{x,y}= {
     {10^-6,10^-15},
     {10^-5,10^-14},
     {10^-4,10^-14},
     {10^-3,10^-13},
     {10^-2,10^-11},
     {10^-1,10^-9},
     {1,10^-8}
    }

xy = {{10^-6, 10^-15}, {10^-5, 10^-14}, {10^-4, 10^-14}, {10^-3,
    10^-13}, {10^-2, 10^-11}, {10^-1, 10^-9}, {1, 10^-8}};

ClearAll[intf]
intf = Interpolation[xy, InterpolationOrder -> 3];
Plot[intf[t], {t, 0, 1}]

ClearAll[fitf]
fitf[x_] := Evaluate@Fit[xy, {1, x, x^2, x^3}, {x}]
Plot[fitf[x], {x, 0, 1}]