Liquid etchant composition for thin film resistor element
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid etchant composition for use in fabrication of a thin film resistor element in an electronic circuit, comprising chromium, silicon and oxygen (hereinafter referred to as &#34;Cr.SiO 2  &#34;), more particularly to a liquid etchant composition for use in etching the Cr.SiO 2  resistor material having an aluminum layer applied as conductor thereon, with an undercut-free shape at the pattern edge of the aluminum layer and a process for etching the Cr.SiO 2  resistor material in an easy and controllable manner. Furthermore, the liquid etchant composition of the present invention is more suitable and useful when the resistor film is formed on an organic insulating film such as a polyimide film. 
     2. Description of Related Art 
     Electronic integrated circuits used in a computer are terminated with resistors having the same resistance as that of a transmission line impedance for the purpose of obtaining impedance matching between the circuit elements and the transmission line to inhibit reflection noise generated in transmitting high-speed digital signals. That is, a so-called impedance matching by signal termination resistors has been adopted in the integrated circuit application systems. Therefore, termination resistors must be arranged in or about a package at the same time when the integrated circuits are to be packaged on a board, thereby terminating the transmission line. 
     Hitherto, resistors for the impedance matching have been externally mounted as independent parts on printed circuit boards or multichip module boards. However, the external mounting of the independent parts is improper for miniaturization of the integrated circuits and improvement of packaging density. 
     Built-in resistor elements contained in packaging boards could greatly improve the packaging density of the integrated circuits. In a method therefor which has been proposed, a thin film resistor is formed on the surface of a certain layer of a packaging board having a multilayered structure, and the thin film is then fabricated by photoetching into an element shape to have a desired resistance. According to this method, the packaging density cannot only be improved but also a high resistance-accuracy termination resistor element can be formed, and further the length of wirings can be shortened so that the signal-propagating speed is greatly improved. 
     As a resistor material useful for built-in resistor element in the packaging board has been developed. The material is a composite of SiO 2  and Cr metal which is deposited as thin film by RF magnetron sputtering (see H. Matino and T. Ushida, &#34;Effect of Substrate Bias on Properties of RF-Sputtered Cr-SiO 2  Films&#34;, IBM J. RES. DEVELOP., November 1977, pp. 576-579). The liquid etchant composition for this resistor material has been demanded. 
     In order to secure an ohmic contact between the terminating resistor element and the upper wiring, it is desired that the resistor film and the wiring film are formed in a continuous deposition process. On the other hand, this method has a problem that an adequate pattern-forming accuracy is hardly obtained when the wiring film is thick. It is known that this problem has been overcome to secure formation of the high-accuracy resistor pattern by providing a thin wiring film and then forming a two-layer film pattern of the resistor material and the wiring material in a continuous manner by a photoetching process and thereafter forming a film of the wiring material which has a desired thickness to form a termination resistor pattern. 
     Moreover, large-scaled computers or supercomputers have a tendency to use a thin film multilayered wiring board using an organic insulating material having a low dielectric constant, such as polyimide, in a packaging board such as a microchip carrier or multichip module board for the purpose of shortening the package-originated signal propagation delay. Now, the abovementioned termination resistor element has been necessitated to be formed on the organic insulating film, for example, polyimide. 
     As prior etching compositions for etching the Cr.SiO 2  resistor material film is known a liquid mixture (or mix) of nitric acid and hydrofluoric acid as disclosed in Japanese Patent KOKAI (Laid-Open) No. 60-83301. However, this liquid mixture has two problems when used for etching a laminate comprising the Cr.SiO 2  film and an aluminum layer on the film (hereinafter referred to as &#34;Al/Cr.SiO 2  laminate&#34;): the first problem is that the resistor film is undercut so that the aluminum layer forms an eave (see FIG. 1D attached hereto) and the second problem is that the etch residues are easily produced when the Cr.SiO 2  film is etched by the prior etchant. In the first problem, an aluminum electrode film in cracked at the Cr.SiO 2  pattern edge when applied in the subsequent electrode deposition step onto the Al/Cr.SiO 2  laminate. The cracks propagate from the eave region through, whole Al film to finally reach the top Al surface. FIGS. 2A to 2D show an etching process showing schematical cross-sectional views of a sample at each step. In these figures, reference number 1 denotes a patterned photoresist, 2 an aluminum layer, 3 a Cr.SiO 2  layer, 4 a polyimide layer, 5 a ceramic or glass substrate and 6 etch residues. FIG. 2A shows the sample having a patterned photoresist film and unetched Al/Cr.SiO 2  laminate. FIG. 2B shows the sample having the etched Al/Cr.SiO 2  laminate. FIG. 2C shows the sample having the photoresist removed. FIG. 2D shows the sample having a thick aluminum electrode formed thereon and illustrates that microcracks 8 are produced in the Al electrode during or after the deposition of the electrode when the resistor film has undercut. It shows that the microcrack occurs because of insufficient Al coverage caused by the undercut of the resistor film. These microcracks may be a cause of the open failure of the aluminum electrode, which deteriorate the reliability of integrated circuits. If the aluminum eave is large enough, it falls down and therefore the Al electrode is badly deformed. In the second problem, the residues are more easily produced particularly when the Cr.SiO 2  film is deposited on the organic insulating film of polyimide or the like. Therefore, there arise problems caused by the etch residues, that is, a bad effect on pattern-wise etching of underlying polyimide or adhesion damage between the polyimide and a next thin film on that. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide a liquid etchant composition for etching the Cr.SiO 2  resistor material of the Al/Cr.SiO 2  laminate used in an electronic integrated circuit, with the aluminum layer being free of undercutting so that microcrack propagation in the aluminum layer at the edges of the resistor film pattern is inhibited during the subsequent step of the aluminum electrode deposition and with no etch residues being produced on the board. 
     The secondary object of the present invention is to provide a method of etching the Cr.SiO 2  resistor material, particularly of the Al/Cr.SiO 2  laminate as mentioned above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A, 1B, 1C, 1C&#39;, 1D and 1D&#39; are schematical cross-sectional views of a sample showing a flow chart of the etching process for etching the Cr.SiO 2  resistor material with a liquid etchant composition according to an embodiment of the present invention and a prior etchant composition. 
     FIGS. 2A, 2B, 2C and 2D are schematical cross-sectional views of a sample illustrating the mechanism for producing microcracks in an aluminum electrode. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with the present invention, a sintered material comprising a silica matrix and metallic chromium fine particles dispersed in the matrix, referred to as Cr.SiO 2 , is used as sputtering target material for resistor film deposition, and aluminum is used as electrode and wiring materials. 
     A laminate comprising a resistor film and a thin electrode film must be subjected to the patterning process by a photoetching to form an isolated pattern. From the standpoint of simplifying the process, it is desired that the photoetching is conducted in one step to form the laminate patterns. Furthermore, the liquid etchant composition should be such that it can form the Al/Cr.SiO 2  laminate with an undercut-free shape at the edge of the laminate pattern to secure an aluminum electrode film coverage at the edge of the laminate pattern and inhibit the formation of cracks in the aluminum layer. 
     In order to achieve the abovementioned objects of the present invention, the present inventors have found a novel etchant composition for etching the Cr.SiO 2  resistor material of the Al/Cr.SiO 2  laminate. 
     The present liquid etchant composition should have (1) an adequate etching power for both chromium and SiO 2 , (2) a proper etching power for aluminum as well, which realizes undercut-free shape of Cr.SiO 2  resistor film together with normally tapered thin Al electrode 
     edge at the pattern end of Al/Cr.SiO 2  laminate, and (3) a proper etching rate. 
     In accordance with the present invention, the acceptable maximum undercut is 0.5 μm though no undercut is favorable. 
     Firstly, a reagent for etching each of the three components, aluminum, chromium and SiO 2  is selected as a base component. To the mixture of these reagents is added ammonium fluoride which is experimentally known to effectively increase the adhesion of a photoresist during the etching procedure and further added phosphoric acid which was determined to be effective for promoting the etching of the Cr.SiO 2  material, whereby the object of the present invention has been achieved. 
     The etching action of the liquid etchant composition according to the present invention will be in detail explained below. 
     Etching of the Cr.SiO 2  material with a prior etchant composition proceeds in two stages. The etching rate is higher at the earlier stage to remove a major part of the material and at the subsequent stage the etching rate is lower because of a slowly-etched layer and, therefore, a major part of the etching time is consumed to remove the slowly-etched layer. That is, the film of the Cr.SiO 2  material is composed of two kinds of stacked layers which are different in their properties from each other. The presence of the slowly-etched layer causes the unstability of the etching time and the formation of etch residues. Particularly, when the undercoat is of an organic material such as polyimide, this tendency increases. To completely remove such obstinate etch residues, the overetching period tends to exceed to finally bring about the great undercut in the Cr.SiO 2  layer. 
     The liquid etchant compositions of the present invention can provide an undercut-free shape at the edge of the Al/Cr.SiO 2  laminate pattern and have an adequate etching capacity even when the laminate is applied on the organic insulating film of, for example, polyimide which facilitates the formation of the etch residues. The liquid etchant composition of the present invention is characterized by containing hydrochloric acid for etching chromium, hydrofluoric acid and ammonium fluoride for etching silica, phosphoric acid for strengthening the etching composition and ammonium fluoride for improving the adhesion of resists. 
     The present inventors, as a result of their extensive study, have found that the etching liquid composition of the present invention should preferably have the following concentration for each of the components: 
     
         ______________________________________
hydrochloric acid  1.92-2.64
                            mol/liter
phosphoric acid    0.26-0.77
                            mol/liter
hydrofluoric acid  5-10     mol/liter
ammonium fluoride  3.2-5.4  mol/liter.
______________________________________
 
    
     If the concentration of the hydrochloric acid is lower than the range above, the undercut amount of the Cr.SiO 2  material increases so that the undercut-free shape cannot be formed (see Example 8 below). The concentration exceeding the range, on the other hand, brings about the same results (see Example 10). If the concentration of the phosphoric acid is lower than the range above, there occurs a problem of etch residues of the Cr.SiO 2  layer (see Example 11). If the concentration of the phosphoric acid exceeds the range, on the other hand, there arises no etch residue at all but the undercut amount increases. (see Example 12). If the concentration of the hydrofluoric acid is lower than the range above, there is a problem of pattern edge deformation in the Cr.SiO 2  resistor film. (see Example 6). If the concentration of the hydrofluoric acid exceeds the range, on the other hand, the undercut amount of the Cr.SiO 2  resistor film increases. (see Example 4). If the concentration of the ammonium fluoride is lower than the range above, the adhesion of photoresists is poor and a small amount of the Cr.SiO 2  resistor film is undercut (see Example 13). If the concentration of the ammonium fluoride exceeds the range, on the other hand, a large amount of the etch residues appear, although the undercut-free shape of the pattern edge of the Al/Cr.SiO 2  laminate can be obtained (see Example 14). 
     The Cr.SiO 2  etchant composition of the present invention is also effective for the Cr.SiO 2  thin film material applied on a substrate of an inorganic material such as glasses, ceramics or silicon semiconductors, as well as on the organic material such as polyimide. 
     The present invention will be illustrated below with reference to some examples. 
     EXAMPLES 
     The testing method used in these examples are as follows: 
     An alumina ceramic substrate or a glass substrate of 10 cm×10 cm square has been used as a base board. Polyimide vanish was spin-coated on these substrates, then baked and cured at a suitable temperature. The final thickness of the polyimide coating was 14 μm. The coated substrates were placed in a sputtering apparatus and dried by heating under vacuum. Then, a Cr.SiO 2  (Cr:SiO 2  ratio=66 wt %:34 wt %) film and an aluminum film were continuously deposited in this order by sputtering. The sputtering conditions were as follows: 
     
         ______________________________________
1)  Cr.SiO.sub.2 sputtering
    RF Power Input          1.0 kw (2 min.)
                            0.5 kw (2 min.)
                            0.25 kw (2 min.)
    Argon pressure = 0.2 Pa
2)  Aluminum sputtering
    DC Power Input (DC) = 4.0 kw (8 min.
    and 40 sec.)
    Argon pressure = 0.2 Pa
3)  Thickness of Cr.SiO.sub.2 film = 2000 Å
4)  Thickness of aluminum film = 4000 Å
______________________________________
 
    
     A positive-type photoresist pattern was applied on the Al/Cr.SiO 2  laminate under predetermined conditions. Then, the aluminum film was etched using the resist pattern as a mask. The etching conditions were as follows: 
     
         ______________________________________
1)    Liquid Etchant Composition:
      phosphoric acid    78 parts by volume
      nitric acid         2 parts by volume
      acetic acid        15 parts by volume
      water               5 parts by volume
2)    Liquid Temperature:
                         40° C.
3)    Etching Period:    Just etch time + 30
                         seconds
______________________________________
 
    
     The aluminum etchant was prepared as mentioned above, and heated to a predetermined temperature in a water bath, into which the substrates were dipped and left to stand to thereby etch the aluminum film. The procedures above were the same for all the sample substrates. Thereafter, the various Cr.SiO 2  etchant compositions as shown in Table 1 were prepared, in which the aluminum-etched sample substrates were etched under the abovementioned conditions while being vibrated. Then, the sample substrates were subjected to the washing, drying and resist-stripping by a resist stripper. 
     
                                           TABLE 1
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Component                      Etching
                                    Etch Adhesion
                                              Undercut
                                                    Shape of
HF        HCl
             H.sub.3 PO.sub.4
                 NH.sub.4 F
                     HNO.sub.3
                         KCl
                            H.sub.2 O
                               time sec
                                    residues
                                         of resist
                                              amount μm
                                                    pattern
__________________________________________________________________________
Example
1     7.5 1.92
             0.77
                 4.32          30   none ◯
                                              0.2   ◯
2     &#34;   &#34;  0.51
                 &#34;             &#34;    &#34;    &#34;    0.3   &#34;
3     &#34;   2.40
             &#34;   &#34;             &#34;    &#34;    &#34;    0.0   &#34;
4     &#34;   2.64
             &#34;   &#34;             30   &#34;    ◯
                                              0.2   ◯
Comp. Ex.
1     2              3      60 105  yes  X    10    X
2     1          7             (Etching could not be effected.)
3     1                        90   yes  X    7     ◯
      1.75 *
4     12.5
          1.92           2.01  30   &#34;    X    0.0   X
5     11.3
          &#34;      4.32    1.07  &#34;    &#34;    ◯
                                              0.0   ◯
6     7.5 0.96
             6.12              30   none ◯
                                              5     ◯
7     10.0
          1.92
             1.02
                 4.32          &#34;    &#34;    ◯
                                              1     ◯
8     7.5 &#34;  &#34;   &#34;             &#34;    &#34;    ◯
                                              0.8   ◯
9     5.0 &#34;  &#34;   &#34;             &#34;    &#34;    ◯
                                              0.4   Δ
10    7.5 2.40
             0.26
                 &#34;             &#34;    yes  ◯
                                              0.0   ◯
11    &#34;   &#34;  0.77
                 &#34;             &#34;    none ◯
                                              1.5   ◯
12    &#34;   &#34;  0.51
                 5.74          &#34;    &#34;    X    0.1   ◯
13    &#34;   &#34;  &#34;   5.40          &#34;    yes  ◯
                                              0.0   ◯
__________________________________________________________________________
  in parts by volume (the other in mol/l)
 *amount of polyethylene glycol added
 
    
     The etching process as mentioned above is illustrated in FIGS. 1A to 1D and 1A to 1D&#39; showing the schematical cross-sectional views of the sample in each step. In these figures, reference number 1 denotes a resist pattern, 2 an aluminum layer, 3 a Cr.SiO 2  layer, 4 a polyimide layer, 5 a substrate, 6 etch residues and l 1  an undercut amount of the Cr.SiO 2  layer. The shape of the edge of the Al/Cr.SiO 2  laminate after the Cr.SiO 2  layer was etched is shown in FIG. 1C for etching by a prior liquid etchant composition and in FIG. 1C&#39; for etching by the liquid etchant composition of the present invention. the resist-removed state of the Al/Cr.SiO 2  laminate is shown in FIG. 1D. In this figure, l 1  is a length of the eave, i.e., a distance from the end of the aluminum layer to the end of the undercut Cr.SiO 2  layer. This is referred to as the &#34;undercut amount&#34;. According to the present invention, a novel liquid etchant composition giving l 1  =zero (0) has been found. 
     In the present invention, the presence of the etch residues of Cr.SiO 2  is observed on the polyimide surface by an X-ray microprobe analyzer. The adhesion of the resist is visually inspected after the etching of Cr.SiO 2  or during the washing of etched Cr.SiO 2  with water. The pattern shape is observed by SEM. 
     In Table 1, Comparative Example No. 1 uses a prior liquid etchant composition composed of hydrofluoric acid and nitric acid. This example presented no satisfactory result in any etch performance item. The mixture of hydrofluoric acid and ammonium fluoride used in Comparative Example 2 did not work for etching Cr.SiO 2 . In Comparative Example 3, the composition is a mixture of hydrofluoric acid and polyethylene glycol which is a viscosity enhancer. The etching of Cr.SiO 2  was possible with this solution but the undercut amount was 7 μm and the eave was formed in the aluminum layer and the etch residues of Cr.SiO 2  were observed on the polyimide layer near the etched pattern edge. In order to completely remove the etch residues, the overetching time was obliged to get longer so that the undercut amount of the Cr.SiO 2  layer was greatly increased. The adhesion of the resist was poor so that the resist came off during the washing with water. 
     The other comparative examples were poor in any one of the inspected properties. In Comparative Example 4, a liquid etchant composition comprising hydrofluoric acid, hydrochloric acid and potassium chloride was used in order to strengthen the etching power for chromium and avoid the formation of the eave of aluminum. The eave was not produced but the aluminum layer was excessively etched to damage the edge shape of the Cr.SiO 2  layer. The adhesion of the resist was poor. The etch residues could not be completely removed. Then, in Comparative Example 5, ammonium fluoride was added, which is experimentally known to improve resist adhesion. In this case, the undercut amount of Cr.SiO 2  was zero (0), and the undercut-free Al/Cr.SiO 2  laminate was obtained. However, there was observed a large amount of etch residues. Comparative Example 6 used a liquid composition containing phosphoric acid added thereto to raise the viscosity of the composition and inhibit the undercut of the Cr.SiO 2 . The etch residues were not observed at all. However, an undercut of about 5 μm was unexpectedly observed. Thus, the addition of phosphoric acid gave good results for the etch residues but bad results for the undercut when the amount of phosphoric acid was large. 
     Comparative Examples 7 to 9 were intended to obtain the synergetic effect of Comparative Examples 4 and 5. Particularly, the concentration of hydrofluoric acid was changed in each of these comparative examples. In both of these comparative examples, the etch residues of Cr.SiO 2  were not observed at all. The pattern shape became poorer and the undercut amount was decreased as the content of hydrofluoric acid was increased. 
     As is clear from the foregoing, the present invention provides a high performance Cr.SiO 2  -etchant composition, particularly liquid etchant compositions for wet-etching an Al/Cr.SiO 2  laminate with the Cr.SiO 2  resistor layer therein without undercutting the Cr.SiO 2  layer and with forming an undercut-free shape at the end of the Al/Cr.SiO 2  laminate, and further a method of etching the Cr.SiO 2  resistor layer. The liquid etchant composition of the present invention is particularly effective for etching integrated circuits containing an organic insulating film of, for example, polyimide therein. That is, little or no residues of Cr.SiO 2  are produced when etched. The liquid etchant composition of the present invention allows the etching process to be carried out with good reproducibility. The undercut-free shape of the Al/Cr.SiO 2  laminate pattern edge does not allow any microcrack formation in an aluminum electrode formed in the subsequent deposition step at the edge of Al/Cr.SiO 2  laminate pattern. Therefore, the present invention can provide terminal resistor elements with high reliability in packaging boards, such as a microchip carrier or multichip module boards, for integrated circuits. 
     With the liquid etchant composition of the present invention, etching of the Al/Cr.SiO 2  laminate can be continuously performed by using Al etchant and then Cr/SiO 2  etchant with only one photoresist pattern. The photoresist does not come off during the series of etching. Therefore, use of the liquid etchant composition of the present invention permits to provide the greatly simplified process. 
     Furthermore, since the liquid etchant composition has a strong etching power, this etching power is not lost even for the Cr.SiO 2  resistor film formed on the organic insulating film. Therefore, the liquid etchant composition of the present invention can cope with some change of the Cr.SiO 2  film properties under mass production conditions and then has a wide margin of tolerance for Al/Cr.SiO 2  film deposition processes and is independent of substrates.