ 10 nm thick gate oxide layer with a defect density < 0.2 defects/cm² can be formed by implanting silicon ions into the substrate through the grown gate oxide layer with a dose of ≧ 10¹⁶ atoms/cm² and by subsequently annealing the structure at a temperature of ≧ 1150°C. The anneal at the high temperature guarantees, that the flatband voltage of the device does not shift. Since this anneal can be very short, i.e. in the order of 10 seconds, the high anneal temperature is not critical even if the device has received a doping in a previous process step where the doping profile must not change. Other advantageous embodiments of the inventive method are disclosed in the subclaims. The invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawing. The Figure shows in a diagram the defect densities of an about 10 nm thick silicon dioxide layer plotted against doses of silicon and BF₂ (not part of the invention) ions being implanted through the silicon dioxide layer where after the implantation the layer was annealed at 1000°C for thirty minutes. The substrate to be used with the invention is silicon which can have undergone a doping process i.e. contain differently doped areas including p/n-junctions. The surface of the substrate need not be flat, but can contain for example steps or trenches. On the substrate the dielectric layers according to the invention are formed. Studied were layers consisting of silicon dioxide, silicon monoxide or silicon nitride or mixtures of the above compounds and also stacks of layers containing at least two differently composed layers where the compositions were selected from the above compounds or mixtures thereof. The layers were grown using thermal wet and dry oxidation. The thickness of the layers ranged between 8 and 20 nm with the thickness being measured conventionally with an ellipsometer. The defect density of the layers as grown (the method to measure the defect density is described below) is not uniform. As can be seen from the Figure the yield of 10 nm thick silicon dioxide layers grown by thermal wet oxidation on silicon substrate can take values like 36 and 64 %. Yields of 36 and 64 % correspond to defect densities of 10.6 and 4.6, respectively. The substrate, a silicon wafer, with the grown layer was subjected to ion implantation. For the experiments the ion implanter model 350D marketed by Varian was used which is a conventional medium current implanter. The ions being implanted were, silicon. The energy of the ions is chosen in such a way, that the r p (projected range) exceeded the thickness of the grown dielectric layer. The energy of silicon ions implanted through a 14 nm silicon dioxide layer was ≧10 keV. The applied dose ranged between 10¹³ and 10¹⁶ ions/cm² for all the applied ions. Subsequently the substrates with the grown dielectric layers were subjected to an anneal step in an inert atmosphere. The purpose of the anneal is it to heal the damages caused by the ion implantation. To accomplish this treatment at a temperature higher than 500°C in an inert atmosphere is sufficient. The time is not critical however at the lower temperatures (i.e. ≦800°C) a duration of between thirty minutes and about two hours is preferred. In case the dielectric layer serves later on as gate isolation an anneal temperature of about 500°C is too low. It will be sufficient to improve the breakdown voltage yield but the threshold voltage will shift and become instable. To avoid this an anneal at a temperature of ≧ 1150°C is required. At this temperature p/n-junctions migrate rather fast, therefore when p/n-junctions were formed in the substrate in previous process steps the high temperature anneal has to be very short. Actually - as pointed out above - the anneal time is not critical and an anneal time of ten seconds is already sufficient. Such an anneal can be made in an RTP (rapid thermal processor), e.g. the model AG 610 or AG 4100 marketed by AG Associates in which the substrate temperature can be ramped up at a rate of 200 to 400°C per second by means of heat radiation emanating from tungsten lamps to which the substrates are exposed. To cool down the substrate temperature with a fast rate it is sufficient to switch off the tungsten lamps. For short anneal cycles the inert atmosphere can consist of nitrogen if the anneal cycles take longer the nitrogen must be replaced by argon. The result of the described treatment is illustrated in the Figure for the cases where silicon or BF₂ ions respectively were implanted into a 10 nm thick silicon dioxide layer followed by an anneal at 1000°C for thirty minutes. The Figure shows that independently of the ion used at doses of ≦10¹⁵ ions/cm² the yield decreases or remains about constant and that when the dose is increased above 10¹⁵ ions/cm² a dramatic yield increase decrease occurs with the yield at a dose of 10¹⁶ ions/cm² being about 93 %. The defect density or the yield respectively is measured by a double ramp method. For the measurement the samples, like silicon wafers having a diameter in the order of 100 mm, are prepared by producing on the grown dielectric layer 110 aluminum dots. The dots having a diameter of 3.5 mm and a thickness of 0.7 µm each are distributed regularly over the wafer surface. At the aluminum dots the breakdown voltage is measured in the following way: A voltage is applied between the substrate and the aluminium dot with the voltage being ramped up at a rate of 1 volt per second. If at a field of 9 MV/cm the current does not exceed 1 µA the breakdown voltage at the location of said aluminium dot is considered sufficient. The ramp up of the voltage is repeated (therefore the expression "double ramp method") in order to trace failures that have occured but were not detected during the first ramp up. The measurements revealed the dramatic decrease of the defect density as a result of the ion implantation with doses > 10¹⁵ ions/cm². It was also found - which is however not so important - that the inventive method does not cause an increase of the absolute value of the breakdown voltage. The effect of the high dose ion implantation cannot be explained yet. It was found, that no change of the refractive index or the density of the dielectric layer occurred as was found after the treatment disclosed in US-Patent 4 001 049. The excellent dielectric properties of the inventive layers make then suitable for VLSI-applications in the semiconductor technology field. VLSI-applications concern mainly gate-isolation where with increasing miniaturization the thickness of the dielectric isolation must decrease where it is however essential, that the defect density does not increase. The inventive method, i.e. the formation of a gate isolation, will be described in even greater detail below by way of an example. Five silicon wafers were processed in exactly the same way. The wafers had a homogeneous doping of 4 to 6 x 10¹⁵ boron atoms/cm³. On the five wafers and on an additional sixth wafer a 10 nm thick silicon dioxide layer was blanket grown by thermal oxidation where the wafers were subjected at a temperature of 750°C to an atmosphere containing air and water vapor. The uniformity of the layer thickness was verified ellipsometrically. On the additional sixth wafer the defect density was measured at this state by using the double ramp method as described above, i.e. by forming the 110 aluminium dots and then measuring the breakdown voltage or the field respectively at them. The yield was 49 %. In addition at the sixth wafer the flatband voltage was measured using the conventional procedure. Into the thin silicon dioxide layer on the five wafers silicon ions were blanket implanted with a dose of 10¹⁶ ions per cm² and an energy of the ions of about 10 keV using a model 350D Varian implanter. Subsequently the wafers were annealed in a rapid thermal processor (model AG 610 marketed by AG Associates) where the temperature of the wafers was ramped up with a rate of about 350°C/sec up to a temperature of about 1150°C that was held for about 10 seconds. Subsequently the heating was switched off to allow a rapid cooling of the wafers. During the anneal 3 l of N₂/min. streamed through the RTP. The five wafers were prepared for the BV-measurement exactly as described in connection with the additional sixth wafer. Finally using the double ramp method as described above the breakdown voltage on the field respectively was measured at the five wafers. The defect density was 0.5 ± 0.2/cm² corresponding to a yield of 95 ± 2 %, i.e. the results were very uniform and reproducible. In addition the flatband voltage was measured such as on the sixth additional wafer. It was found that the threshold voltage had not been changed by the ion implantation and the anneal.

Claims: A method of forming a silicon dioxide gate isolation layer having a thickness of ≦ 10 nm on a silicon substrate, comprising the steps of: forming said silicon dioxide layer by thermal oxidation, implanting silicon ions into said substrate through said silicon dioxide layer with a dose of ≧ 10¹⁶ ions/cm² and annealing said silicon dioxide layer at a temperature of ≧ 1150°C. The method of claim 1, wherein said annealing step is carried out for 10 seconds in an inert atmosphere. The method of claim 1 or 2, wherein during the annealing step, the substrate temperature is ramped up at a rate of 200° to 400° C per second, held at the anneal temperature for 10 seconds and cooled rapidly. A silicon dioxide gate isolation layer having a thickness of ≦ 10nm on a silicon substrate, prepared by: forming said silicon dioxide layer by thermal oxidation, implanting silicon ions into said substrate through said silicon dioxide layer with a dose of ≧ 10¹⁶ ions/cm² and annealing said silicon dioxide layer at a temperature of ≧ 1150°C wherein said resulting layer has a defect density of ≦ 0.6 defects/cm².

Inventors: EUEN WOLFGANG, HAGMANN DIETER DR, WILDAU HANS-JUERGEN, EUEN, WOLFGANG, HAGMANN, DIETER, DR., WILDAU, HANS-JUERGEN, Wildau, Hans-Jürgen
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1988
Main classification: H01L21
IPC codes: H01L27, C23C14, H01L21