Trench etching process

Abstract: A trench etching process comprises the steps of: preparing a substrate, forming a mask pattern for the trench etching having a material different from that of the substrate directly on the substrate, detecting changes in results of emission spectroanalyses generated by etching the mask pattern and the substrate, and determining that the trench etching is completed when a predetermined change in the spectroanalysis results occurs. Having the mask material deposited directly on the substrate enables a uniform trench depth to be achieved.

Description: The present invention relates to a trench etching process. With the enhancement of the degree of integration of IC's, the trench etching process for, for example, silicon, has become extremely important when miniaturizing the area of the isolation and capacitor cells. In a trench etching process for producing a trench in a silicon substrate, a precise and reproducible control of the depth of the trench is difficult, since there is no layer which can act as a stopper layer, and accordingly, there is no known effective means for controlling the distribution of the depth of trenches over an entire surface of the silicon substrate. A prior-proposed trench etching process is shown in Figures 1A to 1C. As shown in Fig. 1A, a phosphosilicate glass (PSG) layer 2 having a thickness of, for example, 1.0 µm is formed on a silicon substrate 1, as shown in Fig. 1B, an opening 3 by which the silicon substrate 1 is exposed is formed in the PSG layer 2 in which a trench is to be formed, by photolithography. After forming the opening 3, as shown in Fig. 1C, a trench 4 is formed in the silicon substrate 1 by a reactive ion etching (RIE) process using the remaining PSG layer 2 as a mask. The RIE process for producing a required trench is usually carried out by a time control, i. e. the time is predetermined by calculating the etching rate of the RIE of the silicon substrate. The RIE process for producing the required trench can also becarried out while monitoring the depth of the trench by calculating a difference between the time when laser beam is reflected from the surface of the silicon substrate and from the bottom surface of the trench. Such a trench etching process, wherein a time control is carried out, is disadvantageous in that there is no guarantee that a required etching has been carried out, since no warning is given if a required etching is not realized due to unforeseen circumstances. Further, when the depth of trench is monitored by using a laser beam, etc., the monitoring precision is reduced due to the shape of the trench. Japanese Unexamined Patent Publication (Kokai. No. 61-232620, published on October 16, 1986, discloses a process for etching a semiconductor substrate wherein an etching end point is detected by monitoring the emission intensity of an atom or molecule or an ion of components of an etching mask. In this etching process, a completion of a trench etching can be precisely and reproducibly detected, but when many trenches having the same depth are formed in a silicon substrate, a uniform distribution of the depth of the trenches can not be obtained over an entire surface of the silicon substrate, since the etching mask is not directly laid on the silicon substrate and the etching rate of the film disposed between the etching mask and the silicon substrate is not the same as that of the silicon substrate. An embodiment of the present invention may provide a trench etching process wherein a completion of the trench etching can be precisely detected. An embodiment of the present invention may provide a trench etching process wherein a precise trench etching can be carried out reproducibly. An embodiment of the present invention may also provide a trench etching process wherein trenches having substantially the same depth can be obtained over an entire surface of, for example, a silicon substrate. According to the present invention, there is provided a trench etching process comprising the steps of: preparing a substrate; forming a mask pattern for trench etching, comprising a material different from the material of the substrate, directly on the substrate; commencing etching of a plurality of trenches in the substrate using the mask; detecting the results of emission spectroanalyses generated by etching the mask pattern and the exposed portions of the substrate; and determining the completion of the trench etching using a predetermined change in the spectroanalysis results indicating that the said mask pattern has been removed from the entire surface of the substrate. In the present invention, a silicon or other semiconductor substrate is preferably used. Further, the mask pattern is preferably made of phospho-silicate glass or thermal oxide of silicon. For a better understanding of the invention, an embodiment will be described with reference to the accompanying drawings, in which: Figures 1A to 1C are cross-sectional views explaining prior-proposed processes; Figs. 2A to 2D are cross-sectional views of an embodiment of the process of the present invention; and Fig. 3 is a graph illustrating a relationship between etching time and the depth of trenches in different regions of the silicon substrate. As shown in Fig. 2A, an etching mask of, for example, phosphosilicate glass (PSG) film 2 having a thickness of 1 µm is formed on a surface of a silicon substrate 1. Note, Fig. 2A shows cross-sectional views of two regions of the silicon substrate 1, i.e., REGION A and REGION B. P denotes a surface of the substrate 1. Then, as shown in Fig. 2B, a trench etching process is carried out under the following etching conditions, using a reactive ion etching device (RIE device). Reactive gas ..... carbon tetrachloride (CCℓ₄) + chlorine gas (Cℓ₂) Pressure in reaction chamber ..... 13 Pa (0.1 Torr) High frequency electrical power ..... 700 W In Fig. 2B, 4 denotes a trench and 5 denotes ions of the reactive gas. As the RIE etching progresses, the PSG film 2 may be completely removed at a portion, for example, region B, of the silicon substrate but still remains at, for example, region A, due to, for example, a deviation of the thickness of PSG film 2 on the surface of the silicon substrate, or the surface roughness, etc., as shown in Fig. 2C. When the etching has progressed beyond the step shown in Fig. 2C, the PSG film 2 is completely removed from all portions of the surface of the silicon substrate, including the regions A and B, as shown in Fig. 2D. At this point in the RIE process the 603 nm spectral emission, which is present until all of the PSG film 2 has been removed, disappears or decreases markedly. Even when a silicon oxide film is formed during the etching process on the side wall portion of the trench 4, the 603 nm emission is markedly reduced, and thus, when changes in the results of emission spectroanalysis are generated, e.g. the spectral emission has vanished or markedly decreased, it is determined that the trench etching is completed. The time from the removal of the mask, i.e., the PSG film 2, at one portion to the time of the completion of the etching varies in different positions of the wafer surface, and the depth of trench formed by etching after the mask has been removed varies due to the difference in etching speeds for the trench per se and for the surrounding surface of the silicon substrate 1. Nevertheless, since the material of the inner portion of the trench and of the surrounding surface is the same, i.e. silicon, there is no difference in the etching speed thereof. Further, the period from the removal of the mask to the end of the trench etching is very short, compared with the total etching period. Therefore there is no deviation of depths of trenches during the period from the removal of the mask to the completion of the etching, with the result that the depths of the trenches are substantially the same depth over the entire surface of the wafer, e.g., the silicon substrate. The depth L₁ is substantially the same as the depth L₂ , and thus the depth of a trench in region B becomes the same as that of a trench in region A, and thus, according to this embodiment, a control of a distribution of trenches having the same depth can be carried out. The reason why the depths of the trenches in regions A and B are substantially the same is due to the mask thickness (D₁ and D₂), the etching rates of the masks (M₁ and M₂), and the etching rates of silicon (S₁ and S₂) in each region A and B, as explained below. If one supposes that the etching rates of the silicon substrate 1 and the PSG film 2 (mask) in the region B are larger than those in the region A, the ratio of the depths T₁ and T₂ of trenches in the regions A and B will be as shown by the following expression T₂/T₁ = S₂/S₁ x M₁/M₂ x D₂/D₁ and provided that the mask thickness is same (i.e., D₁ = D₂, and if the ratios of the etching rates are expressed as S₂/S₁ = 1 + α, M₂/M₁ = 1 + β, then T₂/T₁ =(1+α)/(1+β) = 1 + (α-β)/(1+β) Since α and β are in general both positive and of similar magnitude, α is larger than (α-β)/(1+β), and consequently, T₂/T₁ is nearer to 1 than S₂/S₁, with the result that a more uniform trench etching depth can be obtained. The above theory is supported by Fig.3, which illustrates a relationship between the etching time and the depth of trenches in region A and B of the silicon substrate. The steps I, II and III in Fig.3 correspond to Figs.2B, 2C, and 2D, and show the first stage, the middle stage, and the completion of the trench etching. In the first and middle stages I and II, the depths of trenches in the region B are slightly larger than those in region A, but on the other hand, upon completion of the etching, the depths of trenches in the region B are slightly less than that in region A. Various etching processes can be used with the process according to the invention, such as ECR (Electron Cyclotron Resonance) etching or MRIE (Magnetron Reactive Ion) etching. As will be clear from the above, the present invention may provide a trench etching process wherein a completion of a required etching process in a trench can be determined by detecting a change in emission analysis results.

Claims: A trench etching process comprising the steps of: preparing a substrate (1); forming a mask pattern (2) for trench etching, comprising a material different from the material of the substrate, directly on the substrate; commencing etching (5) of a plurality of trenches in the substrate using the mask; detecting the results of emission spectroanalyses generated by etching the mask pattern (2) and the exposed portions of the substrate (1); and determining the completion of the trench etching using a predetermined change in the spectroanalysis results indicating that the said mask pattern has been removed from the entire surface of the substrate. A trench etching process according to claim 1, wherein the determination of the completion of the trench etching also uses etching ratios of the mask pattern and the silicon substrate. A trench etching process according to claim 1 or 2, wherein the substrate (1) is a silicon substrate. A trench etching process according to any preceding claim, wherein the mask pattern (2) is made of phospho-silicate glass or thermal oxide of silicon. A trench etching process according to any preceding claim, wherein the trench etching is carried out by reactive ion etching. A trench etching process according to claim 5, wherein the etching is magnetron reactive ion etching. A trench etching process according to any of claims 1 to 4, wherein the etching is electron cyclotron resonance etching. A trench etching process according to any preceding claim, wherein the predetermined change is a reduction to substantially zero of the intensity at a predetermined wavelength.

Inventors: ABE NAOMICHI, MIHARA SATORU, MOTOYAMA TAKUSHI, ABE, NAOMICHI, MIHARA, SATORU, MOTOYAMA, TAKUSHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: H01L21
IPC codes: H01L21